TXT250 revs won't settle.

[gazdaman]

So I got my TXT 250 with an overheating problem, this was narrowed down to a shot head O ring.

So I've replaced the O rings, and got a new head. But now when I start the bike it'll only start from cold with full choke, and the revs will shoot to maximum, I can knock the choke off and keep it running with tiny throttle inputs.

But if I rev the bike at all, the revs shoot right up and take AGES to settle back down. And the bike won't idle.

I've had the carb apart, all jets clean, and the choke plunger moves freely.

I did a compression test, 130psi

I sprayed WD40 along the inlet track to try and highlight any air leaks, but none found.

Anyone got any ideas of what to check?

Gaz

[steve1979]

Is the throttle cable running smoothly, try disconnecting the cable at the throttle end to eliminate its not snagging any where. Other than that are the reeds okay. Apart from the cable or an air leak i cant think of any thing that would give the symptoms you are describing

[rev3]

Sounds like you need to strip and rebuild carb again, something must have been put back slightly wrong.

Assuming the revs dropped ok before stripping the carb.

[jse]

Is the throttle cable running smoothly, try disconnecting the cable at the throttle end to eliminate its not snagging any where. Other than that are the reeds okay. Apart from the cable or an air leak i cant think of any thing that would give the symptoms you are describing

I'm suspect of the throttle cable also. I always think about what parts are moved/changed when repairing something (and another problem comes up) and in this case, it's the fuel tank. I'd closely inspect the cable routing and see if it got kinked somewhere. Make sure that there is a little slack and maybe take the cable off to check for a frayed end. This usually happens at the throttle end and it only takes one wire to come off the strand to catch inside the housing or inside a ferrule or fitting.

Jon

[nzralphy]

I'd say you have an air leak between the carb and cylinder

[jse]

I'd say you have an air leak between the carb and cylinder

That was my first thought too, but the "sudden-onset" symptom pointed

to the most recent "activity" as possible cause, which was the replacement

of the head. Of course, it always is a possibility that several small

causes will exhibit as one big symptom so it could be both an air leak

and cable malfunction or even the rubber tip of the choke plunger coming

loose and holding the enrichening circuit open to a slight amount.

Jon

[gazdaman]

Y

That was my first thought too, but the "sudden-onset" symptom pointed

to the most recent "activity" as possible cause, which was the replacement

of the head. Of course, it always is a possibility that several small

causes will exhibit as one big symptom so it could be both an air leak

and cable malfunction or even the rubber tip of the choke plunger coming

loose and holding the enrichening circuit open to a slight amount.

Jon

It exhibited this before I stripped the carb, and has run like this ever since I've had the bike running. I'm quite an accomplished spannerer so I'm happy the carb is assembled properly.

The carb slide is moving freely and returns back to its stop nicely. I don't think its a cable issue.

I haven't checked the reed valve yet, ill do that! I would also expect its an air leak,between carb and cylinder but spraying wd40 around with the engine running didn't highlight anything.

Do you guys think a poorly sealing reed valve could exhibit this problem?

I really appreciate the input

Gaz

Edited April 2, 2011 by Gazdaman

[legionnair]

I am leaning towards an airleak but instead of WD40 try carbcleaner higher flash point or put a rubber hose on a propane torch end and turn it on slowly this works great looking for vacuum leaks on modern cars.

good luck

[gazdaman]

Yeah, the WD40 I used was squirting more of a liquid than a gas that might get picked up by a small air leak.

I'll pop the inlet rubber off next and give it a good going over, and then I can check the reed valve at the same time.

My Dad seems to think the crank seals could be at fault. I guess it's possible, I've not used it enough to notice it using any gearbox oil.

Gaz