Vintage Dirt Bike Q & A

I have a 1974 MT125 that I ride alot on and off road. But anyone that owns or has rode a MT125 knows they do not have enough low end power. The bike goes good up small hills as long as you keep the rpms above 6,000 but if you let it drop below that, your done. I have dropped the front sprocket 2 teeth, and it helped, but now its not fast enough on the rode. So what I would like to do is some light modifaction to the engine to improve the low end torque. I have thought about shaving the cylinder head some (the bike only has 7 to 1 compression ratio) but I dont know how much to remove. Or use a CR cylinder head. Also, I think the copper head gasket on the CR's are thinner then the head gasket on the MT's. If anyones got any ideas, please let me know. Thanks

The absolute easiest things you can do are:

1) Use the .020" copper CR125M head gasket (the MT125's is .050" thick, but seals better). The CR and MT combustion chambers are very similar in volume and design, so I would stick with the MT head.

2) Advance the ignition timing by widening the point gap.

If you need lights, you're pretty much stuck with the points ignition - which is too bad, since the CR CDI ignition kicks in an extra 12 degrees advance at lower rpm and really wakes up the bottom end, and revs much quicker.

You can also remove the pipe baffles and run a silencer on the end, but this isn't exactly an easy mod. My simulation program says it will work quite well, though (the stock pipe is tuned for high rpm, and is super restrictive).

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 alot Ray, ill try the copper head gasket and see what that does. As for the ignition, ill have to stick with my stock system because I do ride the bike on the rode also. If you or anyone else have any more ideas, let me know

You might want to coat both sides of that new head gasket with "Gaskacinch" to improve the seal.
That is what was recommended with my new gasket.

Good luck
~JM~

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Well I finally got around to geting the cr125m head gasket on my MT125, and it helped the bottom end power alittle.
Should I change the jeting in the carburetor? The bike seems to run fine
and what should I do with the ingnition timing. advance it?