Fun with carb base gasket

[rpmerf]

Ongoing story of my fun.

1970 C20 350, Quadrajet from a 1973 truck. The truck had been owned by my uncle from 1987 through 2011 when he gave the truck to me.

I took the truck to a car show at work about 3 weeks ago. It barely made it home. Sputtering, doesn't want to accelerate. Intuition told me vacuum leak, so I grabbed the carb cleaner. I found a leak around the passenger's side of the carb. Guess it needs a base gasket. Pull the carb off, look at the gasket, search around for a couple hours and determine it's a 1975-80 gasket. Don't know why I have a 70 manifold with a 73 carb and a 75-80 gasket, but whatever, it's been like that for the past 8 years. I find one at a local Advance and buy it. I ordered 3 off RockAuto, so I have some spares. Install it. Still changing RPM when I spray the pass side of the carb base, but it's better than it was. It drives like it always did. Figure I'll give it a couple heat cycles and retorque it and see if that helps. That takes us to today. Check it again, still leaking. Retorque it, still leaking. Figure I'll try coating the gasket in a thin layer grease to help it seal. Still leaking. At this point I realize something is not right. Take off the carb, line up the gasket, and there is an exposed vacuum port on the pass side near behind the secondaries. I haven't removed this carb since I got the truck in January 2011. This carb has had a vacuum leak for at least 8 years! The truck and it has run pretty good. Maybe a little lean at cruise, but always fired up and got down the road. Just went on Rock Auto and ordered 2 different gaskets - an open plenum one and a 4 hole one. Both on wholesaler clearance, so I got 3 of each shipped for $12. Not sure which one I'll use but I figure the 4 hole one will work better with the stock manifold. Can't wait to see how much differently it will run without a vacuum leak.

[MJN]

Hopefully the gasket fixes it for you, over tightening the carb down to the intake can also cause issues and warp the base plate and cause leaks. The GM service manual calls out 75 in lbs (6.25 ft lbs) for the 4 bolts that hold the carb to the intake. If you have trouble finding the correct base gasket it may be worth a call to Quadrajet Power, www.quadrajetpower.com Also, can you post the intake casting number and the carburetor number?

[Ironangel]

When you say "base gasket" do you mean the 1/4" thick one? Sounds like that carb is due for a rebuild including new bushings in the primary throttle shaft openings. Over torqueing the air cleaner wing nut can and will warp the carb base plate. Couple that with over tightening the carb nuts and bolts and they'll suck air as well...

[rpmerf]

Yes the 1/4" thick gasket between the intake manifold and carburetor. I know the carb is 7043208, don't know the number on the intake. One of the gaskets I ordered is the correct gasket for a 73 Chevy Truck 5.7L 4 barrel according to RockAuto.

I was torquing them 'to feel.' I found a torque spec of 36 in/lb (3 ft/lb), then 144 in/lb (12 ft/lb) and tried that on my last run. 12 ft/lb was a bit tighter than I had them before. I think I prefer going be feel.

The carb was rebuilt by a professional at some point, but it might had been 20 years ago at this point. Once I get the new gasket in, I will see if the shaft bushing is leaking.

[rockyrivermark]

Originally Posted by MJN

Hopefully the gasket fixes it for you, over tightening the carb down to the intake can also cause issues and warp the base plate and cause leaks. The GM service manual calls out 75 in lbs (6.25 ft lbs) for the 4 bolts that hold the carb to the intake. If you have trouble finding the correct base gasket it may be worth a call to Quadrajet Power, www.quadrajetpower.com Also, can you post the intake casting number and the carburetor number?

Take the carb off and take a fine flat file and run it evenly acrossed the bottom
If it’s warped you will see the low spots the file doesn’t hit.
Do that until it’s all even. Common for this to happen from over tightening the carb down.
12”-14” file works well

[demian5]

I have ran into a few instances where there was an EGR intake on an older vehicle and the gasket barely covered the ports in the intake. When they split or were marginalized, there was a huge leak and it wouldn't run.

[rpmerf]

Still waiting on my gaskets from RockAuto. I looked though a rebuild kit in my gasket pile and found a paper gasket. Lined it up on the carb and it looked good. I figured I'll run this gasket until the 1/4" gaskets get here. Installed it, sprayed around the carb, no change in RPM. Progress! I took it out for a drive and watched the A/F gauge. A LOT richer. I had to take the idle screws out 1.5 turns.

Now it's time to start looking through what jets, rods, and springs I have to get this carb tuned.

[MJN]

Originally Posted by rpmerf

Yes the 1/4" thick gasket between the intake manifold and carburetor. I know the carb is 7043208, don't know the number on the intake. One of the gaskets I ordered is the correct gasket for a 73 Chevy Truck 5.7L 4 barrel according to RockAuto.

I was torquing them 'to feel.' I found a torque spec of 36 in/lb (3 ft/lb), then 144 in/lb (12 ft/lb) and tried that on my last run. 12 ft/lb was a bit tighter than I had them before. I think I prefer going be feel.

The carb was rebuilt by a professional at some point, but it might had been 20 years ago at this point. Once I get the new gasket in, I will see if the shaft bushing is leaking.

Whether you prefer tightening by feel or not there is a torque spec there determined by GM for a reason and why torque wrenches are and should be used. I'm not saying 12 ft lbs will warp the base plate or air horn but going by feel is not a repeatable method and could cause additional damage and vacuum leaks. Pdf page 475 or manual page 6M-33 outlines the torque values. http://www.73-87chevytrucks.com/tech...ice_Manual.pdf