Me and my teacher are having a bit of confusion about which vacuum port to plug my vacuum advance into... I can get a pic if it really helps anyone but i dont see how it would.
I hooked a vacuum guage up to both ports as well as to the brake booster just to see what it read. heres what i came up with. This is a 350 btw... With the port that it is in right now the vacuum reads 0lbs at idle and when the throttle is snapped it goes up towards 15-20. I plugged the vacuum advance in and turned the distributor untill i got the most vacuum at idle in the other vacuum port. In the other port and the brake booster hose the vacuum reads about 21lbs and when the throttle is snapped it goes down towards 0lbs. Which port should i be in, is it right the way i have it now, i confuse my teacher with my car alot and this time he just isnt sure since he gets all of his curved.
Tia,
Dennis

 

It should be on Port #1.

And, set the timing with the vacuum disconnected from the distributor. Using a timing light.

 

The engine didnt come with a plate that tells me where the timing is, I was pretty confused when i went to use the timing light i couldnt find where to point it, that is why i am setting it to most possible vacuum, i am also adjusting my air fuel ratio this way.

Do you have any other suggestions what to do about my timing?

Side Question: I have an edlebrock 650CFM, i think the part number is 1806? This has the vacuum secondary, I read on these boards once that using the vacuum secondary with a manual transmission is not recommended. Why is that? What carb is recommended with a manual tranny? Should I consider selling mine and getting a new one? Is the cost of a new carb worth the reasoning behind why it isnt recommended?

 

a manual trans works best with a mechanical secondary carb, better response and overall power. For drag racing. Worse mileage usually too.
On the street, in your case, it's not worth the hassle to sell yours and buy a new one.


adjust your carb to most vacuum, then setup your timing as per damons recommendation on the "HEI primer" on the main page of TGO. Then after timing is setup right, go back and recheck your idle screws, idle mixture, etc. 'nuff said.

 

i have done it both ways, full manifold vacuum & ported vacuum, it depends on the motor.

you can run more timing at cruise than you can at full throttle, at full throttle you don't want full advance on the timing.
generally a motor wants a max of 36~42 total timing advance at cruise, somewhere around 26~34 at full throttle.
you may need to play with vacuum advance limits & add or remove centrifugal advance to find whats best.

base timing is set different for each way, ported vacuum needs a little more base timing than full manifold vacuum. with full manifold vacuum i like more centrifugal advance that comes in a little faster along with the vacuum advance limited.

it doesn't matter if its a 1928 Chrysler or a 2002 LS1, it comes down to Grumpys' rule #1, give the motor what it wants.

 

By the way you are not reading vacuum in POUNDS, you are reading it in INCHES OF MERCURY.

 

not always. i get around 22mpg on the autobahn doing 70mph

timing tab:



nebu.

 

I realized that after looking at my first post again sorry.

Nebu, I do not have that timing tab on my engine like every other engine i have worked on does. That is where half of my problems come into play.

I read the article on the HEI setup, i never noticed that one, it is a very good write up. I will be doing most that stuff within the next few weeks

 

yea, that crane 96001 or whatever is money WELL spent, fully (FULLY!) adjustable vaccum advance, with a lockout tang, nifty alright. Run a long vacuum line from manifold vaccum on the car, into the car (or at the wipers) so you can see your vaccum as you drive around. That'll help you setup your timing and carb actually.

the only thing wrong with manifold vacuum at idle, is it *can* cause a hunting idle, as your vacuum changes, so does your idle speed.... anyway, ported vacuum came about during the era of smog, so, take that as you will.
more advance at idle allows you to back off your idle screws, and get a nicer idle.
You should see a timing tab like that, if you look straight down at your water pump, right behind it, in a little slot basically. Kinda hard to see, if you're looking there, and there's no tang, then you're SOL.

Anyway, now i'm just rambling, tune 'er up!

 

If you dont have timeing tabs then you can get timing tape from any auto parts store and line it up when the engine is at TDC.

 

Get the proper timing tab and set the timing with a timing light. You cannot set igntiion timing "by ear" or by using a vacuum guage alone. You'll end up with over-advanced timing and likely damage the motor.

 

hmmmmm.... timing without a light.
this doesn't cover EVERYTING, but I have done it before....

advance it till it pings while driving, then turn the dist back small bits at a time till it stops pinging.

do that until you can get a light on it and set it right.

also be nice to it till you do, as pre-detonation doesn't always make enough noise to be heard....

 

Did you look behind the water pump? You can always static time your engine by putting #1 cylinder at TDC and make a mark on the timing cover where the dampner groove is at. Making that your TDC mark, you should be close enough. This is an acceptable way to check timing for California, and you know how tough California is.

 

ported vacuum, is a stupid idea from the early smog era.
best vacuum for dist is, normal manifold vacuum.

the vacuum can on the distributor, should
be 'full advanced', at idle

the best vac cans, are low total advance,
with full advance at lower vacuum{that typical}

 

What you passionately state may be true, if it can be assumed a proper vacuum can will be used.

That isn't a very good assumption.

Others will argue that you're completely wrong.

Personally, I've seen acceptable/good results both ways. Since the Proform main body currently on my 396 doesn't have provision for ported vacuum, I'd better be a fan of manifold vacuum for it, or I've really got a problem.

