OS MAX FX 91 Test Runs After Service.

I was asked to service this engine after it was bought second-hand. This engine was still in good condition. I cleaned the bearings but decided to replace them, as they just feel too rough. I had to replace the carb o-rings, and I think the engine runs very well and should be a good flyer. The sound you hear while the engine is running at a specific RPM that sounds like a rattle is actually a muffler exhaust beat that is quite distinct on the OS 91 engine with the standard muffler.