Vintage Dirt Bike Q & A

I pulled apart a very gummed up MT250 carb, and after pre-soaking, & re-soaking in carb cleaner and dissasembly I now seem to have 1 extra part. The extra part looks like a mini brass pipe about 1/8" dia. x 1/8" long. The only thing I can figure is that this part seems to fit over the idle speed screw, and may have come out with the screw during dissasembly. If this is correct, then I'm guessing this brass piece was originally a press fit into the carb body??

This part does go back into the carb body, but it certainly isn't a tight fit anymore. So assuming I'm correct on this, does this piece provide the seal between the body and the idle screw? Can I put the carb back together and expect it to be reliable or will it develop an air leak thru the idle screw? Or any other way to repair or just look for another carb body?

On other stuff, does the air screw supposed to have a o-ring on it to provide a tight seal? (this one didn't have one, but it almost looks like there is a space on the screw for it)

Also, any recomendations on what position the needle should be set at? On this carb the clip was in the very top position, which seems rather odd. If it helps, the needle ID is 271004, the pilot jet is a #58, main jet #122, air jet in the throat #200, and the air jet in the bowl is #90.

The original owner says he never touched the bike. It was parked at approx 2000 miles and has been sitting ever since.

Thanks in advance for any help!

The brass tube could be a spring spacer, to keep spring tension/friction high when the air screw is turned way out; if so, it would be a sliding fit, not a press fit. It may be related to the needle clip being in the #1 (leanest) position; I'll get to that in a second. It doesn't show up on the parts diagram, and may have been added by the owner.

The pilot air circuit in these carbs is pretty low-precision, and some air leakage is normal. Neither of my MT250 carbs (there are two different types, with different pilot circuits and pilot jets) have O-rings on the air screw.

The #90 jet in the float bowl is actually the fuel jet for the starter (choke) circuit.

Possibly this carb has the same problem most of these old Keihins have - a worn needle jet (long jet the main jet is screwed into) making the 1/4 - 1/2 throttle mixture really rich. Usually the first thing people do is to drop the needle down (which doesn't help the problem much, but makes the 1/2 - 3/4 mixture super lean), then back the air screw way out (when sometimes it falls out due to lack of spring tension), then maybe park the bike when it just won't run right.

When your bike gets running, try adjusting the idle speed to about 1,200 rpm and then turn off the fuel tap and let it run out of gas. Stay with the bike, with your hand on the kill switch! On a good-running carb the idle speed might pop up to maybe 1,500 rpm just before the motor dies, but the MT250 I recently worked on (with worn needle jet) zinged up to 8,000 rpm before I got it shut off.

Dropping the pilot jet two sizes may help a lot - you may even have to raise the needle one notch to compensate - but it depends on which carb you have. If the pilot jet is in line with the main jet along the centerline of the carb, it's still available; if you have the shorter pilot jet (off at an angle from the main jet), you'll have to modify the carb bowl to allow use of the longer pilot jet, as the shorter ones are impossible to find.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I dont know if you found the solution yet but the little brass tube fit into your float valve with a rubber oring to seal the valve when it is shut.

I didnt bother reading the othert reply or all of your post..lol

Matt