12. I notice that while running the engine rpms change without my doing anything to the throttle. Sometimes it runs faster and some it slows down. Any suggestions as to what could cause this?

This condition is almost certainly caused by some small anomaly within the fuel system.

I would check your performance under load (in forward, pulling on the dock lines). If the RPM smoothes out under load, you probably just have a bit of crud in the idle system of the carburetor. If the changing RPM continues under load, with the engine fully warmed up, I would more suspect a problem in the fuel supply itself (pump, clogged filter or line, etc.)

If the former is the case, I would take the carburetor apart and clean it using an aerosol can of carburetor cleaner, paying particular attention to the upper half of the carburetor. If you have, or can borrow a copy of our service and overhaul manual, chapter 4 will help you with photos of the internal configuration of the carburetor.

If the latter is the case, I would disconnect the fuel line from the carburetor and operate the fuel pump, checking for a good flow of fuel from the tank. Pump at least a pint of fuel during the check.

If the fuel supply appears normal, then I would go back to the carburetor and clean it as suggested above for RPM variations at idle.

Another, although quite rare, cause of this condition could be a vacuum leak between the manifold and block. Some folks have reported success in diagnosing vacuum leaks in this area by spraying a bit of starting fluid or carburetor cleaner between the head and the manifold while the engine is idling. If the RPM changes while applying the fluid, it would indicate that the manifold gasket is leaking.

There is an obvious fire hazard in becoming too aggressive in spraying starting fluid around a running engine. I would use the thin plastic tube that comes with most aerosol cans of carburetor cleaner and starting fluid. In this way, you can localize the spray more effectively.

Wandering RPM is not a frequent condition, and one that is not particularly easy to diagnose. In at least one case, one of our customers had to change his carburetor before curing the problem. If your problem only occurs at idle, you may simply decide to live with the anomaly. - Updated: November 4, 2003