mmadden55

I think your cam is in wrong. The crank will show top dead center and if your cam is timed wrong it will still run like crap. Running that much advance at idle and then dying when it is in gear is typical of that.

skinny z

Any adjustment that I've worked with has always been accessed through the vacuum nipple. As you know, not all vacuum cans are adjustable. Maybe yours isn't and that'll make it difficult if not impossible to dial in without the problems you're experiencing. At least it has been for me with any OEM or aftermarket non-adjustable part. The mechanical adjustment under my cap is to set the total amount of vacuum advance available.
Just to keep it clear, this is taking into consideration that this is what's causing your idle issue. I'm only relating my experiences. It could be any of the suggestions offered up.

3rdgenmaro

iTrader: (9)

Unless it has jumped time, it shouldnt be. I havent ruled that out, but right now I know that there is room for improvement in the carb and distributor that needs to be straightend out first.


Yes and this is part of why it is so aggrevating to me. My camaro's HEI has the adjustable vac can. This evening Im going to do what JaBoT said and see what happens. But I really believe that part of the problem is the unadjustable vac advance. You should never need 25+ extra degrees vac advance.

skinny z

Relating my experience from earlier this week, I was running 14° of initial and ported vacuum. Making the switch to full manifold vacuum advance has brought in another 22° for an idle total of 36°. Not too far off what you might end up with. My car behaves like a completely different machine since the change.

3rdgenmaro

iTrader: (9)

Hmm. I will mess around more this evening and see what happens. Thanks again.

skinny z

I thought I'd post this from another thread. It may be useful to you (unless of course you've read it already) or others that are following what you're doing. It's a long read however the best part is in the vacuum can info section.
The following is copied directly from that thread:

Here is a great info on vaccum advance from one of the nova sites I use .. I copied and pasted so you don't have to log in to the site. It came from a corvette site originally, two guys contributed to parts of the article John Z and Lars Grimsrud .

3 parts...

Part 1


As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

skinny z

Vaccum Can info:

Distributor Vacuum Advance Control units
Specs and facts for GM Distributors

Background
The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load increases, this “over-advance” condition must be eliminated to produce peak power and to eliminate the possibility of detonation (“engine knock”). A control unit that responds to engine vacuum performs this job remarkably well.

Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec:

How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection.

How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit. If the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance unit can only be allowed to pull in 16 more degrees of advance.

Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.

Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in mind that the control unit is somewhat of a “finesse” or “final tuning” aid to obtain a final, refined state of tune; we use it to just “tweak” the car a little bit to provide that last little bit of optimization for drivability and economy. The vacuum advance unit is not used for primary tuning, nor does it have an effect on power or performance at wide open throttle.

With these general (and a little bit vague, I know…) concepts in mind, let’s review a few concepts and terms. Then it’s on to the master listing of specs and parts…..:

Part Number
There are many different sources for these control units. Borg Warner, Echlin, Wells, and others all sell them in their own boxes and with their own part numbers. Actually, there are very few manufacturers of the actual units: Dana Engine Controls in Connecticut manufactures the units for all three of the brands just mentioned, so it doesn’t make much difference who you buy from: They’re made by the same manufacturer. The part numbers I have listed here are the NAPA/Echlin part numbers, simply because they are available in any part of the country.

ID#
Every vacuum advance control unit built by Dana, and sold under virtually any brand name (including GM), has a stamped ID number right on top of the mounting plate extension. This ID, cross referenced below, will give you all specifications for the unit. So now, when you’re shopping in a junkyard, you’ll be able to quickly identify the “good” vs. the “bad” control units.

Starts @ “Hg
Vacuum is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to as “Hg. The “Start” spec for the control unit is a range of the minimum vacuum required to get the control unit to just barely start moving. When selecting this specification, consideration should be made to the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t want the vacuum to start coming in until 9 – 10 inches Hg. For most street driven vehicle performance applications, starting the vacuum advance at about 8” Hg produces good results.

Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet, will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.

Max Advance @ “Hg
This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine never produces 20” Hg, you better not select a control unit requiring 21” Hg to work.

