Confuzed1

iTrader: (4)

Thanks JaBot for the reply....I already have a secondary blade adjustment screw. But if I adjust it to the almost or closed position on the secondary blades, doesn't that render the rear idle A/F adjustment screws useless?

I would pretty much end up closing those, and most likely adjusting the fronts down a bit also (below the square setting) in order to idle at a reasonable rpm...but if I totally block off the transfer slots in the rear, won't it idle rich? I suppose it won't hurt to try it!!

Skinny - If you're getting 20 MPG on the road, you must have it dialed in fairly close. Much more so than mine for sure! I'm lucky to smack down 12 on the highway...I'm afraid to even calculate it....and I use premium only at 4 plus bucks a gallon.

I've yet to mess with any of the air bleeds on the carb. Everyone I mentioned that to says you'd better know what you're doing or you can really end up with a mess!

skinny z

Below the transfer slot there's another orifice which is the idle discharge hole. With the butterflies closed this port is still active. For what it's worth, on my Speed Demon carb there's a recommendation for adjusting the secondary butterflies. For engines that idle above 1000 rpm, the butterfly is adjusted the same as the front. On engines that idle below 1000 rpm, it's recommended to close the throttle blade to the point where the transfer slot is no longer visible. Yes, this changes the idle speed as well as the mixture.
Regrding my mileage, 20+ mpg steady state hiway cruise. The afrs are in the 15+ range, with rpms around 2500 at 70 mph. Vacuum is approaching 20".
Even with the poor idle quality of this current carb, I still have excellent hiway mileage. In and around town is a different story with a blend of around 15 mpg depending on the type of driving.

JaBoT

yea what he said

Confuzed1

iTrader: (4)

Well an update- I discovered my carb is screwed up. JaBot I tried shutting the secondary blades all the way, and shut down on the primary blades also...until I couldn't shut them down any more...and I was still idling over 1k RPM's...

Both blade screws were backed off and not touching the linkage arms..then I forced the throttle arm further towards closed using my finger, and the idle dropped down.

Something's warped or whatever...my throttle blades are not shutting fully, and the return spring is not weak. Time for another carb...

JaBoT

That actually makes more sense now. It's not often you cant' get the idle low enough when everything is set up right.
Do you have a cable or the blower throttle linkage. I have seen the linkage cause issues like that.

Confuzed1

iTrader: (4)

Good thought, but I'm still using the stock cable, and it wasn't holding it...I could remove the cable with the return spring still on and it would still idle high with both screws backed off.

Confuzed1

iTrader: (4)

I think it's time to try out this 800 CFM Edelbrock Thunder AVS carb I've had sitting around. Got it for a good price on fleabay a year ago. Looks brand new, but I know it'll be hard to set up.

If this turns out to be a fail, I'll drop coin on a new 750 DP and sell the Edelbrock off....

skinny z

Here's a response to the original question of "timed vacuum vs full vacuum, what's the difference?"
Here's the ignition and carb tune I did just yesterday.
You can see my engine specs in my signature.
I just made the change from ported vacuum to full manifold vacuum, which I should add, has made a night and day difference in some preliminary road testing.
Base timing set at 14°. Idle vacuum (with no vacuum advance connected) was 9-10" at 800 rpm. I also have a MSD distributor and have the two blue springs installed. This means I'm "all in" by about 2800 rpm. I have the total advance bushed to 18° maximum advance. This limits my total, without any vacuum to 32°. I'm keeping this value deliberately low until I get to the track and do some testing. I have 10.25:1 compression with iron heads so I'm staying conservative on the mechanical timing at the moment (I could go to 34° with these heads).
Now for the vacuum advance. Prior to the switch, I ran ported vacuum which has no effect on idle timing. At cruise, I was all in with 13" of vacuum which was a maximum of 22°.
I'll mention here that I got over 20 mpg on the highway. It's also a 12 second 3700 lb street car.
I have the Crane fully adjustable vacuum can. It allows adjustment for both vacuum set point as well as total amount. Since my idle vacuum was at worst 9", I adjusted the can to have all the advance supplied before that value. The total available was 22° so I kept it at that. This means at idle, I have 14° base timing plus all the 22° of vacuum advance for an idle total of 36°. Idle rpm went way up because of this as did my idle vacuum. It's now at 12". I have several adjustments with the Barry Grant carb that allow me to reduce idle speed. These include the idle stop screw and the secondary idle stop screw as well as the "Idle-Eaze" feature (which is akin to drilling holes in the primary butterflies), which is unique to BG carbs. I brought my idle speed back down to 750-800 rpm.
Prior to this change, the car idled reasonably well but there was an off idle flat spot that I had yet to address. Not sure if it was going to be a combination of idle mixture and main jetting, IABs, HSABs or an accelerator pump change however, will my idle timing now at 34°, the flat spot has all but disappeared. Idle quality has improved noticeably and idle vacuum has risen to 12". This will allow me to move up on the PV setting which should further enhance mid throttle acceleration. I keep my tune lean to enhance fuel economy however this hurts the performance to a certain degree. The PV change should help that.
It'll be interesting to see any change in fuel consumption. My bet is that both highway and mixed driving mpg will go up.
Consider me converted.

