I believe I got this tip from one of the posters on this forum. But I wanted to mention it again. I have shaken the muffler loose on my YS .91 AC sever times. If it hadn't been safetywired on it would be sleeping in the bean field. Well use teflon tape like you get at the pluming section of the hardware store. 2 times around seems to do the trick.
Wow is this one powerful engine! i only wish it didn't shake so much.
Sparky

Thanks for the tip!

If you alter the rpm slightly you can improve on the shake problem. Sometimes moving the peak rpm 2 or 300 one way or the other helps a great deal. Try a different prop size.
Also make sure that you are not running the engine too lean.

Good Flying,
Dave