The following listing (Non-HEI) is as follows: The first two part number listings are the two numbers that are most commonly used in a Chevrolet performance application. The “B1” can is the most versatile and user-friendly unit for a good performance street engine. As you can see, it was selected by GM for use in most high performance engines due to its ideal specs. The “B28” can was used on fuel injected engines and a few select engines that produced very poor vacuum at idle. The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. The Chevrolet applications are listed first by application, followed by a complete listing of all of the units used on any GM product (all GM units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do).

3rdgenmaro

iTrader: (9)

Yes I had read it before, but always good to refresh. I havent messed with the truck lately. Had some other things that needed to be done and spent some time with the family. Should get to mess with it some later this week.

rusty vango

people waste more time and money on a holley than they will EVER be worth,unless you are one of those guys that enjoy constantly having to tune,tune, and tune some more. I find a edelbrock carb to be a better choice as they are a "set it and forget it" kind of carb

3rdgenmaro

iTrader: (9)

Why not read the thread next time instead of making a useless "X brand is better then Y" statement.

JaBoT

I have no idea what you are talking about. Holley is like any other carb. If it's set right in the beginning it will always be set right.
I don't know why you think holley carbs mysteriously go out of tune. I have tuned dozens of holley carbs and not one came back out of tune for no reason.

3rdgenmaro

iTrader: (9)

Okay. Transfer slot is squared. Opened the secondaries back up about a 1/8 turn and when the motor was warmed up it let me square the transfer slots back up.

Reset a/f screws.

Hooked up the timing light and was sitting at 50 degrees, idling about 1200 rpm. No more coming in. Shifting into drive and it drops to 600 and almost dies but will barely idle. Try to retard the timing back down and it will not idle any lower. Lowest it will idle out of gear and stay running is 44 total. Will not idle in gear at that point.

Seriously driving me insane. Wonder if it is something in the idle circuit that is causing it or if its just the timing curve in the distributor.

May through the 600 from my Camaro on it this weekend just to see if it makes any difference.

Vac advance was unhooked entire time.

3rdgenmaro

iTrader: (9)

I just wanted to add that this aint something that has just suddenly happened.

It has always just been "tuned by ear". It ran and ran pretty well like that. But I know the transfer slots were WAY off like that and the a/f screws were way out. Who knows where the timing was.

After reading on here and seeing the sticky I wanted to get it dialed in right to see what I was leaving on the table.

It just makes me wonder if there is a problem in the carb that Ive always been able to cover up by tuning by ear and when you actually try to tune it, it shows itself?

skinny z

I haven't looked back to check but are you sure that your timing marks are accurate?
While it could be a problem with your carb, and your idea of swapping it out is not a bad idea, it may be that the problem lies else where. Even the previous post about your cam timing is a valid one.
You make a point about the tuning by ear and setting things up that way. It's not the point you think though. Without any data from the time it ran well and comparing it to what you're trying to do now, you end up lost.

3rdgenmaro

iTrader: (9)

Yes I reverified the timing marks.

I do know that when it did run good last, the secondaries were closed a lot more and the primaries were open WAY too much. If I remember right, the transition slots were probably open 1/4" or so.

It would be nice to have a distributor that was easier to adjust. I think getting it right and maybe a different carb on there but I don't know.

Only thing Im worried about doing is having the motor pinging and not be able to tell it.

skinny z

I agree on the distributor. I've spent hours over the last couple of weeks making changes and observing the effects. It's a pain removing advance springs and checking the curve with a timing light and tach rather than simply plunking the thing into the distributor machine and working from there. Same thing with a fully adjustable vacuum advance.
If you hadn't mentioned that you did all of your testing with the vacuum advance disconnected, I would have suggested you do so and then invest in the Crane (or equivalent) vacuum can that has both amount of advance and set point adjustability.
One thing I've learned from years of diagnosis and trouble-shooting (not car related necessarily) is that it's essential to eliminate any variables. If you suspect the carb, then swap it out. Otherwise, all of your efforts may end up not amounting to anything.

pancherj

I think to myself: "How is TDC of the piston linked to the firing of the spark plug?" I would start looking into the timing chain and sprockets. It seems you eliminated the distributor itself and the carb should have no impact on what the advance reads. Just my thoughts. This is certainly a tough one...

JaBoT

Whats your converter stall speed?

