just bought a 1999 250 honda fourtax, was smoking badly but would run fine, did a top end rebuild ,new piston and rings, head has all been done stem seals ect
it no longer smokes but when you start the engine it idle just fine but as the engine starts to warm up the engine revs pick up to about 1/4 throttle can ant 1 help ?

 

I assume you are talking about a TRX250EX or a Recon? Both have the same motor so what I will say pertains to both. I'd say you need to either adjust the slow jet or clean the carb. The jet is a PIA to get to but can be done. It is on the front bottom of the carb and you need a slotted screwdriver, but just the bit from a removeable one. The needle should be about 1 1/2 - 2 turns out. Wear gloves because you'll burn your hand on the back of the cylinder if you don't. Adjust it after the motor is warm. Screw it out until the idle comes down to about 1000 RPM. Then put the bike in first gear (it shouldn't move forward at 1000 RPM). Then bring the idle up until the clutch just starts to engage, then back it down just a bit. After all this is done, you should be good. But, when you start it cold, it of course won't idle. You will need choke or 1/2 choke to keep it running until it warms up. This is normal on these bikes. As long as your motor idles good warm, don't worry about what it does when it's cold.

 

You have an air leak somewhere. Where? I have no idea but rest assured you have one. It's the only way an engine can RPM like you say it does.

On some carbs the enrichener can hang up in the bore and give it extra fuel and air and this will make the RPM come up when warming up. This is where I would go first.

Let us know what you find out so I can learn.

 

If ran good before teardown IMO then look that the carb.intake manifold [rubber boot]that mount carb to cylinder head..Has an gasket or oring there!! Make sure oring didn't fall out during re-assembled...

 

^^^^ great advice and good call.

 

i have had a play with it and have found that when i take the top vac hose off the carb it worked spot on but cover the end of the hose with your thumb then it starts to rev , hope this bit of info helps