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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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The best way to diagnose needle and seat sealing problems is to attach a clear tube to the overflow fitting on the bottom of the carb and bend it up perpendicular to the float bowl. Open the drain screw, and you can see the float level from outside the bowl, especially while the engine is running.
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Yes, but I like to see it for myself before shelling out bucks for an OEM float or needle valve. On bikes with a fuel pump, over pressure can seep by an otherwise good needle and seat.
I know, just generalizing. :)
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