I bought an '83 VT500C this summer. I Rode it through November and one morning as I entered a highway I found myseld stuck at about 40mph, and I couldn't get the RPM's up. The problem cleared itself and the bike ran great for the 20 mile trip I took. A few blocks from home on the return trip the same thing happened. Days later I started it up and found that it idled fine but I couldn't get the revs up reliably. Today after taking the carbs apart, cleaning them, replacing the jets I could buy (Slow and Main. I had to re-use the needles) I put it back together and it started right up and idled great. Revving it was smooth and easy for a few minutes, and then it started smoking badly out the exhaust (normal for this bike) and then revving became spotty again. I was told by the guy I bought it from it was running too rich. I don't see anything in the Clymer manual about adjusting the mixture. If anyone has any ideas I would appreciate your input.