General Category > Quadrajet Carb Talk and Tips
Idle bypass air - what I've noticed (lengthy)
Bailey28:
OK. I don't believe any of these base plates match the carb bodies.
The base plate that came with the 6202 has SAE mixture screw threads. It has two screw mounts to the body. The distributor port is manifold vacuum only. It does have lots of holes on the right side near the EGR vacuum source.
The "D" shaped ports next to the idle down wells are open all the way through top to bottom and have small holes above the throttle blades connecting them to the venturi. Bypass air is .080", not .110 for sure.
It has a 1/4" NPT brake/vacuum hose port threaded into the rear.
Frank, it sounds like this base plate is close, but not the one that originally came with this carb. Thanks rebuilder guys! (I got the carb in a parts deal, I had no idea who was into it beforehand)>
Are there any identifying numbers on the base plate that I can research?
Frank400:
The baseplate casting number for a 17059217 should be 17060558 in a small circle.
The baseplate casting number for a 17056202 should be 7041452 in a small circle.
But having the correct casting number does not mean it is the "born with" baseplate. Same casting number was used in a number of applications with different sized machined holes.
But at least now you know if you don't have these casting numbers on your baseplates, you have the wrong baseplate(s).
Having 8-32 idle mixture screws in your 56202 is what it should be, with finer metric idle mixture screws in the 59217 (beginiing in 79 for all Qjets).
Bailey28:
I had to put my reading glasses on for the numbers on the base plates, but I did find them. They are exactly as you stated, so I have the correct base plates.
I also noticed that the base plate for the 9217 was slightly cracked where the drivers side throttle bushing was installed.
Cliff recommended using a ported vacuum source for distributor advance, however the 6202 base plate is not ported as you mentioned above.
I drilled the vacuum passage for the APT in an E4ME base I had laying around. Rebushed the primary shaft bores, and centered the blades.
I also installed restrictors in the air bypass channels in the base plate I drilled at .035".
I will report back with the results of this combination very soon.
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