Engine RPM fluctuates

[lhbradley]

Engine rebuilt by a mechanic over winter - new bearings, valves, rings, etc.
I rebuilt carb and fuel pump myself.

The engine RPMs fluctuate about 100 RPM or so over a period of 3 or 4 seconds. It speeds up and slows down. Weird. I also had gas pooling in the carb. Took it apart and adjusted the float a bit. Still using the old valve and seat, as the one in the rebuild kit stuck up a bit more than the old one, and would have required a major change to the float setup. But the old valve/seat assembly looks good.

Thoughts?

[Don Moyer]

Fluctuating RPM is a tough issue to pin down.

The usual suspicion is that the shaft seals on the throttle valve assembly are worn and allow a bit of air to be drawn into the fuel/air mixture, but in our application the throttle arm seldom gets enough use to cause the seals to wear out. Zenith considers marine engines in the same category as an industrial application, where the throttle remains stationary for long periods of time, as opposed to automotive applications where the throttle valve is in almost constant movement.

My first guess is that your float valve may have something to do with the problem. I'm concerned as to why the standard needle and seat did not fit. Before doing anything more complicated, would you please give us a few more details relative to the float valve issue? The correct index number on the seat of the valve in a late model carburetor should be 25 (stamped on the face of the seat itself).

Don