Vintage Dirt Bike Q & A

Okay 1975 mt250. Sat for 15 years I cleaned the carb and it ran. A little iffy but ran pretty good. Had trouble tuning it because of Rusty tank and no air filter. Cleaned the tank and added a fuel filter and it ran better. Ordered a carb kit and before I got it installed it started acting funny. Air screw adjustment seems to do nothing. It would start and run but just idle even at full throttle. I cleaned the carb (several times) with the same results. I added an air filter with prior to this a 4 inch uni and the same thing happened. It will start and rev for just a couple seconds then just idle. If I shut the gas off and let it almost run out it will run perfect for about 30 seconds until it runs out of gas. I start it again and does the same thing will just run idle until I shut the gas off and as the bowl begins to empty it takes off. At one point when i first started shutting the gas off it hit 9500 rpm at the begining of this problem. If I turn the gas off and on it will keep running but I have no way to keep the flow perfect. Well upon cleaning the carb for the 800th time today and checking and triple checking I broke one of the jets off in the carb putting it back in. I drilled it out but Looks like I may have gotten the threads as well. Anyway I'm soo lost and frustrated. Its the jet just in front of the needle valve. It has a long skinny tip with four holes in the tip and one at the bottom. Not the main just but the other. Does anyone have a spare carb or jet and or some ideas?

That rusty tank is going to cause you trouble until you seal it, replace it, or try some other trusted cure. That super fine rust is great at sifting through filters then binding together while it sits in the carb's float bowl.
Pull off that air cleaner and take a look down the throat of the carb. Is the slide moving all the way up? Is the needle coming up with it? Normally dirty carbs have lots of trouble on the low throttle openings and rev just fine. Your sluething with running it out of fuel would lead back to the fuel system, though not necessarily to a plugged jets. Still, if you do take another apart, make sure to take it completely apart, including the enricher circuit / choke. The main jet and the needle jet that it screws into need to come out, as does anything else that has any sort of passage.
Sorry I can't help you with the broken pilot jet. Buy a new jet, buy a matching tap, and go to town for one last great try would be my approach. I hope somebody else has a better idea for you. Else, look up http://www.bikeboneyard.com and Brendan will probably have a carb for you. I do have one for the CR250M, but I don't know if that might be just different enough to require more fiddling and I sure don't expect you have the patience for that right now. Good luck! Jon

I got a carb if you want a one its from a 75 mt250 that was running and revving alright. The tranny was messed up so I couldnt drive it but the carb seemed fine. I went with a mikuni and dont need the kehin anymore.

Thanks for all the help fellas. It gets so frustrating because it seems like i'm so close and this bike is sooo much fun but every day its something new. Yes I'm interested in the carb for sure. My email is mailto:Forrest@DJIdaho.com I'll try and send you one as well. I'm gonna clean and seal the tank this week and yes despite my fuel filter I've been finding tiny bits in the float bowl.

While your at it, don't forget to check the points. My MR was running great one day, then like crap the next. Long story /short: They way it was running, I thought the carb was the problem, bad idle, fouling plugs, smoking. I cleaned it out, rebuild kit, etc etc, but the points were cooked. Replaced them, and the bike runs like a champ!

I'll check those as well. Thanks everyone for all the help and we'll see how this goes.