It might be that you just need to turn the idle setting up a bit - the silver knob/dial on left side near fuel petcock - rotate clockwise (I think) to increase idle setting (to about 1250rpm) once engine is warmed up. Also, rather than just leave bike on full choke to warm up for 5 minutes, you might try, once it starts and is on full choke, blipping the throttle a bit repeatedly and slowly decreasing choke as you see engine temperature coming up so that choke is off by time you get to full operating temperature. Alternatively once temperature is maybe half way to full operating temp, can ride away SLOWLY with partial choke still on and dial back choke setting as temperature comes up. Basically as temperature comes up, choke setting goes down. Hope that helps.
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-Jim
'93 MC21