Also you should be adjusting the carb while the car is in gear not park. This can make a big difference.
Set the timing to around 16 at idle and hook up the vacuum can. Have someone sit in the car with foot on brake while its in gear.
Set the idle using the secondary stop screw as low as it will go without stalling.
Make sue the transfer slot on the primary is squared. Slowly turn in the a/f screws evenly around the carb. As you do this the idle and vacuum should go up. Does it?
As the idle goes up lower it back down using the secondary throttle stop and keep adjusting the a/f screws until it starts to run rougher or vacuum starts to drop. then turn the a/f screws back out like an 1/8th each.
It may not run great, but it should run in gear.
Keep in mind it's a pretty big cam especially with a distributor that is not really adjustable. It is going to need to idle at a higher rpm in gear.

If it still isn't right:
Check for vacuum leaks. Also check the plugs to see if they all look the same. You could have a vacuum leak from the lifter valley and not know it.
Check the valve lash. If even one cyl is out it will mess things up.
Check the timing chain.
Try swapping carbs.

I would also highly suggest you get a quality distributor that is adjustable.

Also are your cam spec right? I can't find that cam on Lunati's website.

3rdgenmaro

iTrader: (9)

3200 TCI IIRC. Been awhile.

http://www.lunatipower.com/CamSpecCa...rtNumber=40150 But with 1.6 on exhaust, 1.5 on intake.

Ive been wanting to do it in gear, just hard to find the help. Having a 2 yr old that cant sit still doesn't help. I'll try and get my dad to help me in a couple of weeks. My wife is not a car person and if our daughter did sit still, I don't see her having the ability to feather a throttle to keep it running if need be. She might run me over.

I just sold a set of wheels yesterday, going to put that money into a distributor so that I can atleast have one that is easier to adjust.

Thanks for all the help guys.

JaBoT

looking at the cam specs it is starting to make a little more sense. That cam probably needs to idle at least at 1000 rpm in gear maybe more. It's an oval track cam. Those cars usually idle way above 1000 rpm and are manual trans. Plus most of the circle track guys have their distributor locked so they have the same timing at idle as wot.

3rdgenmaro

iTrader: (9)

That's what happens when you are 22 and don't know a lot. You call different cam companies and then pick one of their recommendations. Probably would have been a better choice out there. But it runs good when you get the rpm up.

JaBoT

Try adjusting it to idle at around 1000 in gear and see how it is. But it is going to need a lot of timing at idle.

skinny z

I thinks it's important to stress here that if you're using manifold vacuum then ALL the vacuum advance must be in at the lowest vacuum you see when the car is in gear. Otherwise, it'll never run properly. Fact.

That's high up on the list for sure.

JaBoT

This is not a fact. I have tested it and done it numerous times in the past. It will only fluctuate the timing a degree or two. That is nothing. Not even enough to change 50 rpm. Fuel injected cars come from the factory with idle timing that changes way more than that.

Now it may only pull enough the can to give you a degree or 2. That's an different story.

3rdgenmaro

iTrader: (9)

Looking at either an MSD 8360 or Mallory similar. Any suggestions? Must be small cap distributor. I do not have an ignition box and would prefer not to have to get one at this point.

skinny z

It is a fact and I'll give the circumstances where it's so.
An engine idles at 900 rpm is park and produces 13" of vacuum. The vacuum advance can will start to initiate advanc at 11" (0 degrees) and it's all in at 13" (15 degrees). Total advance at idle is initial plus vacuum advance. The vehicle is placed in gear, idle decreases by 150 rpm and the vacuum drops to 11". What happens to the advance? It's gone. All that remains is the inital.
You can move those values around to suit whatever engine you're working but the point remains that if the can is the wrong spec, you'll have trouble.
I don't think you can find an OEM can in the following list that'll give you a wider spread from onset to all in than a couple of inches.
I know the Crane unit has about a 3" spread.
Fuel injection is irrelevant here. We don't have a computer that can change timing 100 times a second to compensate for idle rpm and vacuum fluctuations.

