Rough Idle help
 

I recently had a Marvel 4-5 carb rebuilt for an O-360 and it runs fantastic... above idle. If I'm below about 1300 RPM the engine will surge up and down by about 50-70 rpm. Does anyone know what would cause this?

At any normal flight power setting the engine runs great, no issues. I have a wood prop, which I know is known for making idling more difficult, but I feel above 850ish rpm it should have enough momentum to idle smoothly.

Any help is appreciated.

1) Check primer is closed and locked.
2) Close off primer port(s) to exclude a leaking primer pump.
3) Check for induction leaks. MAP should be ~12" at idle. Higher could indicate a leak.

Check for a missing carb air temp port lead plug or a threaded plug there, if it was tapped at some point.
Good Luck,
Mahlon