Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Frankencarb?
Shiny:
Thank you Kenth!
I conclude I have the wrong baseplate and will plug those holes!!
Doh!
Shiny:
Can anyone tell me what the "correct" number I should find on the 7029240 baseplate?
I have another core, 7040244, and the baseplate on it has a different number (38753) that also doesn't match the Delco reference for the 7040244 baseplate assembly (7036831). I suspect the numbers on the baseplates may be machined casting part numbers and not assembly numbers but don't know where to look this stuff up.
Would be nice to know if the 7040244 baseplate is actually and magically correct for the 7029240 carb. But I don't want to deal with it if not... the APT screw was a major hassle the first time and I was lucky to salvage what I now know is the wrong baseplate.
I have also posted on PY and although my "wrong baseplate" has bypass holes, I don't yet know if they actually connect to a vent or other air source. It is NOT obvious and I haven't yet pulled the baseplate off the body. It may turn out the holes do nothing and all is well with the idle circuit.
Lots for me to learn!
Thanks,
Mike
Cliff Ruggles:
Having the wrong baseplate isn't a big deal as far as idle bypass air is concerned. IF your main body bypass air passages in the voids in the casting aren't drilled you can simply drill them. You may also need to "notch" the airhorn as well to complete the bypass air system.
Bypass air is used to keep throttle angle down at idle speed and helpful if you have "low" vacuum from a larger than stock camshaft or the engine combination just isn't making a lot of vacuum at idle speed, or anytime you are pulling fuel from the main boosters (nozzle drip) but the carbs idle system is adequate for the application.
You can still get into trouble with the wrong baseplate when it comes to gaskets, so make sure you are using the correct gasket between the main casting and baseplate for the parts that are being used.
There are other potential issues with swapping baseplates from one carb part number to another. APT height is one, transfer slot location and length is another, not to mention the size of the holes under the idle mixture screws and the size of the idle bypass air holes (if used).
One has to keep in mind that Q-jets were application specific and set up at the factory so all three components worked well together. If/when we come in and start swapping parts around more times than not it throws things off some as far as the calibration is concerned. This is why I supplied some "basic" calibrations in my book, but it is impossible to provide exact specifics for any particular carburetor part number being used on a "custom" engine build.
Even so knowing the "basics" is the first step toward getting a good end result.
I'll add here that the Buick 350 carburetors made from 1969-1974 are EXCELLENT starting points for a really nice high performance unit. They don't need a lot of help anyplace either and it is best to start out at or very close to the stock settings for most engine builds they end up on.......
Shiny:
Cliff, thank you!
I opened it up today and find the body was never drilled for bypass air, consistent with original design.
So even though the mismatched baseplate has idle bypass holes, they weren't doing anything.
I will scrutinize the gasket and walk myself through all the circuits tomorrow to make sure the mismatch isn't hurting anything. All the gaskets and parts I've replaced came from you, so I expect they match the body. Just need to make sure the body and baseplate passages are aligned.
I will likely ask again about the tuning before I re-assemble...
Also good to know the Buick 350 carbs are decent, as I have a 7040244 core that might need to be rebuilt to replace this frustrating POS I'm battling. The 7040244 is rough and missing a lot of parts, but maybe they'll all transfer from the 29240 MD Frankencarb if I decide to toss the one I've got.
Kenth also posted on PY that the transfer slots are different between this Buick 430 carb and a Buick 350 carb. Is that a big concern for use with my mostly-stock 1970 350 Pontiac?
The "never-drilled" holes are circled in the photo.
Mike
Cliff Ruggles:
Transfer slot length and location can be an issue with some set-ups. If you are exposing a LOT of transfer slot I've seen scenarios where you can completely seat the idle mixture screws and still WAY too rich at idle......
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