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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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Hi Bruce, hope you get yours figured out, my tank is surprisingly spotless, no fuel filter(will get one soon), and new line, flows perfect and bowl stays full. I have no fuel pump.
Thanks for the response Sam, I did not, never used the compression tester before, didn't know that you need to open the throttle to get an accurate reading. All replacement parts OEM, ordered through honda dealer, tried swapping multiple times old and new, same deal. The boots are rock hard, but they still seal perfectly, checked with ether while running. Diaphragm is good. Timing:I have no idea how to check that..
Thanks for the reply Goob, that's what I thought, but I'll have to set up a bottle of clean fuel with a filter and try it that way, will do that later today. I think I did, did it the way i always do and it seems to work, i use torch tip cleaners, which are basically the same as Honda's "jet cleaners", didn't change seals, but none leak, and the only o ring was on the pilot screw, seemed pliable and fit nice, let the carb soak in napa's carb dip for a good while, then sprayed it, and cleaned it pretty well. And counted the turns of the pilot screw and put it back to where it was, and tried turning the pilot screw a few ways to see if I could get it to run differently, no change. I did nothing to the choke, all I checked was when I engage the choke, that the black plastic with a needle retracts and lines up straight, which it does, what else is it supposed to do?is there a way to check if the choke is not working properly? The plug was wet, smelled like unspent fuel, which I found odd.
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