Vintage Dirt Bike Q & A

I have a freshly rebuilt 76 cr250m.

It starts on about the third kick with the choke on, and runs out real nice once you get the rpm up a little. It seems really fat to me on the bottom. It loads up pretty easily at lower rpm and will not idle. I'm guessing the main jet is to rich.

I don't know what size the factory main and pilot jets were, so I don't know how far from stock I am. This is definitely not an area of expertise for me, appreciate an advise here.

My setup:

Main Jet MX140
Pilot Jet 55
NGK BR9ES plug
Red Line synthetic at 32/1
Premium 92 octane fuel
Near sea level

Will changing my main and or pilot jet help cure this? What would you recommend.

Thanks

I have been told that the bike is acting like the pilot jet is clogged, but it is not.

From what I can tell, your bike came with 148 main and 55 pilot jets, and a 2.5 throttle valve. A 140 main seems pretty lean to me. I'd put the needle clip is in the center (#3) position to start, if it's not there already.

Have you set the float level? This can make quite a difference at low to mid throttle settings. My Clymer manual indicates the correct height is 20mm. and the adjustment procedure is in the link below, page 42 (actual page 46):

http://honda-elsinore.alp-sys.com/manuals/MR25076/ShopManual76mr250.pdf

Does adjusting your air screw have any effect? It should help with your idle and low-throttle operation, possibly a lot (2.5 turns out max). A 52 pilot might help even more.

I've only jetted this carb for a 125, so someone with 250 experience should have better info.

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

Thank you for the advise. I adjusted the float level and switched to a 50 pilot and it is running much better now.

Ken