This is from personal experience. NIX the Cool Power. I originally ran my 1.60 with 10% Cool Power and fried the engine with just one inadvertent lean run, i.e., it ran rich the previous day and ran lean the next day without any mods. Lucky for me they repaired the engine under warrantee -- had to replace ring and sleeve.

I now run Wild Cat 10-15%, 18% Synth/Castor mix and have had no more problems. This is the best engine in my inventory -- she starts on the 1st flip even when cold and even when unused for couple of months.

Your problem description sounds familiar -- does this happen only while flying, i.e., not while running it on the ground? If my memory serves me correctly that's the way my engine sounds when I transition from full power to just below half. I've gotten use to this and it doesn't appear to be a problem. I do, however, have a problem going from idle to full power quickly. Its been a while since I've used this engine (will be running it on an Ultra Stick this weekend - I hope) and have not worked the low-speed needle adjustment at all in its entire history. I'm hoping that is all that is needed to smooth out the transition.

What happens is if you allow the engine to idle for an extensive period of time on the ground you have to flick the engine a few times before you can go to full power or you may stall it -- especially if it is the 1st flight of the day. In the air if this happens the engine wont stall but will hesitate before transitioning to full power. This almost caused me a problem when I had to do a landing go around -- barely cleared those trees at the end of the runway. Again, I've been living with this problem for a while and plan to eventually adjust the low-end to correct it.

I've own this engine for almost 3 years and have had it mounted on 2 Ultra Sticks -- the 3rd Ultra Stick is currently being prepped for flying this weekend. I once buried it in the Florida sand; had to give it a water, then alcohol bath, to remove the sand. The 1st time I started it after burying it in the sand I ran an experiment. It had not been started in over 6 months following the water and alcohol bath. I primed the carburetor real sloppy wet, applied heat to the glow, and back flip the prop -- she started immediately. Unfortunately that Ultra Stick met its demise on its 1st flight due to either a radio interference or momentary battery disconnect problem (everything worked fine after the crash) -- the worst feeling in the world is to watch your $500.00 airplane nose into the trees while your flailing on the sticks to no avail. BTW, this was exactly the same problem that put it in the Florida sand with the 1st Ultra Stick -- I hope my 3rd one is not a repeat.