Vintage Dirt Bike Q & A

'74 MT 250 Carb Setting
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Author:  ruouthere [ Tue Aug 08, 2017 12:45 pm ]
Post subject:  '74 MT 250 Carb Setting

I have a question regarding my newly rebuilt '74 MT 250 and it's carb setting. Took it out for my first ride and found when reaching 4500 rpm or so the engine stops accelerating and starts running rough. It's been 40 or more years since I worked on a two stroke so not clear if I'm running into a rich or lean situation? Any thoughts?


Author:  Killer Canary [ Thu Aug 10, 2017 7:02 pm ]
Post subject:  Re: '74 MT 250 Carb Setting

Hello Kevin,
I have a '74 MT250 on it's first overbore;it runs well with the needle clip in the bottom groove. I found this by trial and error because Canadian model carb specs seem to be quite different from U.S.
Be sure that the 6 holes in the endplate of the muffler upstream of the removable spark arrestor are clear and unplugged or performance will be anemic whatever the carb settings.

Author:  ruouthere [ Thu Aug 10, 2017 8:15 pm ]
Post subject:  Re: '74 MT 250 Carb Setting

So a bit of an update.

I purchased a new spark plug, the dealer didn't have the exact NGK plug but had the resistor type plug. Come to find out they don't work well in an older vehicles such as the MT as they need more voltage to function. Switched to the non-resistor (the correct plug) and the bike runs much better at higher rpm but still has the same but less of a problem between 4 & 5K rpm. The plug looks good maybe a little on the dark side.

Not clear if I have a timing problem or point gap or ? Going to bring it to someone and see what he can find in a few weeks. In the mean time I'll look at where the clip is placed and maybe experiment with that first.

Author:  ruouthere [ Mon Aug 14, 2017 10:35 pm ]
Post subject:  Re: '74 MT 250 Carb Setting

Update #2

Moved the carb slide needle jet clip to the bottom cutout, that actually made it run worse. So moved the clip to the top setting, now I have ok low end power and smooth performance between 4000 and 5500 rpm then gets rough.

But something still needs tweaking as the power band is very uneven and lacks the punch it should have.

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