JaBoT

Glad to hear you have seen the light
It's a shame that some people will argue to death about how this way to set up the timing is wrong because they have been doing it different for ever and refuse to hear or learn anything new.
Props to you for trying it and enjoy your better running car

skinny z

Thanks for that. I'm an old timer by most of the forum members standards and what I learned more than 30 years ago dies hard. I've been back in the hobby about the last half a dozen years or so (had to raise a wife and kids) and dragged a lot of the old school with me. Don't get me wrong, the old school still has a lot going for it but trying something different, even against the advice of my old school racing friend, is what experimenting and learning is all about.

JaBoT

I to am not a young, although at times i forget and like to think i am.
Trust me with out learning from the old school guys I wouldn't have the foundation to get to where I am today. Not to mention all the little tips and tricks. But unfortunately with a lot of people they reached a point somewhere when they stopped taking in new information or do anything different, and that's fine. I just have a problem when they tell everyone "that's wrong" or "he doesn't know what he's talking about I've been doing it like this for years".
Just out of curiosity, what did you old school racer friend say after he saw how it idled and ran this way?

skinny z

My old friend is a smart guy. No doubt there. Mechanical engineer and very hands on with all of his projects. He work is immaculate too. What I find though is he's often like the character you've described and insists on this or that because he's experimented, researched and arrived at the conclusion that suits his situation. He'll often insist that this is so and don't bother with that way. When I get his attention on a point, he goes very quiet. I can tell he's listening and absorbing the information. This was his reaction to my experiment and ultimate conversion to full manifold vacuum. I won't get to see make the switch himself though. He's last carbureted car, a 67 Chevelle, has recently had a 454 LSX transplant and he's using the latest FAST EFI 2. His other car, an 81 Monte Carlo was converted to EFI years ago. We may still get to experiment with the 502 TA which at this point I jokingly consider to be mutual property (he owns it but it's been at my place for half a dozen years). GMPP 502 with an 850 Holley. With a 5 speed no less!
You can teach an old dog new tricks. I'm learning all the time.

I'm guessing Token (the original poster) won't mind my hijacking his thread. He hasn't posted since 2011.

Confuzed1

iTrader: (4)

Skinny- I too tried running ported vacuum when my engine was NA and now with the blower. Off the line performance and idle was better using full manifold vacuum in either case for me. Engine runs a lot cooler too. I had issues with it hooked up to ported vacuum and running hot if I idled for long periods. I still have that issue, but not as bad - a fan shroud "should" help with that I hope.

With this blower, it likes a lot of initial advance...I set mine anywhere from 19-21 degrees BTDC with the can disconnected and plugged. Then reconnecting the can bumps it up another 12 degrees, so I'm at 30-32 degrees advanced at idle (I pull around 17 inches of vacuum. At idle.). Some NA engines with big cams - like a lot of initial too, so it's not just a blower thing exclusively.

You should feel a lot more "pep" with no flat spots on full manifold vacuum. I think engines with smaller cams can tolerate ported vacuum much better than engines with bigger, longer duration cams. At least that's what I've found.

My Edelbrock carb swap didn't work out at all....I started a thread about it. Not sure if I'm keeping the carb even after I go back through it, but maybe one of you guys can help me figure out what the heck happened to it.

Glad you're dialing your carb in!!

