Air Cleaner Popping

I recently bought a 1950 TO-20 and have put to much money into it to let it sit. After I got it running it is popping through the air cleaner. Seems like when I idle it down it stops popping or backing fire. Do you think it might be a cracked head or bad valves? I hate to say this but looks like some oil in water. Any help is greatly appreciated. November 19, 2014.

Here are some possibilities: There is a buildup of carbon in the combustion chambers. There is a cracked distributor cap. The plug wires need to be replaced. There is a bad spark plug. The spark plugs are the wrong spark plugs. The timing needs to be reset. The advance mechanism in your distributor is not working. The point gap or dwell angle needs to be reset. The carburetor needs to be adjusted.