Edit: Jabot, I wanted to post up that I'm not disregrarding your work or findings. My experiences are different than yours obviously and I'm just giving a heads up to 3rdgenmaro that he may encounter a problem with just any old vacuum can should he try to go the full manifold route.

skinny z

Non-HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC680 B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18
1964 Corvette exc. FI
1964 Impala, Chevy II
1965 396 High Perf.
1965-67 283, 409
1966-68 327 exc. Powerglide
1967-68 All 396
1969 Corvette 427 High Perf.
1969 396 Exc. High Perf.
1969 Corvette 350 TI
1969-70 302 Camaro
1970 400 4-bbl
1970 396 High Perf.
1970 Corvette 350 High Perf.
1973-74 454 Exc. HEI

VC1810 B28 1965 409 High Perf. 3-5 8 @ 5.75-8
1965 327 High Perf.
1966 327 High Perf.
1964-67 Corvette High Perf. FI

--------------------------------------------------------------------------------------------------------------------------------------------

VC1605 B9 1965 impala 396 Exc. High Perf. 7-9 10.3 @ 16-18
1965 327 All Exc. FI
1969 327 Camaro, Chevelle, Impala
1969-70 Corvette 350 Exc. High Perf.
1969-70 350 4-bbl Premium Fuel
1970 350 Camaro, Chevelle, Impala High Perf.
1971-72 350 2-bbl AT
1971-72 307 All

VC1675 B13 1968 327 Camaro Powerglide 9-11 8 @ 16-18
1968 327 Impala AT
1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy II
1970 350 Camaro, Chevelle Exc. High Perf.

VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl 5.5-8 12 @ 14-18
1969-70 350 2-bbl

VC1765 B20 1965 396 Impala High Perf 5-7 8 @ 11-13
1966-67 Corvette Exc. High Perf.
1966-67 Impala 427 Exc. High Perf.
1966-68 327 Powerglide Exc. High Perf.
1969 307 All
1969-70 396, 427 Camaro, Chevelle High Perf.
1970 400 2-bbl
1970 307 MT
1973 Camaro 350 High Perf.

VC1801 B21 1971 350 2-bbl 7-9 10 @ 16-18
1971-72 400, 402
1971-72 307 AT

VC1802 B22 1971-72 350 4-bbl 7-9 8 @ 14-16


Other Part Numbers & Specs:

VC700 B3 8-10 11.5 @ 19-21
VC1415 M1 6-8 10 @ 13-15
VC1420 M2 5-7 11 @ 16-17
VC1650 B12 8-10 10 @ 15-17
VC1725 B18 8-10 12 @ 13-16
VC1740 A5 6-8 12 @ 15-17.5
VC1755 A7 8-10 12.5 @ 18-20.5
VC1804 B24 6.5-8.5 10 @ 12-14
VC1805 M13 6-8 12 @ 14.5-15.5
VC1807 B25 5-7 8 @ 13-15
VC1808 B26 5-7 8 @ 11-13
VC1809 B27 5-7 9 @ 10-12
VC1812 B30 5-7 12 @ 11.75-14

The following listing (HEI) is as follows: The first four part number listings are the 4 numbers that are most commonly used in a Chevrolet performance application. The “AR12” can is the most versatile and user-friendly unit for a good performance street engine. The AR 15 and AR23 are almost identical, with only slight variations in their “start-stop” specs. The “AR31” can is the HEI equivalent to the “B28” Hi-Perf can used on the early engines: The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. All GM HEI vacuum advance units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do.

HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC1838 AR12 1975 350 Buick 7-9 7 @ 10-12

VC1843 AR15 1977 305 All Exc. Hi Alt, Exc, Calif. 3-5 7.5 @ 9-11
1974 400 All w/2-bbl
1977 305 El Camino
1976 262 Monza Exc. Calif
1976 350 Vette Hi Perf, Incl. Calif
1975 350 Z-28
1977 305 Buick Skylark

VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc. Calif
1975 350 4-bbl
1974 350 All w/1112528 Distr.
1978 350/400 Heavy Duty Truck, Exc. Calif, Exc. Hi Alt.

