Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Rebuilt 4988S Not So Good.
Ozzmann:
Cliff as well as answering my above questions, could you please advise on these 2 as well.
My carb looks exactly the same as the one on page 23 of your book. The opening on top at the rear of the air horn, should there be vacuum there?
I only just noticed today that I'm getting about 3" there.
To answer your earlier query I am getting a full vacuum reading of 11" at the front port on carb.
My next query, when I removed the bung from the front port the rpm increase a lot, , and I was able to turn mixture screws out another turn and back off the idle screw to the point that it didn't contact the throttle linkage at all.
Does this mean I could use more idle air, if so what do you recommend?
Cliff Ruggles:
Very few q-jets will have vacuum at any rear ports in the airhorn. Most of those ports were to take in fresh/filtered air for the hot air choke. I've worked on several early 70's Old's carbs that had vacuum in the airhorn at one of the rear fittings, so it was used on some models.
In any case, make sure that you plug all vacuum fittings and correct any vacuum leaks.
Take a look in the carb at idle speed and see if any fuel is coming off the boosters (nozzle drip). If you don't have nozzle drip, and have full control of the idle fuel via the mixture screws, you do not need any addtional bypass air (in most cases). Some engines still like a lot of bypass air, for one reason or another, so adding it can help clean up the idle and further reduce how much transfer slot is exposed to gain more control of the idle fuel via the mixture screws.
Experience has taught me when to do this, over leaving it alone, so it's pretty much a case by case basis.
In most cases I do NOT use manifold vacuum to the advance at idle speed. A well chosen ported source is used instead. Quite a bit of information about this on the NET, and some folks will say that you MUST use MVA or you just don't know how to tune or what you are doing.
From my experience, having to run the initial timing clear off the scale to get the engine to want to idle well, tells me that the basic components (compression/cid/camshaft) were poorly chosen, and even more likely the carburetor does NOT have enough idle fuel capabilities at idle speed.
What I recomend to do first, is to set the carb up for the application, then do some tuning to see if the engine likes/wants MVA, or is fine with a lower base timing setting.
The real trump card in attempting to use MVA with heavily cammed engines, is that the timing falls out easily at low vacuum readings, requiring an adjustable advance or one with a really low spring tension.
Most folks, even some "experts" who debate this topic on the NET, so not even fully understand how the vacuum advance works. The ONLY difference between ported and manifold vacuum as far as the vacuum advance is concerned, is that timing is applied at idle and coasting with MVA. A well chosen ported source does EXACTLY the same thing everywhere else.
I chuckle when I read threads where folks try to indicate that a ported source continues to add timing beyond where a manifold source would fall off. Common sense would tell anyone looking at this topic, that ALL sources under the throttle plates at any give throttle angle, engine speed/load, would show the same reading if a gauge were placed on them.
An even bigger laugh comes when folks try to indicate that the advance could be applied at heavy/full throttle. When the throttle plates are on end, the reading(s) are near or at zero, or at least well below the spring tension found in any vacuum advance every produced.
The biggest laugh of all comes when we read a thread where the owner of a car switched from ported to MVA, or visa versa, and now his engine makes a TON more power at full throttle.
In any case, I ALWAYS recomend for the tuner to work with each individule set-up, to see what settings they like the best?
For most N/A engines with a decent static compression ratio and well chosen camshaft, about 8 to 14 degrees initial (base timing) is sufficient. That setting must be tested with a well heat soaked engine to make sure it doesn't "buck" the starter on hot restarts.
We then set up the mechanical curve to add about 18-22 degrees, all in by apprx 2800-3000rpm's. The curve must NOT start till just past idle speed. This is extremely important. IF any of the timing from the mechanical advance is coming in at idle speed, it typically falls out when the trans is placed in gear. This can cause dramatic drops in engine rpm's at idle speed, and is almost ALWAYS blamed on the carburetor not working correctly.
We set up the vacuum advance to add about 10-15 degrees of timing, then choose what source to apply the advance by testing to see what the engine likes/wants.
Some engines will buck and kick profusely with a LOT of timing at idle speed. Some respond well to it. It's boils down to a case by case basis on what the engine wants. The tuner should keep in mind at this point, that the ONLY difference is that he has the choice to add the timing at idle speed, and when coasting via MVA. A well located ported source adds in the same amount of timing at every other point.
