PatM,
first of all are all the bolts and screws on the carb. and intake manifold tight. having to have the choke on means that you are not getting enough fuel (too much air). 2nd, when last where the valves adjusted. this can cause rough idling. 3rd, is the oil level in the crankcase correct. I was on a forum thread where someone had a similiar problem and it ended up being that they didn't have enough oil in the crankcase they also just didn't seem to be able to get the idle mixture/idle speed correct. last, the float level in the carb. can also affect the idle mixture.
I found that adjusting the carb. is a matter of getting the idle speed as well as the idle mixture correct. if you get the idle mixture right but the idle speed set too high the machine will idle erratically. by this I mean that if you rev it it will take a long time for the motor to come back to idle speed or the motor will just continue to run at a higher speed and never come back to idle. too low and the machine will tend to stall when put into gear.
I am not at the machine now but from memory I'm going to say that idle mixture is between 1 and 2 turns out from seated. you should notice that if you turn the idle mixture screw in any further than 1/2 turn from seated that the machine will stall. if you turn the idle mixture out any further than say 3 turns the machine will run rough.
another thing to remember is that you will probably have to make some small adjustments to the idle mixture and idle speed from summer to winter. when the temperature changes the air density changes and this will cause the mixture to be off some (same with altitude). good luck.
I've copied a paragraph from an article on carb tuning. he used it to determine the correct jet size and I used this method to get my carb. tuned and it seemed to work pretty good. you can find the whole article at
http://justkdx.dirtrider.net/carbtuning.html.
The Pilot Circuit, Tuning from Idle to ¼ Throttle -You can use the air screw to help determine if your pilot jet is appropriately sized. Take your bike for a short ride letting the engine come up to normal operating temperature. With the engine stopped, transmission in neutral and the bike on its stand turn the air screw clockwise until it just seats, gentle now it's delicate and you don't need to torque it down just gently seat it. Now turn the air screw a quarter of a turn out so the engine will fire and start it. Slowly turn the air screw counter clockwise ( out ) until the point where the engine just reaches the maximum obtainable rpm and continuing to turn the air screw beyond this point wont increase the engine speed (rpms) any further. I find it's easier to hear the rpm increasing if you set the idle at its lowest possible position without the engine stalling. You'll want to repeat this procedure a couple times until you're confident that you've found the right spot and that the result is reproducible. When you're comfortable count the number of turns ( 360° revolutions ) you've backed the air screw out to reach this point. The normal operating range is between 1 and 1.5 turns out so if you find the ideal setting is less that 0.75 turns out consider installing the next richer pilot jet (larger number ). If you find the ideal setting is more than 2 turns out consider installing the next leaner pilot jet ( smaller number ).