Motorcycle Repair: 1981 suzuki gs 1000g, compression gauge, piston ring

Question
my bike is not firing on the #2 cylinder at idle although it is getting fuel and spark. Any ideas?

Answer
A number of things could cause this.

The first thing I would try is swapping the plug with one from another cylinder - it is possible for a plug to make a nice spark in the open air when it won't inside the cylinder with compressed mixture around it. If the problem moves with the plug replace the plugs and you should be OK. If it stays in the same cylinder it didn't cost anything and you eliminated the easiest possibility.

If that doesn't help, try switching the coils next. Again, if the problem moves with the coil it is the culprit. It is possible that the coil can't provide enough power to fire under compression.

The next test is compression. You will need a compression gauge for this (or a friend with one). If the misbehaving cylinder has much lower compression than the others you probably have a valve problem - either a worn or cracked valve or seat or one that is really badly out of adjustment and not closing properly.
It could also be a stuck piston ring, but that is usually accompanied by oil consumption.