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Carb gasket confusion (sbc)

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1991 305 converted to Quadrajet carb with a summit intake and stock reman Quadrajet for a 1983 k20 (the vehicle that it is in)

I’m going to preface this by: I have it “fixed” and running well now, but ….

ran fine when parked a couple months ago. Went to use it and it was stumbling and stalling and running very poor altogether. Limped it through the project (it’s a yard truck) and kept it running by cranking up the warm idle.

fast forward to tonight: started it up and was still running poor. Let it warm up and settle into its warm idle. Checked it with brake cleaner and the carburetor gasket was leaking.

so in an attempt to not break anything I put my torque wrench to it and started tightening. Finally got the bolts to tighten at 20 lb-ft. Probably too much but they were already more than 15 by hand. So I kept working them down compressing the gasket. (Stock thick paper gasket with internal stops).

so as I was doing this I kept checking for leaks. Still leaking. Finally got the bolts to stop turning at 20 lb-ft and still leaking. So i reved the engine some for about 20 seconds and the idle came up and stayed up. Reved it again for about 20 seconds and the idle came up again and stayed up. Rinse and repeat a couple more times until it seemed steady. Checked the gasket for leaks and no leaks. Lowered the hot idle and checked again, no leaks. Purrs like a kitten.

so: why was it leaking and then only stopped leaking after racing the engine? Made no other changes between leaking and no leaks. Just raced the engine for about 20 seconds or so.

also: can someone recommend a better carburetor gasket? I’ve lost confidence in this one. It was replaced last fall and Only has one or two snow storms on it before giving up. Really can’t be dealing with that when it’s -20f out.

And: how tight should the bolts be. I’m sure they are over tight, but that is what it took to get it to seal. Again, this is a fresh reman carb and a new manifold when i put the motor together.

thanks in advance
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1991 305 converted to Quadrajet carb with a summit intake and stock reman Quadrajet for a 1983 k20 (the vehicle that it is in)

I’m going to preface this by: I have it “fixed” and running well now, but ….

ran fine when parked a couple months ago. Went to use it and it was stumbling and stalling and running very poor altogether. Limped it through the project (it’s a yard truck) and kept it running by cranking up the warm idle.

fast forward to tonight: started it up and was still running poor. Let it warm up and settle into its warm idle. Checked it with brake cleaner and the carburetor gasket was leaking.

so in an attempt to not break anything I put my torque wrench to it and started tightening. Finally got the bolts to tighten at 20 lb-ft. Probably too much but they were already more than 15 by hand. So I kept working them down compressing the gasket. (Stock thick paper gasket with internal stops).

so as I was doing this I kept checking for leaks. Still leaking. Finally got the bolts to stop turning at 20 lb-ft and still leaking. So i reved the engine some for about 20 seconds and the idle came up and stayed up. Reved it again for about 20 seconds and the idle came up again and stayed up. Rinse and repeat a couple more times until it seemed steady. Checked the gasket for leaks and no leaks. Lowered the hot idle and checked again, no leaks. Purrs like a kitten.

so: why was it leaking and then only stopped leaking after racing the engine? Made no other changes between leaking and no leaks. Just raced the engine for about 20 seconds or so.

also: can someone recommend a better carburetor gasket? I’ve lost confidence in this one. It was replaced last fall and Only has one or two snow storms on it before giving up. Really can’t be dealing with that when it’s -20f out.

And: how tight should the bolts be. I’m sure they are over tight, but that is what it took to get it to seal. Again, this is a fresh reman carb and a new manifold when i put the motor together.

thanks in advance
Those carbs are very sensitive to overtightening the bolts esp the long front ones. It warps the base plate and the top (air horn). I think your paper gasket has come,apart and by tightening and revving it it may have sucked a piece of the gasket back. The best gasket is a tan colored one approx 5/16 Thick. Those carbs don't require much torque when they are flat and have a good base gasket. Google would give you torque spec. Good luck. Ernie.
 

