I have a ST-75 and am getting 8220 RPMs with a attached to a Astro Hog Bipe. 13 X6 prop. I can barely get it up. I would think that I should be getting at least 9/10,000. I have no leaks, new fuel, and new plug. I am useing omega 15% with 16% synthetic. I was told a long time ago by a flyer that works for Hobby bench in Phx that all 15% has 16% synthetic oil. should I drop to a 12 X 8 to get more RPMs??????? Thanks.

Ron