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

6365 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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First off, the Compression seems low.. Did you have the throttle wide open when you performed the compression test?

Where did you source you replacement parts from, eBay or Amazon by any chance?

Could be an ignition issue, if you bought a Chinese cool the spark could well be breaking down when hot and under load.

Inlet (air) leaks, have you tested for this yet?
How about the diaphragm in the top of the carb, is it perished, does it have a pin hole visible?

Timing... Have you checked it is within spec?
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.
If it was a pilot valve (choke valve) issue to would have fuel spilling from the overflow pipe. The fuel jets control fuel inlet.

The tiniest bit of dirt or dust can play absolute havoc with Honda carbs. They need to be surgically clean, literally.. Not forgetting to clean the fuel tank, petcock and install an inline filter while your at it.
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.
In my experience it either leaks or it does not.. Can't say I've ever tried your theory mentioned above however, float level is irrelevant as it's none adjustable.
Replacement of the valve needle and float is the only option
300's don't have a fuel pump....
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