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300 fourtrax runs good higher rpm, but dies when warms up

6361 Views 14 Replies 6 Participants Last post by  howols
I'll get the year in a bit. 2wd 300 fourtrax. I tinkered with it all day yesterday and got nowhere. Things I've done:
Rebuilt carb.
New oil, oil filter, air filter.
New cdi.
New plug, wire, coil.
Compression is 95-100.
It will start right up cold, after 10 seconds at most will die without choke. Soon after have to give it gas to keep it running. Once fully warmed up, will run worse and worse until it dies. I thought it was possibly electrical or fuel delivery related, but I'm just mechanically inclined enough to get myself in trouble. Never messed with valves, and only rebuilt top end on a 2 strokes. But I have tore 4 strokes apart before, I am thinking rings, or possibly valves need adjusted. If someone could point me in the right direction thatd be great, not sure what else it could be. Bought the machine at an estate sale for cheap and it's in almost showroom quality cosmetically, and from what I could gather it had been sitting for a long, long time. -John
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You should verify that the float valve is sealing properly. I mention this since you've noticed the plug was wet which indicates over-fueling. An easy test you can try is to connect a clear fuel line to the carb which can still be attached to the engine or on the bench in the normal, upright position. Fill the float bowl with gas through the clear line which can be viewed (almost to the top). Now all you need to do is watch the level of the gas - does it hold it's level or drop at all? Some carbs will flood into the engine, others will leak out the vent eventually. It can happen very slowly so be patient.
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