VC1862 AR31 2-4 8 @ 6-8

--------------------------------------------------------------------------------------------------------------------------------------------

VC1703 N/A 1978-79 Vette Special Hi Perf N/A N/A
1979 305 El Camino Calif.
1978-79 350 Blazer & Suburban
1979 Buick 305/350

VC1825 AR1 1976 454 Caprice, Impala 3-5 9 @ 6-8
1975 454 Caprice, Chevelle, Monte, Suburban

VC1826 AR2 5-7 12 @ 10-13

VC1827 AR3 5-7 9 @ 9-11

VC1828 AR4 1975-76 350 Buick & Olds 6-9 10 @ 12-14
1976 350 Pontiac

VC1831 AR7 6-8 12 @ 14-16

VC1832 AR8 1975-76 455 Buick Electra 4-6 12 @ 12-14

VC1833 AS1 1975-76 500 Cadillac Exc. Calif. 4-6 14 @ 15-16

VC1834 AR9 4-6 13 @ 13-16

VC1835 AS2 1975-76 350 Olds 5.5-7.5 12 @ 15-17

VC1836 AR10 1977 305 All Hi Alt, Exc. Calif. 3-5 9 @ 11-13
1977 350 All exc. Calif.
1977 350 Vette Exc. Calif, Exc. Hi Perf
1976 305 All Exc. Calif
1976 350 All Exc. Vette, Exc. Calif
1976 350 Vette Exc. Calif., Exc. Hi Perf
1975 262, 350 All w/2-bbl carb
1975 350 All 4-bbl w/ 1112880 & 1112888 Distr.
1977 305 Chev Truck Light Duty
1975-76 350 El Camino 2-bbl

VC1837 AR11 1976 305 Blazer, Exc. Calif 6-8 12.5 @ 10.5-13.5
1976 350/400/455 Pontiac 4-bbl

VC1839 AR13 4-6 12 @ 11-13

VC1840 AR14 1975-76 350/400/455 Pontiac Firebird 6-8 10 @ 9-12

VC1841 AS3 1975-76 500 Cadillac Calif. 5-7 10 @ 13-14

VC1842 AS4 1976 350 Olds Cutlass 5-7 12 @ 13-15

VC1844 AR16 3-5 12 @ 13.5-15.5

VC1845 AS5 1978-79 425 Cadillac w/F.I. 4-6 14 @ 14-16
1977 425 Cadillac

VC1846 AR17 1977 301 Buick Skylark 3-6 13 @ 10-13
1977 301 Pontiac

VC1847 AS6 1978 403 Motor Home 4-6 12 @ 12-14
1977-79 350/403 Buick LeSabre Hi Alt, Riviera, Olds
1977-79 350/403 Pontiac Hi Alt

VC1848 AR18 4-6 12 @ 9-12

VC1849 AR19 4-6 12 @ 7-10

VC1850 AR20 1977 350/400 Pontiac 4-6 10 @ 8-11

VC1851 AR21 1977-79 350 Buick LeSabre, Century 5-7 12 @ 11-13
1978-79 350 Pontiac

VC1852 AR22 77-78 305/350/400 Chev Truck, Heavy Duty 7-9 5 @ 12-14
1975-76 350/400 Chev Truck Heavy Duty

VC1854 AR24 3-5 13 @ 10-13

VC1855 AS7 1977-79 260 Olds Cutlass 3-5 15 @ 10-12

VC1856 AR25 3-6 15 @ 10-14

VC1857 AR26 3-6 12 @ 13-16
VC1858 AR27 1978-79 305 All 3-6 9 @ 11-13
1978 350 Camaro
1978 305 Chev Truck, M/T, Light Duty
1978 350 Chev Truck Hi Alt
1978 305/350 Buick & Olds
1978-79 305 Pontiac

VC1859 AR28 1979 350 Vette Exc Hi Perf 3-6 10 @ 9-12
1978-79 305 w/1103282 Distr., Incl. El Camino A/T
1979 350 Camaro, Impala, Nova, Malibu, Monte
1979 350 Suburban
1979 350 Buick Century
1978 305/350 Buick & Olds
1978-79 305 Pontiac Hi Alt.

