found this post by maxim,

is it that straight forward?

It's quite a straightforward job on the 21. Remove the seat unit and tank, followed by the airbox if you have one. Make sure you get a good mate with the airbox seal screws, using a good Philips driver as they are made of soft blue cheese and can round off very easily...

Once these are out of the way, access to the carbs is easy. You will need to remove most of the connections to take them off, although you can usually leave the throttle and choke assemblys in situ. The thing to try first is to clean up everything in the float bowl, which is at the base of the carb and held in place with 4 screws. Have a PU container ready to catch the fuel contained in them as you turn the carbs upside down. Once you have the bowls off, you will see some brass jets. Have a look at the float bowl and check to see if there is any build up of a residue. The jets can be taken out gently, using an 8mm spanner for the mainjet holder, and a small flat head/slot driver for the main jet and slow jet.

Clean all of the bits using a good carb cleaner and a toothbrush and re-assemble. At this point you can also check the float height, which are the black plastic pieces in the float bowl, 2 per carb. Holding the carb upsidedown, they should rest with 13mm showing from the base of the carb.

If that doesn't help then we can look at other bits. Wink