Vintage Dirt Bike Q & A

Hi, I picked this bike up and rebuilt with new piston, rings, all seals, all bearings, gaskets, and .020" over bore. I regapped points and cleaned the carb. bike will idle great but once u try to rev it up it sputters. from idle to half throttle seems ok, besides it not having much power. I tried going in and out with the air screw which didn't help and reset the float to stock height 24mm. Now it won't start unless i pour gas down the cylinder, and won't idle. the float height was at 18mm. I'm not using the oil pump but runnung 20:1 premix. could running premix be my problem? should i try a bigger main jet?

thanks

I would check your air jet to see if it's clogged. This is the large jet at the base of the carb mouth, which feeds to the needle jet under the slide. If it's plugged up, the carb will work OK at low throttle openings but will get richer and richer as the throttle is opened. While the carb is off, I would blow out every opening and jet with compressed air to make sure they're all clear.

My Chilton's manual indicates the corect float height is 20mm, and I use 36:1 premix myself (Bel-Ray H1R).

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 have since tried a larger jet but it didn't get any better.

Thank you, I did rip the carb apart and clean all the ports, but I will try it again to make sure. Could timing cause this problem?

Sputtering is usually caused by too rich a mix if all yours jets are known to be good and clear. If your carb and jets are stock then you probably should clean the jets real well or just replace with new of the same size. Check your needle and the needle guide also.

Pilot jets can be hard to clean. Those little holes get clogged and you can't tell really well by visual inspection. Ream them all out with small wire or something and blow out with air and carb cleaner.

What mm and brand (keihin or mikuni) carb does the mt have? You can get a good repacement for cheap with an OKO. They make many different sizes. I just put a 21mm on a 1979 dt100f and it worked out great. Paide $69 + shipping and about $10 more for the right jets. I posted a conversion chart on this site for keihin to mikuni and vise versa swap outs. Oko's are Keihin clones.