Not that I'm necessarily disagreeing with you, I just think you've overstated your case a little.

 

you can buy a timing tab at auto zone or advanced auto for about 5 bucks and it bolts on to 2 of the timing chain cover bolts. plus its chrome and looks good.

 

All of the "good looking" chrome aftermarket timing tabs I've seen are the "2 o'clock" type. If his damper is the "12 o'clock" type, that isn't going to do him any good - unless he re-marks his damper by finding TDC.

He hasn't come back since it was suggested he look behind the water pump (where the 12 o'clock timing tab is located).

I'm putting my money on the existance of a 12 o'clock timing tab on this engine. . .

 

Sorry I never came back, no it was not there and I took it to my dads friend who is a mechanic just to double check me. It is def not there and if anyone needs proof i can get some pics. I am assuming by finding TDC on the first piston take the spark plug out of piston one and crank untill i feel a puff of air come out of that piston? That is how i have done it in the past is there a better way of doing it? I have taken into consideration about putting the timing tab on but I soon might be getting a nice set of heads and while I put the heads on I want to do a cam to match them and when i do the cam i am going to do everything the right way and put the timing tab on. Unless you think i really shouldnt be driving my car the way it is with timing set at most vacuum then maybe i will leave it untill i do the cam and heads.

 

Okay, I'll put it another way: Your car originally had a 12 o'clock timing tab. If it's not there now, an aftermarket tab still isn't going to help unless you determine the TDC mark.

What you described was finding the #1 firing position. Finding TDC is different. You need a "piston stop" or "TDC locator" tool to do this job. It is a drilled/threaded brass rod that is inserted into the #1 spark plug hole. With the piston down (at least away from TDC - that can be done by putting the damper mark somewhere away from the top), screw the tool into the spark plug hole. Rotate the engine by hand until the piston contacts the stop. Mark the position of the damper mark relative to the engine block/front cover/timing tab. Now, rotate the engine back the other way by hand (without moving the tool in the spark plug hole) until the piston contacts the tool again - mark the engine block/front cover/timing tab again where the damper mark is. Exactly halfway between the two marks is TDC. You can also go through this by establishing a marker point on the engine/front cover, marking where it points to the damper in the process above, and putting a timing tape with the zero mark halfway between the marks you made on the damper. The marker point you used on the engine/front cover is now your permanent timing "tab" when used with that damper and timing tape.

(FWIW, when I went through this process with the new damper and timing tab on my 396, the zero mark on the tab was 2 degrees off.)

 

I can't quote this enough without getting banned for spamming!

I've seen so many people say ALWAYS use manifold or ALWAYS use ported. Both are wrong, IMO. I've seen motors that really prefer ported (1971 chevy 3/4 ton with a 350) and motors that REALLY prefer manifold (ZZ4 crate engine, 1979 Corvette) and some that really don't seem to care much (1985 Olds 307 HO).

The only difference between ported and manifold vacuum for timing is at idle up to the transition to off-idle. Once the throttle is cracked more than slightly, the vacuum produced is the SAME.

Also, another common misconception is that manifold vs. ported vacuum affects timing at WOT. This isn't true at all, if you have 0 manifold vacuum you have 0 ported vacuum as well. Some of these people are misinformed, and I think the rest are confusing those two vacuum sources with venturi vacuum, which does INDEED increase as airflow increases. But I've not seen a carb yet that has a venturi vacuum port on the outside

Anyhow, what I always do is set the total high RPM WOT timing (typically 34-36* on a hot SBC), adjust the rate that it comes in at with a spring kit, and then let the base timing be whatever it needs to be (it'll be WOT timing - mechanical advance). Then I figure out what timing the engine WANTS to idle nice and smooth, by advancing the distributor for the smoothest idle. Usually that's 10-15* above the base timing, which on a 20* mechanical advance system gives about 30* of timing at idle. Then I figure out what I need to do to the vacuum can to make it happy. If I get a range--say, it idles happy between 30* and 40* of advance--I'll set the vacuum can on manifold to give it the highest value, then drive it around listening for detonation from the vacuum advance. If I hear any, I'll back the amount of vacuum advance off until it stops.

FWIW, on my ZZ4 crate, I ended up with 35* total timing (20* mechanical advance + 15* base) all in by around 2500 RPM, and 15* of manifold-connected vacuum advance (30* at idle) which gave about 40*-45* of advance at 2000 RPM cruise. It worked well, ran 13s, and got 19city/24.5hwy with 3.89 gears and a 6 speed.

I tried ported advance once on that car, and it had a nasty off-idle stumble when the timing "hit" came in from the ported vacuum. I spent days trying to tune around it (mostly screwing with the carb) until someone suggested using manifold advance.

 

99Hawk120, i have seen a few carbs that had a venturi vacuum port on them. the only domestic one that comes to mind right now is the Carter ThermoQuad. there are also some import carbs that have them.

i played around with it on my ford truck but i gave up because of ping at WOT.
i think they are mostly for emissions proposes.

 

Gotcha. I wonder what the heck you'd use it for? I've never worked on a thermoquad, only Rochesters (various types), Edelbrocks (1) and holleys (1 or 2).

I really only know Rochester carburetors well, though...