After a quick break-in run on a test stand, I installed this engine on an airplane and flew it a total of about 20 minutes during 2 flights. The engine quit both times in flight. It seemed to be running fine when I elected to fly with it. I could tell it was a bit rich and I figured it had loaded up from running at reduced power and that is why it quit. I put the engine back on the stand and after some minor adjustments it seemed to run fine. Nice idle at 2500, but would go no lower no matter what I did. The next session on the test stand the engine acted totally different! Made no adjustments initially. Would not run much lower than 4500 RPM, surged at all throttle settings except wide open.

I have tried everything suggested on this forum, but am unable to get this engine to stop surging and to idle properly.

I used Cool Power heli fuel 30%(orange color)
OS F glo, although I have tried a Saito, and a Hanger 9 4 Stroke plug. OS worked best.
I have tried setting the regulator to flush and on down to as much as one turn below flush....in increments.
Main needle open 2 to 2.5 turns
Idle(low speed) screw 1 to 2 turns and everything in between.
With the throttle plate all but closed...only a sliver of an opening, the engine will run at 4500 to 4800 RPM for 30 to 40 seconds, then abruptly it will drop to 3000-3200, but the engine will die. With so little throttle opening is it getting air from a leak somewhere ??

The fuel tank is holding pressure fine. It will store air pressure 5 minutes after the engine has stopped.

The line from the regulator to the carb will produce a stream of air bubbles when I attempt to throttle back to 4000 and below. NO AIR BUBBLES IN ANY OTHER LINES!

This is my first YS engine, but I have three Saito 4 strokes and 5 2 strokes of varous makes , and 4 diesel engines and I have NEVER encountered this much grief with a new engine.

I have run an entire gallon of Cool Power heli fuel through this engine while trying to get it to run.

I must be missing something here!