Vintage Dirt Bike Q & A

hi everyone ! i have a 74 mt 250 project for some fun times ahead ! i bought it last fall and it ran just fine, and then got her going again this spring, same thing; easy start as well. however, now i kick it over and it races like the throttle is being held wide open. i hadn't adjusted anything and it basically went from 0 to 60 on me.
now, i had put a touch of seafoam in it this spring just to clear any minor gunk in the system. i am thinking that maybe the previous owner(s) had it tuned up according to how it ran with some sludge in its veins , but the seafoam gave it a roto rooting and loosened things up and so everything is tuned too high for the new cleaner carb.
what do you guys think? i am hesitant to start it up again as it revs so fast. that said , if i choke it it will run at a reasonable rpm, but when i take the choke off, look out !
thanks for your help everyone.

Knock down the idle speed with the speed screw, adjust the air screw for max idle speed and then turn it back in 1/4 turn, and see how it runs. You may have gotten lucky with the Sea Foam and it's running well. From what I've seen in person and online, it's far more likely these bikes will run really rich at idle than lean like yours.

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

Make sure your cable or slide is not stuck

You likely have a small air leak somewhere in the intake manifold or it's gaskets between the carb and cylinder that is allowing the engine to suck in air and run lean.
Something like this happened to me on my MT when I forgot to block off the injection line connection on the intake manifold when I switched over from oil injection system to premix.