Vintage Dirt Bike Q & A

I'm having problems with low to no bottom end power in only 1st gear coming out of neutral, usually. More problematic when bike is warmed up it seems. Bike runs well in higher gears. I'm a newbie to motorcycle mechanics so any suggestions would be appreciated. Thanks in advance.

Also, any ideas where I might be able to find the orginal silencer and support bar for this bike?

1976 CR125M in good condition with original carb.

Several things come to mind.

Slow jet may be wrong or plugged.
Engine seals may be compromised. This could be anything from a cracked intake boot to crank seals or base gasket.

If it smokes more then it should then it is [clutch crank seal, cracked case, bad case gasket]. You will be fouling plugs with oil under these circumstances. If it isn't an oily smoke, more sooty with a fur like black soot on the plug, probably slow jet or floats aren't right.

If it isn't the above then it could be [base gasket, left side crank seal]. Engine may run much hotter then you are used to from the lean conditions.

All this can happen with good compression and what feels like a tight engine.

Low compression may means top end needs to be refreshed. Have someone kick it over with the throttle wide open and hold your hand over the carburetor. You should get a pretty strong vacumn like it is trying to give you a hickie.