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Discussion Starter #1 (Edited)
Hi folks,

I have a 2000 Mule 550 with the FE290D engine with indicated 300 hours--actual is less, but the key switch was left ON which ran the battery down while running up the hobbs. After sitting several years in storage it wouldn't run at all without choke. A couple years ago I put a brand new fuel pump, filter, and carburetor on it and checked/cleaned the spark plug and gapped to 0.027" (it was 0.030"). It runs fine under load. Idling is fine for a minute or two, then it starts to lope/waver and eventually quits. When it starts to falter I can goose the throttle and bring it back to life most of the time. I can also immediately restart when it does this. I've just been living with it, but I'm tired of restarting it all the time.

I blew compressed air through the fuel pickup tube and looked into the tank. There's residue on the bottom of the tank (old gas varnish?), but I don't see rust or flaky matter. The pickup appears to be sitting in clear fuel, and I've run plenty through it this season.

I tried setting the pilot air screw 2.3 turns from the stop, but it doesn't make much difference. The screw wobbles a bit in its seat, but I'm not sure that indicates a problem. I checked and cleaned the pilot jet in the top of the carburetor and found the passages open and the O-ring intact. I checked the bolts holding the carb to the engine, and they are snug. I also pulled the cover off the governor linkage mechanism on top of the engine and brushed and cleaned everything with PB Blaster, although everything was in good shape, and I wouldn't expect this to matter when the throttle sits against the idle screw.

Today I ran the Mule in the driveway and noticed the fuel filter has only about 1/4" of fuel in the bottom. I had previously removed the hose to blow air down the pickup tube. I've clipped the old ends off the hoses to ensure a tight fit, and there are no fuel leaks. The air line from the engine puffs air as it is revved and less so at idle, as I would expect. Is the fuel filter supposed to be purged of air somehow?

What am I missing here??

A.
 

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Discussion Starter #2
An update: I checked the valves and found the intake tight. I adjusted intake and exhaust to spec (0.005").

The Mule starts and idles perfectly when cold. Once hot, the idle sort of bogs down and wavers until it quits. It can immediately be restarted. Under load it runs strong.

I pulled the spark plug and found it carbon fouled. New BPR4ES plug on order.
 
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