I would put $5 on a vacuum leak.....

I use a unlit propane torch and while the engine is running hold the leaking propane near ( one icnh)the intake manifold...... if the engine races you found a leak....you may need to use a piece of cardboard to shield the carb side of the engine from the fan blast..... leaks can also be underneath the manifold...... I discovered that I had installed the manifold wrong and the rings.... one anyways ..had pinched and nicked itself..... I replaced it... new gaskets all around and the problem disappeared.

One more thing..... try installing a large face cheap after market vacuum gauge near the carb for ease of adjustment..... and make sure the guage is connected to the wiper port of the intake manifold AND NOT to the vacuum line from the vacuum dizzy advance mechanism..... the vacuum advance connects to the brass fitting at the base of the carb..... if you Tee connect the gauge to that line your vacuum readings will be all screwie.....

Keep us posted of your progress.... that is how we all learn..

Bob C.

Bob C.
CMP Heaven
The Hammond Barn