Topic: '71 LS5 454 intake carb gasket problems
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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Hey guys. Going nuts trying to figure this out. Here it goes.
I have a 71 LS5 454 w/ stock intake 3955287 Q-jet intake. New Holley 4165 spread bore/Q-jet replacement on it.
Of the sources:
Holley carb directions
Holley tech line
Holley Book by Mike Urich and Bill Fisher
Numerous Corvette forums/topics w/ pics
All offer different takes on carb gaskets, how to stack, what to use.
I have an exhaust crossover up front but primaries, but can't find the correct gasket
Holley directions with carb insists on using the and exhaust/header type think gasket and thin metal shim, then carb (in that order).
Holley tech line says to put down thin metal shim first, then gasket and carb.
I even bought the the insulated .25 Q-jet style Holley 108-25 with plastic "o" rings around studs.
I bought two thin metal shims (Holley 108-20), but they are different than my intake style. Comparing it to other BBC intakes in my garage, they match ('68).
Using the exhaust header-like carb gasket included in carb kit leaves room for an exhaust leak.
Nowhere can I find an exhaust-material carb gasket and corresponding shim for my car (though the shim I have will probably work).
Here's 3 pics:
1st, stock intake flange; Second, with the '69 style exhaust-material gasket that would leak (see finger pointing); and 3rd, with the .25 phenelic Holley spacer (I've been using).
Please help.
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#3 is the gasket I have used under all of my Q Jets on both stock and after market manifolds. Never had a problem with one. The heat cross over is for operating in extreme cold conditions, I have always blocked them off with the metal plates included with the intake gasket set when using a stock manifold.
I have been using the think gasket as shown above, with the metal shim below directly over the heat crossover.
Supposedly this is wrong. Even several Corvette mag tech articles have verified this with pics. But they had a '69 and earlier (as those gaskets match).
Been having a fuel leak issue lately (hence I took carb off) but I want to be sure that the gasket won't burn out.
You say block off the heat crossover. I've heard epoxy and other methods. I'm not so sure that that thin metal shim will do the trick, unless it's with that exhaust-material gasket i can't find.
I'm looking more at the 71 flange- I don't think it is a heat crossover.
Take a look at this pic of my other '68-'69 style. Clearly it has holes leading into the crossover track. My 71 doesn't so maybe I don't have to worry about the shim at all.
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You are correct...what you have on the first post is NOT a heat crossover...it's just an empty passage, and has more to do with evening out the vacuum pulses to the secondaries, as well as idle mixtures.
Doesn't really matter what style gasket you use, as long as it covers everything.
NEVER use a thin steel/aluminuminum shim-type thingy directly on the manifold....it will NEVER seal properly.
Are you having carb troubles, or just trying to figure out what kind-o-gasket to use there?
Adams' Apple 2010-08-31 10:25:41
Doesn't really matter what style gasket you use, as long as it covers everything.
NEVER use a thin steel/aluminuminum shim-type thingy directly on the manifold....it will NEVER seal properly.
Are you having carb troubles, or just trying to figure out what kind-o-gasket to use there?
Joel Adams
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Yes, it looks like it isn't a crossover, though I thought it was until i look at my other BBC intakes I had stored.
As you said, it's probably to just even out the pulses or mixture, for a more complete fuel atomization.
I did have a few carb issues- but I fixed them. Thanks for all your help.
I have a 72 LS5 and I put the big #3 gasket on mine. My manifold also looks like that without the holes. I have had no problems with mine leaking. It's been on about a year now, so far so good.
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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