Hi:
I have a new ys .63. I ran it for the first time today, and ihad a problem with it backfiring. I richened the high end to about 3 1/2 turns out and that helped but it did backfire one more time. What will cause the engine to backfire, this is my first YS, and my first 4-stroke. I didn't change any settings out of the box except the high end mixture.I'm running the engine with an apc 12x8 on 30% heli coolpower.by the way it was about 40 degrees today.

Thanks
Stephen

The backfire as you are calling it, is likely from the engine loading up and dying rich. The 40 degree weather isn't helping. Go to the running instructions at www.ysperformance.com and set the engine up properly. 3.5 turns open on the HS needle is way too far open. It should be at between 1.5 and 2 turns.