My YS91AC runs good above half trottle but below it gets mighty rough
down to an idle. It will actually stop if I let it run at this speed.
At idle it runs OK. All the jets are clean, one way valve is clean, no
leaks, no restructions, new plug, have not touched the regulator
adjustment except to clean it, diaphragm has a little bulge in it and
I have one on order. Tried diffenernt jug of fuel. This engine has
been performing flawlessly in my U-CAN-DO 3D for many weeks and
Thanks for the advice Paul. I always put the regulator screw back
flush with the case as they recommend and it works OK. I can't
understand why after running perfectly all of a sudden it won't run in
the midrange. I used these engines for pattern a few years ago amd
never had to fiddle with the regulators. I put my other YS91AC in the
plane and it's OK so I don't think there is anything wrong with the
onboard fuel system. I'll wait for the new diaphragm to arrive. Oh
The regulator works in conjunction with a little plunger which has a
little silicone plug at the end of it which blocks a hole, the plug
can shift out of place or get deformed with age. The plunger also can
get worn on the shaft in which is moves.
Make sure you are running at least 20% nitro in your fuel. These engines are
known to act poorly with lesser nitro contents. Also, how is your glow plug,
is it new or old. Lastly, how old is the engine. If it has considerable time
on it, you may be getting blow back around the piston. If so, YS Specialist
have a mod that help the problem out.