YS120AC not running

[Xavier Mouraux]

Re: YS 61 Long Stroke "need more help"Hello guys,

I have a problem with my 120AC and I didn't found the solution in old messages from the list so I need help.

I have been flying that engine for almost 3 years now, first in a Nova and this year in a Hydeout. It has been reliable for 2 1/2 years. Then, the day before the Oakville contest, it lost power and RPM (down to 6500 from 8300 on a 15x12 on 30%). Checking the engine didn't show anything obvious. However, we changed all the seals and O-ring, repolish the back plate (small mark on it) and rotary valve, clean the valves, clean the carburetor, check the piston and lining, etc. We didn't remove the cam but could see that it looks in good shape. After reassembly,  the engine runs better than ever (8500 with the 15x12 and 30%). It run fine for 2 months and about 50 flights with regular valve checking.

A few weeks ago, it begin being difficult to start and wasn't getting normal RPM after a minute of running on the ground during range check. There was smoke on the exhaust that looks as usual. Checking the engine from the outside again didn't show anything obvious but it didn't even start anymore. I opened it last week and clean the valves (some carbon on it), repolish the backplate (a little mark on it). It still doesn't start. New plug was tried, different glow drivers, turn the plane upside down, remove the needle valve, check the pressure (seems normal - a few turn of the prop by hand get pressure in the system), compression seems good. I haven't disassemble the air chamber but will do it tomorrow night. Maybe I will dismantle the carb also.

What else could I check or try ?

I would like to fly it a few more time before the snow arrive here in a few weeks :-) 

Thanks in advanced

Xavier
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