I just started running my new OS 91 Surpass a few weeks ago. The engine got over 10,000 rpm with a 13x8 APC prop and that is to high. I use 10% Cool Power fuel. I used a 13x10 and got about 7000 so I had the LHS order me a 13x9 APC and engine runs @ 9400. A few on RCU said this engine should run below 10,000 rpm. Also, vibration very high @ 10,000. Mine is in a Ultra Sport 60. My original OS plug (came with engine) blew aftyer about 10 runs, went to a OS#8 and that didn't work well. Went to LHS and shelled out $7.99 for a OS F plug and all is well again. Engine now has 15 to 20 runs on it. Good Luck.