JaBoT

Yea I have found the same thing. Usually if it's a stock cam i run ported just because it doesn't need or sometimes even want all that timing at idle.

skinny z

With about 276 of advertised durtion on the intake, my cam is hardly large however it does present a few twists when dialing it in. (I spec'd a custom grind based loosley on the Comp XR276HR but used the XFI lobe profiles so lift with 1.6 ratio rockers is approaching .590"). As I mentioned, my off idle performance, or more precisely, the low to moderate acceleration away from a stop sign has improved dramatically. There's still a slight stumble on occasion but it varies with ambient air and engine temps. The heat soak plays a part in it as well as we had discussed earlier. I really need to seal up the cowl hood and complete the cold air induction side. It should make a difference in getting a even and reliable tune.
I've also noticed a cooler running engine when idling in traffic or the driveway. Nice side benefit. What I like the most is that the idle vacuum is now up in the 12-13" range, up from 9" or so. Thsi gives a broader choice of PV which I'm looking forward to working with.
What I'm dealing with now is a tremendously lean condition when accelerating from cruise and the engine vacuum drops to the 10" range. I have very lean mains and I suspect that and the HSABs need to be addressed. Perhaps a 10.5 PV will help there. That's a story for another thread too.

skinny z

With about 276 of advertised durtion on the intake, my cam is hardly large however it does present a few twists when dialing it in. (I spec'd a custom grind based loosley on the Comp XR276HR but used the XFI lobe profiles so lift with 1.6 ratio rockers is approaching .590"). As I mentioned, my off idle performance, or more precisely, the low to moderate acceleration away from a stop sign has improved dramatically. There's still a slight stumble on occasion but it varies with ambient air and engine temps. The heat soak plays a part in it as well as we had discussed earlier. I really need to seal up the cowl hood and complete the cold air induction side. It should make a difference in getting an even and reliable tune.
I've also noticed a cooler running engine when idling in traffic or the driveway. Nice side benefit. What I like the most is that the idle vacuum is now up in the 12-13" range, up from 9" or so. Thsi gives a broader choice of PV which I'm looking forward to working with.
What I'm dealing with now is a tremendously lean condition when accelerating from cruise and the engine vacuum drops to the 10" range. I have very lean mains and I suspect that and the HSABs need to be addressed. Perhaps a 10.5 PV will help there. That's a story for another thread too.

JaBoT

That's normal when you adjust a holley to cruse in the 14's or higher.
The 10.5 power valve should help it a lot if not cure it all together.
Usually when i cruise the car lean I like to have the power valve kick in around a few inches below cruise vacuum. Unfortunately a lot of engines have cruise vacuum in the mid to high teens. So the only option is a 10.5 valve.

skinny z

Depending on the ambient air temps, I've seen my cruise afr bouncing of the 16:1 mark. Mostly mid to high 15s. No lean surge either. That's with rpms between 2000 and 3000 depending on the gear and the vacuum will be between 15 to 20". As a consequence, when I apply the throttle and drop the vacuum to the 10" range, the afr drops off the gauge and of course there's a flat spot.
Lots of tuning left to be done.

Confuzed1

iTrader: (4)

I've never used "advertised" duration as a means to compare cams...I know a lot of people do though. I got this from Comp's website:

"Advertised durations are not taken at any consistent point of lifter rise, so these numbers can vary greatly. For this reason, advertised duration figures are not good for comparing cams. Duration values expressed at .050" lifter rise state the exact point the measurement was taken. These are the only duration figures that are consistent and can accurately be used to compare camshafts."

I'm guessing you're running a cam fairly simular to what I have (NX276)...intake duration somewhere around 224 or so at .050. Although you've added a lot more lift. My exhaust duration is 236 at .050 though which helps when using boost (so I've heard)..lift on intake is .510, exhaust is .520. I dunno...I think the cam you're running is fairly aggressive...

I've never had any luck running the 4.5" PV most of these carbs come with, and I've always stepped it up to something in the 8-10" range. I think you'll see an improvement like JaBot said. I kinda surprised you're seeing 16:1 A/F ratios...isn't that too lean? I've always been told 14:1 is near perfect.

Man - I should just drop the coin and get a wideband. I'd probably see all kinds of issues with my setup..

skinny z

224/230 @ .050" with lifts of .574"/.569". Yes, fairly aggresive however they are profiles from Comps catalogue and designed to work with my valve train components. My thinking was to keep the duration values conservative, particularly the overlap, while getting the valve open as far as possible to compliment the better flow of my new heads. I had previously been running Vortecs which argueably start to suffer at lifts approaching .500" so my XR276HR worked reasonably well (despite going beyond the .500" spec).