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1991 305 converted to Quadrajet carb with a summit intake and stock reman Quadrajet for a 1983 k20 (the vehicle that it is in)

I’m going to preface this by: I have it “fixed” and running well now, but ….

ran fine when parked a couple months ago. Went to use it and it was stumbling and stalling and running very poor altogether. Limped it through the project (it’s a yard truck) and kept it running by cranking up the warm idle.

fast forward to tonight: started it up and was still running poor. Let it warm up and settle into its warm idle. Checked it with brake cleaner and the carburetor gasket was leaking.

so in an attempt to not break anything I put my torque wrench to it and started tightening. Finally got the bolts to tighten at 20 lb-ft. Probably too much but they were already more than 15 by hand. So I kept working them down compressing the gasket. (Stock thick paper gasket with internal stops).

so as I was doing this I kept checking for leaks. Still leaking. Finally got the bolts to stop turning at 20 lb-ft and still leaking. So i reved the engine some for about 20 seconds and the idle came up and stayed up. Reved it again for about 20 seconds and the idle came up again and stayed up. Rinse and repeat a couple more times until it seemed steady. Checked the gasket for leaks and no leaks. Lowered the hot idle and checked again, no leaks. Purrs like a kitten.

so: why was it leaking and then only stopped leaking after racing the engine? Made no other changes between leaking and no leaks. Just raced the engine for about 20 seconds or so.

also: can someone recommend a better carburetor gasket? I’ve lost confidence in this one. It was replaced last fall and Only has one or two snow storms on it before giving up. Really can’t be dealing with that when it’s -20f out.

And: how tight should the bolts be. I’m sure they are over tight, but that is what it took to get it to seal. Again, this is a fresh reman carb and a new manifold when i put the motor together.

thanks in advance

10 to 12ft lbs. Way to tight I be surprised if you haven't warped the base plate of the carb.
 

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Discussion Starter · #5 ·
Where can I find the tan gasket? What is it called so I know what to ask for?

i can try to check to see if it is warped. It may well be. When I first put the engine together, I just had a single layer paper gasket. Over a period of time it started to leak as well (could hear it howling as it was sucking past the gasket). It took quite a bit is torque on the front bolts to get it to seal up that time as well. But it did stay sealed until I pulled the carburetor to find different leak (deteriorated vacuum cap).

tired of vacuum leaks….
 

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A carburetor does not need any more then about 6 pounds of torque or basically just a little bit hand tight once the bolt starts to get tight. About a 1/4 of a turn or a hair more past when getting tight will do. Also you want to use a thick enough gasket to help seal things. I use gaskets that are at least an 1/8 inch thick.

Many factory ones that come in rebuild kits at least for Qjets depending on where you get them from and also using stock type gaskets they are so thin they will almost always leak. I don't think they are even 1/16th of an inch thick.

You might want to check your carb base plate bottom or your intake both and see if they are flat with a straight edge if you can get access to one.

You are lucky if you have not broken the ears of the corner of the base plate on those as on Holley carbs they would most likely have broke.
 

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Discussion Starter · #10 ·
Thank you for the reply. I’m hoping I didn’t warp the base too badly by cranking down on it.
They had the thick felpro gasket like what I had, but kind of didn’t want the same one since it failed so quickly.
The fact that the cometic was half the price didn’t hurt anything either.

the last time I had a 4-hole gasket like this one I trimmed it out to match the manifold (dual plane). Is there any need or advantage to opening it up? I was just afraid of the gasket getting weak over time and collapsing into The intake. Any need to worry about this?
 

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If that was a Holley at 20 ft pounds the mounting ears would have bent and you‘d be in trouble.

With a Q-Jet you probably got away with it but not something I‘d recommend.

Some gasket materials dry out without regular use. Probably the shortest fix would be to sandwich the positive stop gasket with a simple single layer gasket on each side.

Bogie
 

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Skim a layer of number 2 around 1/32nd thick on the top of the intake and bottom of carb. I find using my finger works better then a putty knife etc.

Clean your threads while your here then apply the DRY gasket and carb. Tighten bolts hand tight around 10 pounds. Let the number 2 gasket material dry for 24 hours(50-70F).

Have a few extra gaskets on hand and just clean the surface(new gasket and number 2) each time you install a carb.

Apply the number 2 to the metal surfaces not the gasket. The gasket is doing the work. The number 2 is just helping seal any razor blade gouges or slight warps where that gasket meets the metal.

The number 2 is just enough to fill in any imprefections in the machined surfaces.

 
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