Dave,

I've been having a peculiar problem with my YS91FZ which is in a UCanDo3D. It randomly dies. It doesn't matter if at idle, mid, or full throttle. It will fly a couple of minutes and then just quit without warning. No particular pattern noticed. It will always start up again right away with a single back-flip against compression. I have changed the plug, readjusted the needles/regulator several times, checked the fuel lines. I checked the regulator chamber for debris as well. It runs perfect otherwise - no hesitation, slight smoke trail, great idle. I checked the valve clearance which was slightly enlarged but not much. I adjusted them but haven't flown it again yet. I also considered it a heat problem and flew it without the cowl but the problem continued. The fuel filters are clean. The system holds pressure fine. I can't get it to happen on the ground.

This engine was in for service with you a few months ago (cracked air chamber) and worked great when it came back. Any ideas?

Thanks,
Chris