Cast iron is funny stuff. It can crack at the drop of a hat. If it was mine, I would clean up that carburetor pad up real good and get a magnifying glass and inspect it very closely. It may have a crack somewhere. Also, the carburetor pad on those is really low.....close to the top of the intake. I would also double and triple check that nothing on the carburetor is hitting the intake manifold and keeping it from tightening down all the way. I have run myself ragged in the past on that same intake and carburetor combination, only to find out that a carburetor fitting was turned so that one of the points of the fitting was holding the carburetor off the intake pad. All I had to do was turn the fitting so a flat was down and it went right on and sealed up. It may also help if you have the thick carburetor insulating gasket. They are over 1/4" thick. I would not run without one, because those carburetors are bad about heat soak.
