check the diaphragm, may be split or pin holed, if no holes are visible then the diaphragm could have swelled if it has been out before or if you had it to bits before, check the needle jst height normally thee is notches at the thick end of the jet, check to make sure it set to the correct height.
check the the float needle jet, the height of the float, and check the floats for any pin holes.
its very hard to check to see if the needles etc are worn, i done a jet kit on my gs 250 granted 4 stroke but principles are the same, and the kits were not very expensive.
check the spring in the throttle body part of the carb where the throttle cable screws into the top of the carb, check that the throttle cable is adjusted correctly,
check and clean all jets and air ports in the carb, they can normally be removed with a flat blade screwdriver, be careful though they can round off.
if everything ok, check correct grade spark plug fitted / or worn or if any arcing or tracking present. check the timing is correct, check to see if the points and condensor for correct adjustment and pitted or worn if not transistorized set up.
check the inlet manifold to carburettor rubber boot these normally perish, dependant on how bad it is you can see it visually small cracks in the rubber when you remove it and bend it you should see splits in it, this causes the same problems, as does the boot from carburettor to air intake
when checking the throttle cable ie when unscrewing it to inspect it from the top of the carb, take not of which way round it gos as there are grove either side of the round cylindricar attatchment on the end of the throttle cable, i have seen it before people put them in the wrong way bike will fire up and rev to the max, realise what they done turned it back around the correct way and have similar issues to you, as the brass guides inside the carb are damaged and thus make the cylindrical metal body stick every so slightly(i will remember the name of the cyclindrical thing probs after i wrote this)
these are just a few of the things to look for and some of the things ive came across when messing around with carbs.