The PV will be interesting experiment. On the previous engine (Vortec headed) I had a Holley 670 VS which I'd modified with a .059" PV channel restriction. It was very apparent when that valve kicked. Bring the vacuum down to about 8" and the afr would fatten up and the car took off. Great stuff and a lot of fun without having to put the pedal to floor. I like steering with the rear tires sometimes!

The wide band and in dash vacuum gauge are probably two of the best additions I've made to the car from a technical perspective. Lots of data collected daily which is invaluable when going through a tune like I am now.

TreeFiddy

Skinny - I haven't posted in this thread since the first page, but glad you finally tried full vac advance and found similar benefits to what I did. I too found it a bit of a leap of faith to make the switch and go against the 'common wisdom', but my engine clearly likes full vac better.

I agree with the in-dash vac guage, I've run one for a decade, it's a great way to keep an eye on your engine's health and diagnose problems once you learn how to read it. Must get a wb setup myself and try tune for some better mpg - about my only complaint.

JaBoT

If your looking for better mileage try sofas sticky. Since different cars can tolerate different lean afrs. By doing it like in the sticky you will be as lean as you engine wants to be regardless of the actual afr number rather than spend the extra cash on a wideband. Now with that being said I think a wideband is one of the most important tools for tuning and if you have the money for it go for it. Because like a vacuum gauge it can also tell you if there are any issues with your engine even after you have tuned it.

skinny z

Sofa has a post about mileage? That would probably be an interesting read.

skinny z

Not so much "common wisdom" for me as it was "old habits die hard".
I've always known any of the engines I've assembled or help to tune have liked lots of initial advance. Problem was, back in the day, we didn't use an adjustable vacuum can and had to go through a pile of OEM parts to get something that would work. It doesn't take long for that to get really tired and you just go for the ported vacuum deal. The idle hunting as the vacuum fluctuated because the can spec wasn't right was aggravating to say the least.
My engine clealy likes the additional advance at idle too.
Personally I can't see how anyone could get even a reasonably close tune in all the apsects of the fuel curve without an AFR gauge. Reading plugs? Forget about it for details like I'm getting into. Not only impractical but I don't think it would even be possible. Probably why with my first car, a 68 Chevelle w/ 400sbc, fuelie heads and a holley 600 VS got a whopping 14 mpg hiway. That was a lot of years ago....

JaBoT

lol no i meant the holley tuning sticky. If you follow it you'll basically wind up with the leanest your engine likes to cruise at.
And yes, that would be an interesting read!

evilemokid94

iTrader: (6)

Quick question on this debate. I recently rrtimed my motor and set the timing from 8-10 BTDC. I have it hooked up to ported now but normally run manifold because smoother idle and sound. But when I Con ft manifold vacuum I start getting white smoke out my exhaust? Anyone else have this issue?

redneckjoe

i want to thank you guys. after reading through all this, yesterday i switched to full manifold vacuum. i had been running slightly rich at idle, it leaned the mixture a little after switching. i threw a vacuum gauge on the ported vacuum while it was running. no vacuum reading til about 800-1,000 rpm. manifold vacuum in 15 at idle. a major side benefit to me was a cooler running engine. yesterday was a hot florida day, and after 20-30 minutes of running in the driveway, my coolant temp was down 15 degrees compared to ported! seems to have a lot more bottom end grunt or responsiveness too. i consider my car to be in great tune with awesome throttle response, i didnt really think it would run any better.
im glad i took that "leap of faith". i'll be running full manifold vacuum from now on.
thanks all

JaBoT

Great to hear.

TreeFiddy

Interesting! All these anecdotes seem to verify the 'GM engineer' article from the first page - that all these benefits we are discovering were sacrificed to ported vacuum, in a band-aid early attempt at emissons control prior to the introduction of catalytic convertors, at the expense of performance, mpg, and possibly engine longevity.

