I have found that on most bikes I test, not just Mikuni VM24 but all VM series carbs idle quite a bit faster on full choke. Once the bike starts, it will idle at appx. 3500 rpms on full choke so you can decrease the choke arm position slightly which cuts off a bit of air and the rpms will drop to about 2000. It should only take a minute or so to be able to come off choke alltogether. The word choke is a misnomer in this case, as has been pointed out... the circuit richens the pilot mixture but there is also additional air entering via the small passageway set back about 1/2" from the venturi edge on the plunger side. That hole is a straight shot into the plunger well. I use the term choke because that is what folks refer to and that is what is stamped on the arm of the choke lever... If your rpms don't change when you change the position of the choke arm, gas is being obstructed in the choke circuit or even possibly in the pilot circuit. If it takes 15 minutes to warm the bike, something is definitely wrong.
Two things to check:
1. remove float bowls and use some spray carb cleaner (wear EYE PROTECTION!!!!) and spray in the check valve hole in the bottom/side of the float bowl. It should spray out in a mist out of the bowl edge. If it doesn't the check valve is clogged and the choke circuit won't pick up gas. This is COMMON.
2. The brass pickup tube can be clogged. This tube hangs from the carb and seats into the hole in the bowl edge you just cleared. If the tube clogs, gas can't be picked up. You can remove your choke plungers and spray carb cleaner into the hole where the plunger seats and gas should spray out the tip and side holes on this brass pickup tube or you can use a cleaning wire to poke through the small holes to ensure they are clear.
I always do both things when cleaning the choke circuit of any VM series carb to ensure gas is picked up as it should be.
