Quadrajet Carburetor Question

I have a question, BTW "Larry70GS" Thanks for you information the other day on the ignition and Carb :TU: My new question is what are the holes for on the primary side on the carburetor on the right. This may be hard to see but the base plate on the right has holes on the primarys and the carb on the left does not. The one on the left is a NOS 72 GS 455 4-Speed carb and the one on the right is a 72 GS AT STD carb. I'm in reference to the holes in the cut out area on the primary side. Does anyone know what this is for? AL

 

Quadrajet

Larry, its not much to see just a hole at the end of the cut out, AL.

 

well

I think those are feeds for the vac. ports. Run a pipe cleaner up and see if they come out....

 

These ports, as correctly pointed out, feed additional vacuum ports. Does the carb have additional vacuum ports on the bowl casting? If not, the mis-match is not serious. They will likely be blocked by either the gasket or the bowl casting. Ray

 

Actually, I think those two holes (one per primary) are suppose to provide by-pass air for the idle system. Some Quadrajets did not require this. Not 100% sure though.

 

Carburetor Question

This makes no sense, the one with no holes is a NOS 455 4-Speed carburetor, AL.

 

Carburetor Question

I got two other replies from rebuilders: They are the idle air bypass holes. Most all quadrajet carbs had them after 1969. 455 and some 350 Olds carbs had them plus bleed holes in the plates. A cleaner idle is accomplished by keeping the plates closer to closed in the bore, especially when a long duration cam is used. The bigger the cam, the larger the bleeds. Some of the Buick 350's had no bypass bleeds. The off idle discharge slots were a little higher in the bore and the were also shorter than the 455's. There were a few Buick 455's that according to the rochester engineering sheets I have, did not have the bybass bleeds. I've never seen any of them. And the next one was:It is part of the calibration & design. Not a big deal. What is a big deal is having the right exact number carb for the exact combination. AL o No: .

 

Quadrajet Question

Larry, I saw what you were talking about in my service book on the fixed idle air bleed. The holes I pointed out at the begaining of these post is something different. The idle air bleed is a internal path to the mixture scews. Does Mr. Osborne visit the V8 Board? AL.

 

I have seen these bleeds in several places. They are intended to allow the engine to idle with the throttle blades in the correct place for the idle transfer slots to feed at the correct time. These slots, positioned slightly above the idle feed orifice, is intended to allow a "clean" response to the throttle, without the richer mixtures typically needed. It acts as a transfer between idle, and "off idle," and is again, why these carbs are so well accepted by the community. If the throttles are open too wide, this slot begins to feed, causing idle instability. If they are covered excessively, too snmall an opening, an off idle hesitation may be the result as the signal to fill them is either lost or limited. To those unitialted, these air bleeds may be in one of several places, based on the needs of the engineering staff at the time. All of these engines received emission packages unique to them, based on driveline options, and all had to meet emission standards. These slots operate similarly to the secondary accelerator pump orifices I mentioned earlier. All of the Holey double pumper fans, take note. It doesn't take a fragile diaphragm to make an accelerator pump operate. It just takes a desire to make things right the first time.

 

I think the whole point of the exercise (by-pass air bleeds, relocated idle transfer slots, reduced throttle blade opening) was to reduce the possibility of "nozzle drip" from the main fuel feed at the top of the venturi. This provided a cleaner and stable idle. Some engine combos needed these carbs, others didn't.

 

Actually, the cause of the nozzle drip is different from the reason for the idle slots, etc. The cause for the nozzle drip is the result of the combination of retarded ignition timing, camshaft profile, and the necessary lean mixtures with the advent oif emission controls. This combination led to an engine that would not idle properly at the smaller throttle settings on a "normal" engine. The result was that the throttle openings became larger in an effort to create a "normal" idle. The resulting throttle opening caused a situation where the main system started to feed especially where the manifold vacuum was as low as it was, which also started the metering rods off of the seat. Though indirectly, the off-idle slot was the result of the necessary control of emissions, especially at part throttle. Initially, the idle was rich enough to cover this transition, but as the mixtures continued to lean out, the necessary fuel was no longer there, requiring a different circuit to deliver it. This situation demonstrates the "finesse" the Q-jet provides, allowing such fine tuning. This carb worked well into the late '80's. Its fuel control provided the necessary engine performance until fuel injection became a necessity. Neither Ford's, Chrysler's nor Holley could do that without major redesign. Therefore, emission control was the reason for the additional circuit, it was not directly because of the nozzle drip. Though they were related, it was not directly.