Dave I just got this motor back from you, and had 3 flights on it when the motor was in a vertical climb, and just quit. Upon restarting and transitioning to full power it just backfired and quit. It did this at about 8000 RPM. It did this 3 times before I richened the mixture about 1/8 of a turn, and it seemed to fix the problem. My question is ? Do I have a problem, since most motors give an indication of going lean, before they quit. I am very impressed with the power this engine produces, 9100 swinging a 14-8 Zinger. Any ideas as to what could be my problem, or what will happen if this is allowed to continue.

Sounds like it was just too lean. 9100 on a 14x8 is low. What fuel are you running? What plug?

Dave. Using PM 20/20 OS F. I am just curious why it just sudenly quit with a bang. Every engine that I have seen run a little lean gave the usual slowing, and sagging before quitting. This just accelerated, and quit with a bang. It seems like the timing was off, but since it runs good now that can't be the problem Also don't Zinger props run at alower RPM than APCs?

Zingers run slower due to bad airfoil shape. Look at the leading edge and trailing edge of the blades. Both are square unless you worked on them. YS engines will quit with a bang and usually kick the prop off. If you are using a spinner, make a couple discs of 80 grit emery to put between the spinner bp and drive washer. One with grit facing the drive washer, one with grit facing the spinner bp.