Vintage Dirt Bike Q & A

I have a mt 250 in which i had a new oversized piston and cylinder bored out on. I also rebuilt the lower end with new seals and such. My problem is that no matter what pre mix ratio I run, or points adjustment i do i still foul plugs after several hours of running time. I rebuilt the carb but maybe it is worn out? A cr 250 website for 74 to 78 years @ www. elsinores.com has Mikuni carbs for sale but they do not know if they are the same as the mt250 for that year? Does anyone know that info, and if I am barking up the wrong tree on the problem. Thanks for any reply

Has the exhaust ever been cleaned? Too much premix,not eneough air going through?

I do not think the carb is the problem. MTs have a 28 keihin CRs have a 34 or 36 Keihin I would stay with the original. When the plug fouls does it look like it is oil fouled or carboned up? If the rebuilder did not replace the right main seal or put it in wrong it might be sucking in tranny oil. Also use a B7es plug. The slower reving MT needs the hotter plug. I have redone a couple of MT 250 bottom ends and each time the main seals were the reason the bike was running terrible and fouling plugs.

I would try a hotter plug first. with the bigger bore and cylinder you are using more fuel. have you leaned out the carb any?

Back to the main seals...if you notice the tranny fluid going down over time, that could be the problem. Have you tried using 100% Synthetic oil? I prefer the Klotz brand. But DO NOT use that until your engine is broken in completely. Your rings will not seat properly. Try the other suggestions first, then when it's properly broken in, try this oil. This has worked on more than one bike of ours through the years. ...and yes I'm female, and an ex-Powder Puff racer from the 1970's. My Dad taught me how to work on the bikes back then. You race it, you break it, you need to know how to fix it...Dad's motto. Good Luck!