I just finished my first plane with a 4-stroke engine. I have a .40 size cub that I put an OS .52 4-stroke in it. I just started the engine for the first time today to break it in. It seems like the plane vibrates quite a bit. Is this normal? Am I going to have structural problems in the future? Is it just that the plane was running rich which causes the excess vibration? Should I switch over to a soft mount?

Thanks,
Jim

New engines vibrate more, especially when running rich. It will settle down, somewhat, on you.

If the truth was known, you should have made the first three or four runs on a run-in stand until you got the needles set before mounting it on the plane.

Soft mounts sometimes cause more trouble than what they solve.

Enjoy,

Jim

Hello; I try to use vibration absorbing engine mounts in all applications mainly to protect the airframe and radio gear. I would make sure that there isn't something loose. Properly mounted, a 4-stroke doesn't vibrate enough to notice. I do have an OS 48 surpass in a midwest Suhkoi that is ridgidly mounted that works quite well. I have to keep an eye on the mounts attachment to the firewall, they loosened for the first 4 or 5 flights, but have settled down now. I bought quality allen headed screws and tightened them down well and haven't had any problems since.

It will smooth out a bit as the hours pile up.

No mention has been made regarding prop balancing.....An improperly balanced prop will cause excessive vibration. VERY - VERY important to take time to achieve proper prop balance.