Onan engine throttling

[Mike]

I have a Sears garden tractor with an Onan 20 HP twin cylinder engine, model P220G. The problem I'm having is that the throttle is oscillating. Idle or under a load, the engine RPM's rev up then back down, about once a second. The first time this happened, I found water in the gas & filter. I drained the gas tank got a lot of gunk out. The tractor ran fine for about a month. Now its back. I checked the gas and yes, water. So this time I actually removed and flushed the tank, replaced the gas filter with a clear body filter to see any water, and I also cleaned out the carb, found a lot of crap in the reservoir.. Still throttling.. :-( I do have the Sears owners manual, it does have an exploded view of the engine. No instructions, just part numbers and descriptions.. I'm afraid that I may need to remove the cover behind the flywheel to access the workings of the govenor shaft. To access this, I'll need to remove the engine from the frame.. Any other ideas before I jump into this? When I grasp the top of the govenor shaft, I can raise it 1 - 2 mm before it hits a hard stop, is this normal? 8 months ago, I replaced the Ignition coil, condenser and the magnetic pickup for the ignition timing, new parts from Onan. Since I don't seem to have a loss in power or difficulty in starting, I don't see the issue being ignition...

Thanks in advance.. Mike.........

[Travis Jordan]

Mike wrote: /snip/

Small engine RPM hunting on governed engines is frequently caused by fuel mixture problems.

Have you checked the air/fuel mixture (floats, carb jet adjustments)?

[Bob M.]

If there's a fuel bowl with a float as part of the carb, I would suggest you remove that and clean inside there as well, as water could still be floating around in that area.

The slack in the governor shaft sounds about normal.

Most surging is caused by an improper float level or a dirty carburetor. Perhaps some of the junk in the tank found its way past the filter (assuming there is one) and got stuck in a small passage within the idle circuit of the carb.

I have a P24 engine on my generator, but unfortunately is runs on vapor fuel (natural gas) and doesn't really have a float-style carburetor. Also, the mechanical governor doesn't attach to the carb - they use an electronic module to control the speed to maintain a constant frequency.

[Mike]

Well, since I posted here, I checked the plug wires, I found one spark plug connector questionable so I replaced it, same... I replaced the condensor for the ignition, still surging... The carb was the problem. I finally removed the whole carb from the intake and gave it a good cleaning, inside and out. A friend at work mentioned he had an Onan engine that had the same problem, he stated there is a vent hole that tends to get clogged up, he recalls it was in the body of the carb, not in the throat. The engine ran fine when I plowed myself out from the snow storm this weekend...

Thanks for all the help..

Mike