Motorcycle Repair: 1976 mt125, drive sprockets, motorcycle questions

Question
Hello,
i'm trying to make my bike quicker off the line and be able to reach higher
speeds for my short highway stints. I know it's a 125 but the speedo does go
to 80. Besides redoing the top end, i was wondering if, despite the spigot not
being able to be larger on the cylinder side, could i use a larger carb. Also
could I change either drive sprockets and if so do you know what might
interchange. Thanks for your time and consideration in helping others with
their motorcycle questions
nate

Answer
The MT was detuned for more mid-range power and less topend power. The cylinder ports were changes, the carb size reduced and the exhaust was changed. The carb change may not give you much more power without changing the port timing.  I think that the exhaust port is a single oval shaped port so there's not much you can do to modify it.  Raising the exhaust port would typically increase the top end power.  We used to raise the port about 1.5 mm and also put eye brows at the top to widen it a few millimeters but it needs to have a bridged port for those modifications.  Without the bridge in the center of the port it's possible to catch the ring in the port and you'll break a ring.  

You can always change the gearing to make it quicker off the line but you'll sacrifice top end.  If you find a CR125 cylinder and head, you'd have the some hardware that could be modified.  You'll want to be careful because that bike would typically pull the front wheel up almost every time the engine came on the pipe.

Regards
Rich