Hi guys, I have an Onan P220 that surges at full throttle, as the title says. It idles perfectly and even runs fine wide open with the pto engaged. I am working on it for someone and they just don't like the surge and want it gone. Carbs are expensive for this one and I would want it narrowed down for sure before I order one. The engine has a fresh rebuild on it, new rods and pistons. Spark is good and consistent, I have pulled the carb apart several times and cleaned it out with carb cleaner and torch tip cleaners, pulled the welch plug and got to the orifices behind there as well. Pulled the shields and sprayed carb cleaner up and down the intake manifold and at the intake gaskets with no change in rpm. I hooked up an aux fuel tank to bypass the pump and tank with no change in results. Tried seafoam in the tractor tank and in my aux tank, fuel is fresh as well. Any other ideas, or does it sound like it may be new carb time. I see mixed results about the quality of the carb from they are junk, to very rarely do they need to be replaced...
My experience with two Onan P218s surging has been crud in the high speed jet. Although both were P218s, the later production engine had an accelerator pump installed in the carb. As the load on the engine varies, the pump squirts a small amount of fuel into the carb throat when the throttle plate changes position controlled by the governor to reduce RPM surging as the throttle plate opens more.
In the Onan P series parts and service manuals, this pump is not specifically shown. If your P220 carb has this pump, the pump cover has three screws holding a triangular cover, and when you removed this, the parts will come out due to the spring which is part of the pump. Reassembling this can be difficult and if installed incorrectly, the pump will not work. This will cause engine surge at higher RPM and loads.
The only instructions that I have seen as to the correct assembling of the pump is on the sheet that comes with correct Onan carb kit. I very seldom remove the accelerator pump on these carbs, but I squirt carb cleaner thru the fuel passages as I clean the carb.
When I disassemble a carb, I always check the float for condition and buoyancy before resetting the level and reinstalling it if it is still serviceable.
These carbs also have an adjustable low speed jet, but it is covered by a Welch plug that is pressed in over it. I usually remove this plug so I can make low speed mixture adjustments.
Another consideration is the condition of the fuel pump, dirt or malfunction of the crankcase breather valve as the fuel pump is pulsed by vacuum and pressure pulses in the crankcase while the engine is running.
The condition of the fuel supply hoses, fuel filter and vented fuel tank cap are other considerations.
Also check the engine RPM at full throttle, should be 3,600 RPM or slightly more.
Just my thoughts, been a while since I did any Onan servicing.
Dunked the carb in napa carb cleaner for a couple days and it started running better for about a minute, then started surging again. Although not as bad as before. Time for a new carb?
Don't know if anyone is following this yet, but I put a brand new carb on, and it is till surging. Not terribly but not as smooth as I would like it. ANy other ideas?
When the engine rebuilt was the governor spider replaced with a metal one? Are the springs in the right holes on all the linkages? Are the holes in the arms showing wear from age/usage? You may have to move the springs or replace worn arms
