Vintage Dirt Bike Q & A

Hey guys-

I have a problem with my 76 mt250. I just rebuilt the keihn carb with a kit. I believe it came with a 50 slow and 110 main jet...It has a lack of power, and revs out too quick. I believe it to be a fuel problem. Idle is great though...
Other 250cc 2 stokes in the past have used 150+ main jets.
What jet do you recommend?

Thanks!

The info I have seems to indicate a 118 main jet and 55 pilot as stock for these bikes, but I have question marks next to both in my notes so I don't know for sure. I rebuilt the carb on my buddy's '74 MT250 a few months back, but I don't remember which jets were in it.

I'd recommend a 118 or 120 main jet to start, with the needle in the #4 position (the 271002 needle & needle jet in these bikes runs pretty lean). If the bike has a good idle and doesn't surge at low throttle openings, I'd keep the 50 pilot and just fine-tune the air screw as needed.

Also, be sure the oil pump is calibrated properly (you pull the cover behind the countershaft sprocket and make sure the marks on the pump body and lever line up with the throttle off, adjusting with the cable adjuster if needed). If it's dumping lots of oil into the motor as my buddy's was, it will be hard to get the jetting right. And even though it's a pain to do, I'd highly recommend checking the points and timing just to be sure.

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