Vintage Dirt Bike Q & A

I've owned a 74 mt 250 for about 6 months. Bike came with 1600 miles and the original tires and even the title? After some TLC it ran great for about 300 miles mostly riding on the street. Now I have a strange problem. When I start it cold I have to hold the throttle wide open to keep it running. It sputters and smokes like it's on choke and will start to rev normally after a minute or 2. Once the engine is warm it rides ok except at low speeds it bogs and breaks up but revs out ok. Engine is in good running condition good compression and I rebuilt the carb and adjusted the points 300 miles ago. Electrical or carb problem? I'm thinking it's the coil but any suggestions are appreciated. Please don't tell me it's the crank seals. I have confirmed the petcock shuts off and I always shut it off when not riding.

Rob

Rob,

Try turning your fuel tap off while riding the bike. If one of the low or mid-throttle carb circuits became too rich for some reason, the bike should run much better right before it runs out of gas.

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

Thanks Ray for the suggestion. The stock MT250 carb does not seem very tunable. Are they known to be problems? If so can you recommend a cost effective replacement.

Thanks

Rob

Rob,

My MR has the same carb, and it works great! The main and pilot jets are easily available, and even the needle jets can be found. This carb is designed to work with the long intake manifold and airbox boot for good throttle response and low-end power. Yes, it's quite small for a 250, but I doubt it limits top-end power much with the super-restrictive stock MT250 pipe.

It seems Honda intentionally made it difficult to put a larger carb on the MT250 motor, so unless you're planning on installing a CR250 top end I'd stick with the 28mm 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

I managed to fit a 32mm Mikuni on my MT250. It fit fairly easy. A 34mm should fit also. The exhaust will need some work to truly benefit though.

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