Is the engine mount secure and rigid? All four screws or bolts snugged down tight (those on the mount to firewall and those on the engine flanges to mount)? It won't run properly if it's vibrating that badly. Not only will the tank foam but the spray in the carb barrel can be effected with a really bad case of the shakes.

Is the prop balanced? Including any spinner? Is the hole in the prop the correct one for the engine drive shaft? It should be a TIGHT fit.