Vintage Dirt Bike Q & A

I need some help understanding jetting and how to adjust it.

I have three bikes I am currently playing with. Each has a similar problem, they seem to be running rich from 1/8 to 1/2 throttle. It doesn't bog, but gives something of a machine gun with low power. If you shift to 2nd or 3rd gear and give 1/2 to full each runs like a top. One of the MR175's (cherry) practically stands up (it has a martek cdi).

MT250 3.5 slide, 102 main, 48 slow, points - I changed the slow from a 48 to a 50. This had little or no effect, except it feels a little smoother through the whole power band. It is still a little machine gunny up to 1/4 throttle, and a little boggy taking off from idle.

MR175 (White) 3.0 slide, Unmarked main (has a silver sleeve in the jet), 50 slow. I changed the slow from a 50 to the 48. It now hardly wants to run. Very machine gunny on low and I have to run it at near full throttle. Seems to run even richer than before. This bike as a 76 CR125 ignition.

MR175 (cherry) 3.0 slide, 122 main, 60 slow - I switched it to a 48 slow. It now hardly wants to run below 1/2 throttle, and it now does not have any power above 1/2 throttle.

I thought using a lower numbered jet would lean out that portion, but it seems to be doing the opposite. I am also confused by the effect on the top end I am receiving by changing the slow jet.

Any suggestions?

I'd choose one bike and get that running, then apply that knowledge to the next one, etc. Your symptoms sound very much to me like the #1 problem these carbs face - a worn needle jet - but without video I couldn't say.

I'd suggest the cherry MR, as it practically stands up and runs like a top from 1/2 to full - except now it does not have any power above 1/2. Obviously something else happened to it other than the pilot jet change (a 48 pilot is correct IMO) - with that problem corrected, the 3.5 slide, the needle clip at the #1 or #2 position, and the air screw 2 turns out, this bike should run a lot better below 1/2.

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