While having a discussion about heat shields, (did they work , which cars used or didn't use them) got me thinking of the baffle #3884576 I installed when I had some carb (7040207)problems. Is this piece correct for my carb? Is the gasket the same shape as the baffle, and can a gasket be put on the top and bottom of the baffle?
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Re: 70 Q-jet baffle
Dennis----
The GM #3884576 carburetor baffle is NOT correct for a 1970 Corvette. With respect to Corvettes, this baffle was used only on 1968 and 1969 models equipped with Rochester Q-Jets. The stainless baffle was installed on those models BETWEEN the carburetor and the asbestos/metal gasket. The purpose of this baffle was to protect the carburetor from the direct exhaust heat which was designed to pass through the manifold carburetor flange "preheat" passage. This passage is the "slot" seen on the forward(primary) side of the manifold carburetor flange which has two holes on each end, which pass through to the exhaust heat crossover tunnel in the manifold. 1970 Corvette manifolds do not have this heat crossover, so this baffle is totally superfluous and, essentially, ineffective when installed on a 1970 model.
1970 Corvettes with Q-Jets used two different carburetor to manifold gasket set-ups. NON evaporative emission control (EEC) system cars, used a single, thin gasket with a large plenum opening without venturi holes. This gasket, GM #3967470, is GM-discontinued but available in reproduction. No stainless steel baffle was used or is required since, as I previously mentioned, there is NO carb pre-heat slot.
1970 Corvettes WITH EEC(i.e. original Califonia-delivered cars)use a different "gasket". In this case, the "gasket" is an assembly which includes an aluminum heat shield with an attached gasket. The aluminum heat shield extends WELL beyond the periphery of the carburetor and gasket, whereas the attached gasket is similar to the non EEC gasket, but a little thicker. This shield was originally GM #3969837, but this part has been discontinued and replaced by GM #10147945, which remains available. You can use this insulator on your car even if it was not originally so-equipped, but you will have to fabricate a new choke rod. The EEC-equipped choke rod, GM #3970080, is discontinued, nearly impossible to find, and not reproduced.
A better plan would be to use the 71-73 Corvette thick, insulating base gasket. This gasket, originally GM # 3998912, is now available under GM #10069984. You will still have to change your choke rod due to the increased gasket thickness, but the ultimate installation will look more "stock" than the aforementioned 70 with EEC setup, since the large, extending insulator wasn't originally used on cars without EEC( and the installation of EEC is, otherwise, very easy to visually confirm). The choke rod required, GM #3989046, is discontinued, but available in reproduction.In Appreciation of John Hinckley -
Joe
joe, I'm asumming this slot in the manifold is directly under the fuel well plugs in the front of the carb. If I remember right, I have the slot in the manifold, not sure about the holes. Could this be right? (I don't feel like pulling the carb to find out). I've also had several different base gaskets that were supposed to be correct for my car.( some with throttle holes, some without, some covered this slot, some did not). Thanks for clearing that up. I tried the baffle after reading an article in vette mag (july 1998) in the Q&A about idle. Stated much the same as you've said about the purpose, but was an answer to a 70 L-46 problem. Guess everyone makes mistakes. You haven't steered me wrong yet. And as usual, your explanations make the most sense.Comment
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Re: Joe
Dennis-----
You don't need to pull the carb to confirm your recollection of the carb pre-heat passage. Your 1970 manifold should be GM #3965577. This manifold does not have the passage. However, if someone has previously replaced the manifold with an earlier manifold, like the GM #3927184 manifold used for 1969, then it will have the passage. In that case, and if you want to continue to use the manifold, I'd strongly recommend eliminating the passage altogether. To do this, all you have to do is drill and tap the holes on either end of the passage(and, if it has the passage it WILL have the holes on either end)for an appropriate-sized allen head pipe plug(1/4" NPT, if I recall). Then, the "carburetor destroyer" heat passage will be eliminated. I also recommend this mod for owners of earlier Corvettes with the correct manifold and the pre-heat passage. Chevrolet didn't eliminate this passage in 1970 because it worked great and caused no problem.
The original gasket for your car had a single large plenum opening and was externally configured to roughly follow the contours of the periphery of the carburetor base. A replacemnt gasket with venturi holes ought to work just as well and be externally undetectable. In fact, you could use the 68-69 gasket, GM #3884574, but this gasket is also GM-discontinued. The fact that this gasket has the heat passage slot in it would cause no problems for your installation.In Appreciation of John HinckleyComment
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