You should be able to remove a small grey plastic cover on the right side of the cylinder that covers the power valve slider (I don't know what to call it) You can run the engine with this cover off and when reving it up you should see movement in and out. If not something is stuck, broken or misadjusted.This shaft operates two side doors and the main flapper which you can see when looking in the exh port with the pipe off. The ramp and cup (governor asmy) is under the main clutch housing (side cover) should you need to remove it. Make sure none of the parts are gummed up and sticky or broken. Your best bet is to find a parts diagram:
http://www.google.com/search?tbm=is...669.0.9915.22.22.0.15.15.0.63.415.7.7.0.llsin.
Study it and learn how everything functions.
The worn carb slide will create a lean condition....worn slides are very common. You can search for a nice used one or just buy a new one. Make sure the cutout is the same number (located on the angled part on the bottom) or if you know what you are doing you can change this along with other facets of the jetting to create different running characteristics. This is advanced tuning. Above all, make sure the jetting is correct after everything functions properly and all the parts are in good order. The engine should have a crisp "bin bin ba bin bin bin" idle with the air screw out about 1.5 to 2 turns. More and the pilot is too rich, less and it's too lean. At part throttle under light load (needle) it should have a tight sputtering sound and under a load (main) it should pull hard in a smooth acceleration with no hesitation, no blubber and no ooz out of the tail pipe...as long as you ride it hard enough to keep it hot and cleaned out. 32:1 works well for a track ridden 125.
As you roll on the throttle it should be a wave of power along with the hit you're searching for with no bogs or sputts. Use NGK plugs..the standard or V-Power are fine. Don't use anything else. Make sure the reeds are closed and there are no frays or chips. Make sure there are NO air leaks. A little brake cleen works well for this or if you suspect leaks have it pressure tested. Make sure the cylinder bore and piston/ring are in good shape and not worn out.
The carb has to be tuned properly to get the most out of a 125; and this takes some skill and lots of seat time to get just right. Hope this helps.
Let us know how it goes!