The Stovebolt Site Forums The Shop Area Engine Shop Rochester B carb gas leak

I just rebuilt the Rochester B carburetor on my 1952 Chevy 3100 with the 216 engine. But now gas is slowly seeping out of the two big screws that attach the throttle body to the underside of the carburetor. I tightened them up but it didn’t help. When I rebuilt the carburetor I replaced that gasket with the new one that came with the kit from Mike’s carburetors. I am sure I cleaned off those surfaces and made sure the were no burrs. What would cause this leak? It doesn’t look like gas is leaking from the gasket at the top of the carburetor and running down the outside.

Rochester B carburetors are notorious for leaking out of the top to body seam, or in your case the screws. It’s possible the top may be warped. There are only four screws clamping down the top to the body and over time, possibility from being overtightened, the top warps.

When this happens the power valve does not work correctly causing acceleration issues.

Most will suggest replacing it with a Carter, but there may be other factors. I would first check to see if the gasket or carb surface where the screw is leaking is damaged, then check to see if the top is warped.

One of our own members Jon G, has an Tech Tip on some issues and fixes with these carbs, that is well worth reading. You can view that tip here.

The 2 screws holding the throttle body to the bowl do not go all the way through to the reservoir, so they shouldn't leak. Could the bowl be cracked?

Remove the carburetor and drain out all the gas. Use carb cleaner to dry out the fuel inside and out once you remove the top. Set it on a dry piece of cardboard and carefully fill it 2/3 the way with fuel and watch the cardboard for leakage. Modern fuel is really bad about climbing walls and seepage, most likely it's its the top leaking, fuel will migrate up the walls and leak past the gasket and run back down the inside or outside of the main body. The screws you see leaking are just where it lands and leaks from. The main body doesn't have fuel ports or any hole from which fuel can get to the out side, it all has to go up to the top and out into the air stream, or up and back down fuel passage ways, either way it has to be drawn up. Too high of a float and too high of a fuel level makes the migration even worse.

My W1 carters will have fuel residue on the lid and screws, even though the fuel is well below them, fuel migration.

If you find no leak from the cardboard test, look for a wrapped lid, bad gasket, too high of a fuel level, or leaking float.

Very carefully clean and dry the area on the underside of the bowl where the pump link extends through the bowl.

Now, with TWO watching, one on each side; start the truck and observe to see if the fuel is dripping down the pump link, rather than the gasket.

Jon

I think I found the problem and I don’t think you guys would have ever guessed it. With the engine running, I was running my finger around the top of the carburetor to see if it was seeping gas when I noticed the sun glinting off a fine spray of liquid coming from the front of the engine. The plastic gas filter I had installed where the gas line curves around the valve cover was dripping gas and the fan was blowing it back onto the carburetor. I hadn’t touched that when I rebuilt the carburetor so I was focused on the stuff I did touch. It was a pretty big leak - about a drop a second falling down on the manifolds. As soon as I tightened up those hose clamps, I didn’t see any more gas on those screw at the base of the throttle body. But now it’s time to put Raymond in the barn for the winter. Hopefully it’s fixed but I’ll have to wait until spring to make sure.
Thanks for all of the help.

I never use those plastic filters on automobiles for exactly the reason you describe. They're barely acceptable for lawn tractors.

If you must use an inline filter, use an OEM style AC glass bowl filter with steel lines and brass fittings.