Lets step back just a little bit. Grab an extra spark plug, Get the bike up to temp and turn it off. now pull a plug wire and put it on the extra plug, and hold the plug against the block and crank the motor. (this would be best inside under fairly low light conditions so its easy to see. What we are looking for is if you have spark. Any time you have a problem getting an engine to run remember you need 3 things... Gas, Spark, and Air.
If you don't have spark, then the suggestions above are the way to go. However, You MAY have a good spark, but still won't start. I'm thinking about a problem a guy brought into the shop one time. It his case, he had done a "complete tune up" and now the bike would not start when hot. As it happened he had messed up on setting his valves. When it got hot he was loosing compression. As long as the bike was running, it would keep running because it could run with low compression but not start. I only saw that one time because most people simply aren't that exact on setting the valves and at least a couple of cylinders would fire when cranking.
Not saying thats the answer- chances are damn good it IS NOT, but the tracing procedure is the same. First find out if spark is the problem. Another "tool", IF you have spark, spray some starting fluid in the air cleaner and see if you get some fire. also, when the bike is running, try spraying some starter fluid (just a short hit) at he area where the carbs screw into the head. An increase in RPMs would point toward an air leak
If that is no help then it is time to pull the engine from the chasis pull it down, install the biggest big bore kit, racing springs, big cam, deck the block, ... well you know the program <BMFG> (sorry I'm the type that will use ANY excuse to "build" and engine...)
