Vintage Dirt Bike Q & A

Ray,

I believe you suggested replacing the stock Kehin with a Mikuni on an MR175. I just pulled the carb apart on my 1975 MR175 and it was a mess. I have a Minuni 20e 00. It looks to be the same size and I believe it came with the 4 1976's we purchased a while back. Do you know if this model will work, and if so, what jetting changes need made. The motor is completely stock and un-bored.

It was seized because a bearing was stuck.

Matt

Matt,

I'm really not that familiar with Mikunis, although woosh and others have had good luck with them. I don't recognize that number you mentioned, the ones I know are the VM round-slide and TM/TMX flat-slide units. A VM28 looks like it's 5mm shorter than the stock Keihin but otherwise should fit; the VM30 will definitely take some work if you want good full-throttle operation. Mounting a bigger carb to the MR175 can be a real challenge if you want to do it as Honda would (my preference). The intake manifold is a 29mm bottleneck, the cylinder intake flange is a problem when trying to mount other manifolds (CR125M / TM250 / Mikuni / etc.), the air boot will be too small, throttle cable issues, etc. The stock carb can be bored 1mm and the manifold enlarged; Honda did this on the '74 CR125M's optional carb, and it made a real difference on my (modified) MR motor at nearly all throttle positions, not just WOT.

I'd suggest keeping the small-mouth design of the stock carb if possible, which really makes a difference in low end and throttle response, especially when combined with a longer air boot. The computer said so, I didn't believe it, I was wrong.

Can you point me to a web photo of your carb?

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