Some info about using choke.
Activate your choke when your engine is cold, this is a must.
Even if your engine starts without choke from cold, this is an optimum way of getting the most life out of your engine.
Using choke gives your engine a richer mix (more fuel). This means there wont be such a big bang in the head, which means the engine will warm up properly and not too fast which can cause problems.
Use choke for a few minutes, this is all you need.
Also, don't thrash the engine (high revving) whilst it's on choke, or else it makes the whole procedure pointless.
Leaving choke on by accident or on purpose can cause your engine life to reduce rapidly. NEVER forget to turn it off.
Taken from:
http://www.pitbikeboard.co.uk/viewtopic.php?f=20&t=9