Vintage Dirt Bike Q & A

I have a November 1974 MT250 and the throttle response is less than good. Now that I have rebuilt two MR175s, I think I should expect a little more from my MT250. It putters on the idle to 1/2 throttle and does not snap to or snap back when the throttle is revved. I want to check the jetting to see if anything has been changed by the previous owners. I checked the timing with a timing light and it appears to be pretty close to the mark.

Does anyone have the standard jetting for me to compare?

122 main jet, 60 pilot, 3.0 throttle slide, 271002 needle.

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

To save you a ton of time and tail chasing, change the needle jet. Mine was worn, and it would not idle and was uncontrollably pig rich below 1/2 throttle. I changed the needle jet out for one from my parts carb, and it is far better. New ones are available, but only from a supplier in the UK for 25 Euros plus shipping.
I am working on putting a Mikuni on mine, since I have a torn boot anyways. I am having to make a new mount for it, but that's easy.

I wanted to check to see what was original first, because I know at least one "carb kit" was used on it and I don't know what jets were replaced. It had it bored one over and never changed anything, so I figured I would start with the baseline before making any jettting changes. Any suggestions on what I should expect to change in an overbore?

Any reason for going with the mikuni? I have one of those sitting around too.

I'm going with the Mikuni for three reasons: It's larger, parts are more readily available, and I am more familiar with them. As a plus, they do seem to respond to tuning better than the Keihins.
I found a carb mount to go to a tube, so I'm having a flange cut, then just weld a tube to it and clamp everything together. I still haven't decided if I will put an oil port on it or run pre-mix. There's a lot of hills around here, so I may stick with injection.

I'm wanting to put a Mikuni on mine also i am sick of messing with the Keihin. What size of carb are you using? Any jetting recommendations yet?