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Very strange problem Vittorazi Moster 185


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A very strange thing occurs to my Vittorazi moster185.

During my last flight, my max RPM just decreased to 6400rpm instead of 8400rpm. Also have a CHT gauge, max temp is +/- 205°C.

Engine starts after 1-2 pulls, idles great, reaches max rpm but after about 30seconds, the rpm drops at wide open throttle.

I guess there is some kind of leak when the engine gets warm.

I still haven't found the cause, the things I checked and changed:

- fresh fuel (2,5% oil/fuel)

- new spark plug

- swapped carb with a 100% working Walbro WB37 carb (pop off pressure is good, new membranes,...)

- replaced the coil (with 0,45mm gap between flywheel and coil)

- fuel system is ok (replaced fuel lines,fuel pump and filter)

- fuel tank valve is ok

- brass weight at bottom fuel tank is ok

- replaced al the gaskets between carb and reed valve

- reed valves are ok

- replaced exhaust gasket

- motor has 130h, put a new piston, piston rings, needle bearing


Has enyone encountered the same problem? I am running out of ideas :-(

Best regards,


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I had a similar issue, check that the primer bulb isn't drawing in a tiny bit of air. The engine copes and runs normally until it goes wide open for a while.

I had that problem even on a brand new primer bulb - some types of retaining clips cause the pipe to slightly distort on the connection to the bulb.   

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