Or another technique (easier because you don't have to pull the intake snorkle off) is to turn the engine off, swap leads 2 & 3 over, fire it up again and rev it to no more than 2500rpm for about 15 secs. It should backfire a couple of times - once it has, foot off the accelerator again and see if it idles properly. If it does, swap your leads back over and you're done. If it doesn't try another couple of backfires.
Swapping the leads gets the spark plugs in cylinders 2 and 3 firing on the intake stroke - the flame front goes all the way up the intake runners into the carby and (with luck) burns out the crap that's in the idle jet.
Works 95% of the time in my experience
