If, after replacing the plug and trimming the wire the cylinder still won't fire, I have done something that has worked each time I had similar symptoms. With engine off and bike on center stand, I put a piece of tubing on the fuel drain of the carb float bowl that is long enough to reach a clean quart glass jar sitting on the ground. I open the drain for the carb on the affected cylinder and drain it. It should drain about an 1/8 of a cup and stop (leave the drain open if it stops). If it continues to drain, you have a bad fuel petcock and that will need to be replaced as well. After draining the carb, and verifying you do not have a hydro-lock (which you shouldn't if the initial carb drain stopped) start the bike and keep it running. Gas should start to flow into the glass jar. After about 30 seconds shut the engine off and close the drain. Restart the engine and see if the cylinder picks back up. You may have to rev the engine a couple off times but usually it clears up pretty quickly. Also, you may have to repeat this a couple of times. The reason for doing this is to attempt to flush any debris that might be stuck between the float needle and the seat. By draining the carb first, you are dropping the float as far down as it will go, thus moving the needle as far away from the seat as possible. Usually (for me at least) the fuel will flush the offending trash out and into the glass jar, and the the needle will be able to do its job again. IF, this does not work, MY next step would be to do a compression check on that cylinder to make sure it is mechanically sound and capable of producing enough compression to support combustion. HTH
