Vintage Dirt Bike Q & A

New to the site and just finished a complete re-store on a 75 cr125. Bike starts and idles correctly issue is bog when cracking throttle also releasing the clutch in first engine bogs severely and dies if I don't quickly pull in the clutch to get it back in the revs. Engine was sleeved down to 100cc and run in vintage class before I picked it up after it had been sitting for a few years, replaced seals and Carb has been completely re-built with new jets and needle. When problem surfaced I raised and lowered the needle then changed all the parts back to cleaned up originals. looking for suggestions. Thanks.

Scott

Stock carb & cylinder now?

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

Ray,

Using stock carb with stock pilot jet, cylinder is sleeved down to 100cc. Going to dbl. check timing and ground wire today.

Scott

Scott,

It seems like it might be a carb problem to me, possibly low float level or clogged air jet / main jet. It sounds like the idle circuit (pilot jet / air screw) is working OK, but as soon as the slide is raised enough to use the needle / slide / air jet / main jet circuit it leans out big time and the motor bogs. It could also be an air leak. I'd carefully check the cylinder liner for any fit problems to the main cylinder casting and port passages too - the 100cc conversion was a big job, there were a lot of things that could go wrong.

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

Ray,

Thanks for the info. Checked the compression today and got consistent 90 psi. I'm pretty sure thats low compared to what I've been seeing on some of the forums. But it is sleeved down to 50mm so not sure what it should be. So I pulled the top end down to re- inspect cylinder, old piston and rings looked good on initial tear down so I re- installed them at this point I will go ahead and replace both with a wiseco or vertex piston kit. Engine seals have been replaced along with a new gasket kit for the top end, I did not pull the bottom end apart. Carb float set at 20mm from float bowl and carb kit installed before start up, I also re-installed old cleaned up jets with same results.
I'll see what happens with the piston and see where I'm at at that point. Thanks again.

Scott