In the airhorn, at the top of the Q-jet main wells, there are 2 pressed-in brass air bleeds or restrictors (see pic). I've seen some with really large holes, and some with small holes, but haven't noticed a correlation between size and anything else, like cfm. Has anybody ever messed with them, and what is their effect?
A carburetor meters BOTH fuel and air; the air bleeds can be thought of as the air jets. Different engines have different fuel requirement curves. The airbleeds are one of the areas that give the professional tuner additional "tweaking" capability. Jon.
So...any "rules of thumb" for changing them, or is it something best left to somebody with a dyno or flowbench? I'm fairly experienced with working with Weber air corrector jets, are they similar?
Different carburetors will have different size (some even a different number, and in different locations) air bleeds depending on the fuel requirements of the carburetors original application. Best rule of thumb, and this is GENERALLY true for any modification: BUILD IT STOCK FIRST! Now you have a baseline from which to work. Now, armed with an exhaust meter, you can make modifications to jets, metering rods, air bleeds, and hangers; and see the results at different RPM's with the meter on your specific application. The dyno or flow bench is not a necessity, but being able to measure the results of changes IS a necessity. Jon.
I understand what you're saying, and agree totally with starting from stock as a baseline, and making changes one at a time from there. What I'm asking is: What effect do the air bleeds have, and when? For example, Weber air corrector jets tend to have the greatest effect on higher rpm, once the main fuel jets have their main influence. Are the Q-jet air bleeds similar or different? Q-jet design has the air bleeds influencing idle, off-idle, power, and main circuits, but I'm interested in how much influence they have, and when.
The airbleeds can have an enormous effect, depending on the sizes used. The larger the airbleed, the less "negative pressure" in the fuel well; thus the less flow (as well as the mixture for that circuit being leaner) of mixture through the given circuit. As you mentioned, the air bleeds will have an effect on idle, off-idle, and primary metering on the Q-Jet. Some models also have a "timing" adjustment on fuel flow as the secondary side starts to open. For a better understanding, acquire a copy of the new Cliff Ruggles book on Q-jets. For serious performance tuning, Cliff removes the original air bleeds, and replaces them with adjustable airbleeds. Not trying to be ambiguous with the answer, but each carburetor is different; one can only speak in generalities. Jon.
I understand, finding online info on the Weber's is likewise difficult. I'll order me up a copy of Ruggle's book - Thanks for the reminder that it's there! :beer
I'll second that on the new Ruggles book it is great. I did not know that many parts and passages in the q-jet were modifiable until I started reading it. It takes nerve to start driiling things out though. uzzled:
Air bleeds are used to help trim the mixtures as the venturi vacuum increases. You may recall that it is the venturi effect that pulls fuel from the bowl into the circuit in which it is used. As the venturi vacuum increases, so will the fuel flow. If the air bleed is not used, the result can be excessively rich mixtures. The air bleed is used as a tuning aid to help "emulsify," or mix the fuel with air to control the resulting enrichment. The combination of the iincoming fuel and the air bleed helps tune the "curve" or enrichment characteristics of the circuit in question. It is typically used in the idle and off idle circuits to assist in meeting emission control, but without it, the mixture in a particular circuit can become rich enough to create black smoke. Some of this tuning halps with certain rear gear combinations of the drive line. Ray
