Vintage Dirt Bike Q & A

Wonder if anybody can help me with this; I have an MT250, it starts easy warm and cold engine but when cold it revs up to 3000 rpm for about 20 seconds (choke off) before it settles very nicely at around 1000 rpm. Otherwise engine runs great, strong pull all the way from low revs and until red territory. But I can't figure out why it revs at 3000 rpm when cold before settling very nicely at around 1000 rpm. I would appreciate any suggestion.
Thanks, Rolf

Rolf,

It could be your pilot circuit is too rich, as is the case with the MT250 I've worked on.

When your motor is fully warmed up, try turning off the fuel petcock and letting it idle. If the idle rpm climbs higher and higher, your pilot circuit's too rich. You'll have to adjust the idle air and speed screws, go down one or two sizes of pilot jet, or maybe all of the above.

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

Thanks Ray, I'll try that out. I have tried a couple of different idle air screws and adjusted back and forth, but I have not tried smaller pilot jets. Sounds sensible, thanks a lot.
Rolf

hey rolf, how did that work for you: i think have a similar condition with my 250 (see my latest post)

Saw your posting about crazy rpm. Seems it goes wilder than mine. My "problem" is not so urgent, so haven't done any ajustments yet. When warm my engine runs so good I hesitate to touch it, but some day I will. The theori about too rich pilot circuit seems sensible to me. I have tried all variations of air and idle screws, but I have yet to change to a smaller pilot jet.