RPM drop while warming up the engine...

Thank you Bill,

No frost signs. I will check for dripping wet during the missfire, and let you know.

Hi Jim,

I will send picture of spark plugs after checking them.

if I recall, RPM is low, but stable and if I increase RPM vibrations are the same, but I will check it specificly and report back.

Regarding the two recent carburator service Bulletins, I'm not aware. I appreciate ifyou can send me a link to these specific service bulletins, so I can read it and check.

SB-912-073UL

SB-912-074UL

I doubt it is carb ice, and if the service bulletins above are complied with, I have another theory.

Suppose it's a hot day, with a hot engine, probably with MOGAS, then you may be looking at vapor lock. The fuel vaporizes downstream of the fuel pump, and cause intermittent RPM variations.

you shut down the engine, it cools, the fuel cools as its not being compressed on a hot day, hot engine, and it operates normally.

If this is the problem, try putting some AvGas in the tank, if the problem goes away, it's probably vapor lock.

Butch