VC1860 AR29 3-6 12 @ 10-13

VC1861 AR30 1978-79 301Buick 3-5 13 @ 11-13
1979 301 Olds
1978-79 301 Pontiac

VC1863 AR32 2-4 10 @ 11-13

VC1864 AR33 1978 305 Chev Truck, A/T, Light Duty 4.5-6.5 13 @ 11-13

VC1865 AR34 1973-74 350 Vette Special Hi Perf 3-5 15 @ 8.5-11.5

VC1866 AS8 1978-79 425 Cadillac w/carb 3-5 14 @ 13-15

VC1867 AS9 2-4 10 @ 8-10

VC1868 AR35 1979 305 Chev Truck & El Camino 2-4 10 @ 6-9
1979 305 Buick & Olds
1979 305 Pontiac A/T

VC1869 AS10 2-4 12 @ 8-11

skinny z

I've gone the MSD route with a small cap. It was originally run without an ignition box although I've added a 6AL since then. Prior to that, I had a similar Mallory unit. Both had magnetic pickups. Both did what I needed them to do. In your research, compare the output of the modules. Greater output tends to bring better results in a high stress environment. That is, less tendency to drop sparks at high rpm.
I would think that any of the name brands are going to give you satisfactory results provided you pick one with the right specs for your application.
You can tell from the postings that there a varying opinions on whether an adjustable vacuum unit is needed. My vote is yes, especially one that's adjustable for both onset point and amount.

JaBoT

I do agree that an adjustable vacuum advance is absolutely the way to go. But for now until he gets a new distributor setup even a couple extra degrees of timing can make a big difference in idle.
I would also suggest if you can get a few extra dollars together to change the cam to something like a lunati 60141. It will idle better, cruise better, make more power and get better fuel mileage. The cam you have doesn't even start making power untill 3800 rpm. I don't think the car is ever going to run right on the street with that oval track cam in it.

skinny z

I've re-read the entire thread and I haven't see any reference to what the idle vacuum is.
I hadn't paid much attention to how aggresive that cam is either and it explains a lot.

3rdgenmaro, do you know what your idle vacuum is and at what rpm?

3rdgenmaro

iTrader: (9)

IIRC at the point that it wants to idle now (is ~1100 rpm at 15")

Also working on determining what distributor Im going to get.

In hindsight I wish I had choosen a different cam, I just went by what the company recommended. But right now, Im going to keep with it. Depending on how life goes, in the next couple of years, Id like a different combination that was alittle better thought out, (smaller cam and heads or CI increase, roller cam, etc). But for right now, Im sticking with things as is. Ive drove it on the street and it aint terrible with a bad tune, so it should be some better with a good tune.

3rdgenmaro

iTrader: (9)

Going further on the engine vac, that is in park. Put it in gear, it drops significantly. Of course it will drop though if the engine is about to die.

Driving Ive seen it get up to 20" at cruise. Coming to a stop it drops to around 10" but once you hit idle the motor acts like it wants to die and the vac will start bouncing from 3-6".

Of course that was about 2 weeks ago, so I may be off a couple of numbers. But not by much.

skinny z

A light just went on in my head. Maybe JaBot can chime in here.
Last engine I worked on with a cam your size had a difficult time idling below 1000 rpm. The carb was a 670 Holley vac sec. 2 corner idle mixture only.
The ONLY way we could get that thing to settle down was the "old racer's trick" of drilling a couple of holes in the primary throttle blades. We started with 1/16" and eventually worked our way up to 1/8". This allowed us to set an idle speed of 750 rpm with the transfer slot set properly. Idle vacuum at that rpm was about 8" yet it ran very well on the street. Got pretty good mileage too.
Using the secondary throttles to set an idle speed (once the transfer slot has been properly set) has always seemed a little strange to me. Although it's posted here and elsewhere and is generally accepted, I'd rather find another way. For what it's worth, on my current carb, with a 4 corner idle, (Speed Demon 750 VS) the instructions include setting the transfer slot on the primaries and the secondaries. Primaries are typical of what you've done however there were two methods for the secondary slot. Exposed in a similar way to the primaries for engines that idle over 1000 rpm and completely covered for engines that idle below 1000 rpm.
I'm sure the opinions will vary on how whether or not you might want to try this fix but I can say it is effective and has been done by others a million times.

3rdgenmaro

iTrader: (9)

I always took the holes in the butterflies as what people do if the dont know about the secondary screw.