One must make absolutely sure when choosing a ported source, that it is ALL IN right off idle. Many carburetors have ported sources that were designed to run EGR valves, or other emission devices. They have a much higher source location in the baseplate, and do not mimic a manifold source well enough to be used to apply the vacuum advance. Also be aware, that many q-jets have a well located ported source, but it has a bleed-off hole that drops out the vacuum as the throttle angle continues to increase.
I highly recomend to use a vacuum gauge when choosing the source for your vacuum advance, to make sure it is applying the vacuum to the can correctly under all driving conditions......Cliff
Ozzmann:
--- Quote ---Very few q-jets will have vacuum at any rear ports in the airhorn. Most of those ports were to take in fresh/filtered air for the hot air choke. I've worked on several early 70's Old's carbs that had vacuum in the airhorn at one of the rear fittings, so it was used on some models.
In any case, make sure that you plug all vacuum fittings and correct any vacuum leaks.
--- End quote ---
So I should just plug that top port with something? I only noticed it because I could hear it sucking in the air, definately couldn't hear it before I made the mods to carb, maybe enabling the bypass air has caused this leak?
--- Quote ---Take a look in the carb at idle speed and see if any fuel is coming off the boosters (nozzle drip). If you don't have nozzle drip, and have full control of the idle fuel via the mixture screws, you do not need any addtional bypass air (in most cases). Some engines still like a lot of bypass air, for one reason or another, so adding it can help clean up the idle and further reduce how much transfer slot is exposed to gain more control of the idle fuel via the mixture screws.
--- End quote ---
I have no nozzle drip, and and kill the engine by seating the mixture screws. I thought that if you removed a hose and idle speed increased you needed more bypass air, but your saying I would have enough, (in most cases) What should I do, leave the bypass air as is, or add more, I'm confused.
Can I end up with too much bypass air, what is the risk?
Once again I thank you for sharing your time and experience Cliff.
omaha:
I would set the idle with the dist adv. disco'd. You are gonna have to use both the throttle plate adjustment and the idle screws. Probabky wont need too much idle's turned out with the .096 holes. But don't be afraid to adjust the throttle opening. You can always look down in there to see if it pulling fuel from the nozzles. You are not pulling fuel now so open the plates a bit. Even my bypass only carb on my old VW had an adjustment for the air (there was no throttle adj., just air "screw" and the needles). you had to tune both of them. By the way, pulling a vacuum line is exactly the same as adding more bypass air (except for the filtering of the air) think about it. just depends on how big the vac. line is as to how much more air your adding. I have even thought of using some kind of ball valve mounted to a drilled hole in the intake plenum (for air adjustment) and then connect it with a hose to the underside of the aircleaner (for air filtering) it would work like an adjustable bypass air wouldn't it. ( i am nuts i guess}
Pulling a vac. line is the same as adding bypass air just without the filtering. Cliff is 100% correct about having a "ported" source for the dist. advance. Especially for street driving. Race cars dont need vacuum advance but ALL street cars should use vac. advance WITH a "ported" vacuum source. Even if you send the baseplate to cliff to drill it correctly, it would be worth it to have him do that. Always use the ported source. I never seen a Q-jet without a ported source but I guess they exsist. Oh well this is just all my 2 cents worth. Cliff really is the man to inform you. I just write opinions. (but based on some knowledge from my street racing days with a 67 goat and tuning others cars/4x4 trucks round here). Good luck to ya!
von:
Now I realize Cliff has much more experience than me at setting up distributors but I prefer full manifold vacuum for vacuum advance. The only time I prefer ported vacuum is with an automatic trans that drags the idle speed down too much when put in gear (usually with perf cam and stock converter). I set them up on a dist machine and try to match the vacuum can to the engine, plus fab a limiter to limit vac advance to 10-12 (crankshaft) degrees. This is after setting mech advance to start above idle of course and be all in at the 2000-3000 range depending on combo. IMO full manifold vacuum advance will give better fuel economy and the engine will run cooler. BTW some early 70's Chevy Q jets have no ported vacuum source. I had a '70 BBC 7040204 carb that way. I suspect it was that way because '70's used TCS, or Transmission Controlled Spark Advance. That was a trans controlled switch that shut off vac advance to the dist unless trans was in high gear.
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