Since we have had cats for decades now, there would seem little point in continued use of the ported vacuum source for timing advance. Especially when considering the emission levels from any carb setup vs modern fuel injection - really doesn't matter what you do with the carb system, it can't compete. So might as well go with what runs best.

skinny z

While I will say I'm converted I can't stress enough how important it is to have the correct specification vacuum advance unit or one that is fully adjustable. I think the biggest issue people have when making the switch is that with the wrong spec can, the vacuum fluctuates, the advance rolls in and out and the idle will hunt. In the worst case (as has been posted in another thread) the engine will idle in park but will die in gear. With the right unit, all of the advance should be in at the idle vacuum seen with the engine idleing in gear at the desired idle speed.
I've re-posted a couple of links that provide some guidelines. I found them useful at any rate.
http://api.viglink.com/api/click?for...37597241701510

http://api.viglink.com/api/click?for...37597246763011

skinny z

l'll add here that it's interesting to read over the posts on the first page. My own point of view was the same as those that were experiencing what I did and mine was based on the incorrect vacuum advance unit for the engine. The idle was unstable or it would idle in park and die in gear. In retrospect, I can see where my shortcomings were and it's all based on the not having all the facts. I will support the posts that say that it might not be for all engines.
I know it works for mine. Yay!

TreeFiddy

Agreed, a fully adjustable can seems to be very necessary; also for tuning out any momentary pinging at part-throttle tip-in due to lightly loading the engine on the initial high timing, prior to the vac dropping out.

If the vac can is not adjustable your are slave to whatever it's setting is; all you can you is drop out intial timing to compensate which defeats the entire purpose of tuning. Probably safe to say that the ported vac option is much more forgiving of a fixed vac can, but the full vac + adjustable can offers the best tuning flexibility. You've just got to be more exacting with your tuning (which we all love I'm sure ).

Fast355

iTrader: (2)

Every engine I have tried to tune using ported vacumm has a flat spot coming off idle as the timing advances. Move the hose to manifold vacuum and it transitions much better, idles better, runs cooler around town, and helps keep the plugs cleaner.

The last carb engine I worked around was a GM 300 HP 350 crate engine with a performer rpm and edelbrock 1406 on it. Has an HEI/MSD6A running without an HEI module and is sitting in a 1972 Chevelle Elcamino SS car backed to a TH350 and 3.73 gears. Cruising 70-80 mph at 3,500-3,700 rpm it can still get 22+ mpg. The plugs are gapped at .055" compared to the stock HEIs .045" specification. The wider gap and hotter spark help ignite a leaner cruise mixture. I run a 8 in/hg spring in the step up piston and tuned the setup with a wideband. First I removed the linkage that operates the secondars and set the idle mixture to obtain around 14:1. The resulting transition was fairly lean off-idle and got even leaner at part-throttle cruise, up into the 17:1 range. Wide open throttle on the primaries only resulted in about 13.5:1 air/fuel ratio. I pulled the 98 primary jets out and bumped to 101 jets. The wide open throttle mixture dropped to 13:1, still too lean for my tastes but the cruise mixture was perfect at about 16:1. I pulled the .073" x .047" metering rod out and dropped in a .073 x .043" metering rod. This dropped the WOT mixture to 12-12.5:1 on the primaries only. I then hooked the secondaries back up. With the secondaries in operation it once again leaned up to the mid 13s. No surprise since Edelbrock even states this is a lean jetted carb for mileage. I pulled the .095" secondaray jets out and put a .098" in it. Its dropped back to about 12.7:1 at WOT. Once I got the fueling situated, I started messing with the distributor. Seems someone had set the base timing at 14* and it had worn slots and stock springs in the HEI. Was getting around 24* of centrifical advance. The car has a stock torque converter so it was difficult to get up on the advance curve and sluggish and seemed to stop pulling around 4,500 rpm. I welded up the slots and ground them back open to allow 18* of centrifical advance. I then set the base timing at the 14*. I arrived at a spring combination that used a 903 center pivot and 41 weights with a medium and light spring to bring in the full advance curve by 2,800 rpm. Finally I installed a 10* @ 10 in/hg vacuum advance can built for an early L82 corvette and connected it to the manifold vacuum fitting. Compared to the over-timed lean burning mess the car started out as it flat screams now. I think this car would really love a nicely built 2004R transmission with a 2,800 rpm lockup converter, but that is for a different story. I have one in my old 1988 Cadillac sitting on my parents 20 acres and may pull it out, freshen it up and install it someday.

skinny z

Certainly more degrees of freedom now than before. With the timing dialed in, or very nearly so, it allows greater choices in working on the fuel curve. Having made the switch, I've gained more than 3" of vacuum, (as I stated over and over because it's great!) from 9 to 12 and this allows me to work on the PV setting. Not only has the off idle transition improved, I anticipate that moving from the 6.5 to the 10.5 (Yes, Sofa, you were right) will help the mid acceleration lean patch that comes on around the 10" of vacuum level.

Yes. Particularly when idling. That was an unexpected benefit.