Mine is also not a 4 corner idle. So I dont believe there is slots in the secondary.

JaBoT

In most cases opening the secondary is basically the "new way" of drilling holes in the blades. Unless its a really huge cam. Then you may still need to drill small holes, but usually not needed.
I'm personally not a fan of drilling the blades because it's permanent. I have in the past put a bleeder valve on an unused vacuum port to let extra air at idle in instead of drilling the blades.

skinny z

Fair enough.
I've attached a scan from one of my Holley tuning books.
It supports both of your comments.

3rdgenmaro

iTrader: (9)

Okay. I just went back and re read the entire other thread about the ported vs manifold vac and it is starting to make more and more sense about what you are saying.

Let me see if I got this right.

Right now Im having a problem with no vac advance that the engine doesnt want to idle at a lower timing advance. So at WOT, I have too much timing. By hooking the vac advance up to manifold vac, I will be able to back the total timing back down and let the idle timing be higher due to the vac advance being in. That gives me the timing it wants at idle, yet still has the lower total timing that WOT should be.

However, since my vac canister is not adjustable and the cam is what it is, a small vacuum fluctuation could change the idle timing enough that the motor wants to cut out and die. As it starts to cut out, the rpm drops, thus making the situation worse.

Do I have that part right?

JaBoT

yes and no. There shouldn't be much vacuum fluctuation at idle. If you have any decent amount of fluctuation at idle you have other problems. (ie idle to low, miss adjusted rockers, bad cylinder, etc). If you have say 1 inch of fluctuation that will fluctualte the timing a degree or 2 which isn't enough to cause the condition described. And even though the can isn't giving the full advance at idle, even a few extra degrees helps. But I do recommend getting the adjustable can. In the end it will just work better for idle, fuel economy, and a smoother running engine.

Also don't confuse vacuum in park and in gear. In the end all the readings in park are basically meaningless for an automatic trans. Everything has to be done in gear.
You can get the engine to run better with more timing at idle but keep in mind it's never going to be right. You have an oval track cam designed to operate from 3800 to 6500 in a manual trans race car. You have it in an auto street car with a 3000 stall converter.
I hate to say it because i'm not a fan of doing it, but you might want to just lock out the timing at 36 and just get a start retard box.

3rdgenmaro

iTrader: (9)

Also don't confuse vacuum in park and in gear. In the end all the readings in park are basically meaningless for an automatic trans. Everything has to be done in gear.[/QUOTE]

That was going to be the next thing I was going to ask. I need to have it in gear when I try to set the total timing/idle. (Ideally 32-36 deg) And when I make idle screw adjustments, a/f screw, etc. Right?


Im going to beat this dead horse as long as I can. I know I can get it driveable, Ive driven it before with it way off, so its bound to get better.

skinny z

That's how it would work however reading Jabot's post...

That last statement is the clincher.
What I was describing was a stable idle in park, with all of the vacuum advance pulled (I firmly believe that should be the case) however a shift into gear drops the vacuum enough , possibly 3", and your vacuum advance is gone and then the situation you described above starts to happen.
Following Jabot's recommendation of setting your timing with the vacuum advance with the car in gear will get you to where you want to go.
I've found that even a couple of degrees of timing will affect the idle by a few hundred rpm. That's why I like to see all of the vacuum advance in at idle, in gear. Then when you shift to park, there's no additional timing advance and it's only the engine load changing that increases the rpm.

skinny z

Keep at it. It'll all come together.

JaBoT

lol, I understand.
I think if you ask 10 different people to describe drivable or a "street car" you will get 10 different answers!
You might just be better off locking the mechanical advance in the distributor out. Try taking the springs out of the distributor, set total timing to 34 and readjust the idle and take it for a ride.


O and there is a very easy way to see if the can is raising timing at idle (in gear). Just pinch the vacuum line and see if the timing and or idle changes.

3rdgenmaro

iTrader: (9)

Im going to get back at it tomorrow evening and see what I can do. Wont be able to this evening. I will get back with what I find out. But Im seeing the value in having adjustable vac can (which I dont and it isnt the type I can swap) and having an easier to adjust mechanical advance.

Thanks guys for all the help.

