General Category > Quadrajet Carb Talk and Tips

Main air bleed question

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Cliff Ruggles:
If adding manifold vacuum to the advance at idle causes you to loose control of idle speed then move it over to a ported source instead.  The ONLY difference between ported and manifold vacuum to the advance is that ported will NOT add timing at idle and coasting.  It does EXACTLY the same thing otherwise providing FULL manifold vacuum to the advance right off idle, and light load/normal driving scenarios.

Well thought out engine builds do NOT want, need, like, or respond well to butt-tons of timing anyplace, especially at idle speed.

For most engines I get involved with here I shoot for 10-12 initial, 10-11 mechanical (20-21 at the crank) and 10-15 from the vacuum unit, usually using a well located ported source, not manifold vacuum.  This provides around 30-34 total mechanical and mid to high 40's at light throttle cruising. 

Of course with that said those numbers are for efficient engines with optimum compression, tight quench, efficient combustion chambers and well chosen camshafts for the application.  Engine with a lot of quench, for example, or too much cam (overlap) may want quite a bit more timing to be happy.

I do NOT like use or recommend light distributor springs or super-quick timing curves for most of these engines, unless it falls into the category mentioned at the end of the last paragraph.  At that point you are just "crutching" things as inefficient engines like, want, need and respond well to a LOT of timing and fuel.

Anyhow, the timing should start advancing right off idle or 900-1000rpm's and all in by 2800-3200rpm's is early enough.

Some folks are still stuck-in-the-60's and believe all the crap you read on the Forums and install light springs with super quick timing curves which cause nothing but issues with idle tuning as more times than not some of the curve is in at idle speed.  Not necessarily the case here but needs to be mentioned as lots of folks with troubled engine combos will read these threads.

Anyhow, your fuel mileage is WAY off the mark for the combination of parts.  Not sure exactly why since the A/F ratios aren't horrible, but for sure some additional tuning is needed here to get it where it needs to be.......Cliff

Jim H:

Ok, thanks for the insight guys. Cliff, I am monitoring the timing and could very well end up with heavier springs when all the dust is settled.
Jim

Cliff Ruggles:
I'm going to add here Jim is that for the past 10 years or so I have opened up a couple of Saturdays a month to "custom" tuning troubled set-ups. 

Often I get the entire vehicle here and sometimes from great distances.  The record to date is 1800 miles one way, but typically they come from less than 200 miles or so.

Of course they are only coming here because the owner, all his friends, beer drinking buddies, and local shops and "gurus" have been unsuccessful in curing the issues they are having.

Most think it is carburetor related so they contract with me to show up early Saturday morning so I can sort things out with their engine combo.

95 out of 100 times distributor tuning is part of the equation.  I've come to LOVE those cheap POS spring/weight kits MrGasket and others sell to folks for OEM distributors.  I yank that junk out of them and put the factory parts back in which sometimes is most of or all of the cure.  For sure I take the carb apart and correct any and all issues with it and dial it in exactly for the application.

Between those two things I get 99.9 percent of them running like they should be.

What I see with nearly all of these troubled combos is that some of the advance curve is coming in at idle speed making idle tuning difficult if not near impossible.  Before you can proceed to carb tuning you absolutely MUST correct any issues with the distributors mechanical advance curve.  No timing whatsoever can be "in" at idle speed, but it needs to start advancing right off idle (900-1000rpms). 

There is seldom if ever a need for a "super-quick" mechanical advance curve and it is more times than not detrimental to a street driven engine. 

Vacuum is advance is NOT an option either, even though 2/3rd if not 3/4's of the troubled combos brought here are not using it.

Another BIG problem I've been seeing is defective "modules" that replace the points.  Don't think for a second that installing some bug-zapping points replacement unit is working like it's supposed to.  In recent years I suspect many of those companies are "outsourcing" them offshore someplace because I've had to toss out a good many of them and put points back in these distributors.

....continued below

Cliff Ruggles:
Anyhow, timing and fuel curves need to be spot on IF you want the best results from your engine.  When the engine is well thought out it's relatively easy.  It will want about 8-12 degrees initial timing, 10-11 mechanical (20-22 a the crank) and another 10-15 degrees vacuum advance.

When you find yourself going outside those numbers, like having to run 20-30 degrees initial timing just to get the engine to think about idling decent below 900-1000rpms you have a fundamental problem someplace else.  This could include the addition of a camshaft with really tight LSA, too much duration and overlap for the CID/static compression ratio, or the carbs idle system simply isn't up to par for the lower than "normal" engine vacuum at idles speed.

With these things folks need to realize that the better we do with engine choices, compression, duration, overlap, intake closing point, quench distance, combustion chamber shape(s), etc, etc, the LESS timing and fuel the engine will want to make best power and be most efficient on the street, on the dyno and at the track if you are racing it.

The days of flooding these engines down with gas-sucking "pig" rich carbs and distributors with super quick timing curves and no vacuum advance should be long gone......but from what I've seen here folks are still doing that sort of thing on a broad scale.

Not complaining at all as it keeps me busier than I ever want to be and my wallet full at the same time!........Cliff

Jim H:
That is all great advice Cliff, lots to sort out for sure.
Thanks again.

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