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

6344 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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Sounds like fuel starvation (petcock screens dirty) there’s one on the tank inside and one on the bottom if the petcock.
Also, perhaps didn’t clean carb well enough—did you use pipe cleaners swabs and dental brushes in all those ports? Some I know will require a teeny carb wire set.
How did you rebuild the carb—complete carb kit with seals? Proper Carb synchronization and pilot screw adjustment extremely important
Did you replace the choke valve? If worn enrichment circuit will stay open, fouling plug. Is your plug sooty?
Hey john you didn’t say what year quad so I’m gonna post a pic of my Keihin VE90E. Make sure your carb is OEM and that the carb model matches the year of the quad.

you will see all of my seals are worn, cracked and stiff. Your carb likely needs a rebuild kit, a primer plunger and a choke valve. $100 in parts.

In the pic you’ll see the pilot screw; stack the spring first, then washer, oring last. In this pic the washer is hard to see.

inspect the pilot jet hole to ensure the port is not clogged by the tip of an old pilot screw. Insert the screw until it lightly seats—at this point you should be able to see the protruding needle tip and catch your fingernail on it. Back off the screw two full turns; at this point you can still see the tip of the screw but you won’t catch your fingernail on it.

next the lock ring on main jet needle needs to be on the middle setting.

Synchronization is simple. Set the idle screw so it just touches the throttle link. Set the throttle cable adjustment (handlebar) at midway. Install the carb andfasten the cable down, taking most of the slack. Use the handlebar adjustment to take out remaining the slack. When you slightly crack the throttle the vacuum piston should respond. Now you can run in the idle screw.


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