3rdgenmaro

iTrader: (9)

Anybody know of anyone else that sales these at a more reasonable price? $35 is crazy. If I still had access to a lathe I could crank them out very quick. May draw them up and see if I can get a buddy to run them off for me.

My distributor came in over the weekend, hope to get back working on it soon.

jwfirebird

iTrader: (2)

go to ebay and type "msd bushing" there was a ton for cheap

3rdgenmaro

iTrader: (9)

Ha! You are right. Can get them for $9 shipped. Wish I still had a lathe, but oh well.

Thanks.

3rdgenmaro

iTrader: (9)

Well I ordered a 10 degree advance bushing for the distributor, 14 might have been more ideal, but 10 shouldnt hurt anything and will be better then 18 that msd provides. Also ordering a crane adjustable vacuum canister. Plan is to get the stuff put in, wire up the distributor, and set timing initially at 26 static with the extra 10 coming in by 3000. Im going to have to reread all of this stuff to figure out how I want to start with the vacuum can adjustment. Any suggestions or where to start it off at? I figure a max of 14 degrees but Im not sure when to bring it in. May be something I just have to take a guess at and then adjust from there.

JaBoT

Well there is a couple parts to setting up the vacuum can. One is how much timing does my car want at idle and how much can it take cruising without knocking.
To find out how much timing your engine wants for optimum idle is pretty easy. Just keep advancing the timing at idle until the rpms and or vacuum stop going up. Keep in mind it doesn't have to be perfect on the timing number. The car will idle fine with a little less timing also.
So lets say you engine likes 40 degrees of timing at idle. With a total timing of 36 and 26 at idle you will want 14 degrees of vacuum advance with the vacuum can set at around 1" below idle vacuum. This will also give you around 50 at cruise above the rpm of total mechanical advance.
Now that is just the starting point. You will need to drive it and make sure you don't have any cruise or part throttle knock. If you do, back off a couple degrees at a time from the vacuum can until it stops. Also if it doesn't knock you can ad a couple degrees until it does knock then back it off a couple degrees till it stops.
Generally big cams like a lot of timing at idle and cruise unless the car is really heavy

skinny z

To refine a couple of a points JaBot has made (and to clarify them for myself because I'm STILL tuning).
The total advance at idle, in this case 40 degrees has been suggested, is a combination of initial setting of the distributor plus the vacuum advance. That would be 26(initial) + 14(vacuum)=40.
Determine what your idle vacuum is. Keep in mind, that as you add timing your rpm will increase and so will your vacuum. As you reduce the idle rpm (with various means but just be mindful of the idle transfer slot position) you'll find that with the advanced timing, the idle vacuum will have improved. This is why the adjustable vacuum can is so versatile. Once you've zeroed in on the idle vacuum, set the vacuum can so that the advance is "all in" at least 1" below that value. This will help to eliminate any fluctuations in the idle rpm caused by the timing changing.
Total timing wasn't mentioned in the last post however with your bushing installed, you'll be limited to whatever it provides plus the initial setting you put in the distributor with your timing light and balancer pointer. If you use a 10 degree bushing and have set the initial at 26 then you have a total of 36.
If you interested in an example, here are my specs:
Initial (or sometimes called base) timing; 12
Full manifold vacuum advance. all in by 10" (I idle at 11"); 22
Mechanical (all in by 2800 rpm); 20 (via a bushing )
Engine idles at 800 rpm with 11" of vacuum.
Idle advance is 34 degrees.
WOT advance is 32. (Deliberately low until I sort out the rest of my tune)
Maximum available timing when cruising at 2800 is 54 degrees.
Good luck.

3rdgenmaro

iTrader: (9)

Well this weekend I installed the mechanical advance limiter bushing and the crane adjustable vac canister. I need to determine my baseline spring setting and install the correct ones. Hopefully will get that done one evening this week. Plan on installing the distributor this weekend. Going to wire it to run without the box for now and install the box this winter.

For future reference for anyone that may search, yes you can install an adjustable vacuum canister onto a MSD 8360 distributor. 5 minute job. Remove cap and rotor, drive the roll pin out of the distributor gear, Remove the inner shaft, remove snap ring, raise the vac advance up, remove the 2 allen head screws, reinstall the new canister.