ORIGINAL: Charlie P.
It shakes quite a bit and it makes the whole plane vibrate.
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.
The engine mount is secure. Originally it wasn't, and I had to replace a blind nut in the firewall but now it is secure. I've checked all screws and bolts.
I haven't checked the balancing of the prop, but I've tried it with both the original prop (11x5) and also I 've tried with a new prop (11x6) which didn't make a difference. Yes it has the original spinner. The hole in the prop is a tight fit.