Vintage Dirt Bike Q & A

I just bought a 1975 mt250. It has been sitting for 5 year or so.
The bike fires up but will not idle. The idle screw has no affect on the idle.
And the air fuel screw seems to have no effect as well. I cleaned out the carb and all looks ok. One other thing the bike will not start when it is hot.
any help would be great.

If you unscrew the mixture screw all the way, does it have a hole that runs down the middle of it and exits the side?

YES. there are two holes on the side one in the bottom. 'Like a T '.

OK, that is the correct mixture screw for that carb.

Not starting when hot => most likely running lean.

Probable causes are:

Wrong or plugged slow jet. The path can be plugged too. Sitting for 5 years could mean some rust in the tank that is making it through. Stock jet should be a 58.

Float level is set wrong.

There is a leak down stream of the carb. Cracked rubber intake manifold, bad gasket, that kind of stuff. Throttle response will sometimes give a hint for this. When you roll the throttle off the engine gontinues to be reving high for 20-30 seconds before it winds down and dies. What happens is the bike runs off residual gas and the air leak until it dies, so little or no control from the mixture screw.

Thanks for the reply.

Ill check the jet and path to night.
Also do you know what the float should be set at?
Mine seems to hang to low.

Yes, floats are set a 20mm. There should be a picture on the web somewhere that will show the place to take the measurement.

If you are happy with ballpark: What I do is turn the carb upside down, this means the needle valve will be seated by gravity, and adjust the float tab so the floats are perfectly level when the float tab first touches the spring loaded pin in the needle valve.

Good luck.

Hi everyone.
Hmmm... it seems like I have the wrong air mixture screw. When I first bought this bike (74 mt250) the air mixture screw fell out. I tried to find a replacement but the only one I could find that would fit was from a cr80. Would not having the proper screw in there make much difference?
Thanks
Newfie

_________________
Newfie
1974 MT250
1974 MR50
1975 CB750F
http://spaces.msn.com/members/andrewmer ... photoalbum

Yes. Parts must be exact replacements in carbs.

Problem you run into without one of the stock 'hole in it' air screws is you have to have the idle screw so far out it may fall out.

I'm a firm believer in 'if it ain't broke don't fix it' motto. If it idles, doesn't foul the plug, runs strong => be happy and leave it alone. Idle mix screw is really only for the idle. Slow jet/throttle body takes over as soon as you crack the throttle.

Also check for throttle free play, I had zero on my MR and when it got hot it would idle too high, easy fix