After cranking, with fuel in the tank . . . open carb drain screw (2.5 mm hex key, IIRC). Does fuel flow from float bowl out drain? If YES, vacuum petcock and float valve are WORKING; if not . . . problem could be either . . . a good, "WHACK!" of the carb will sometimes free a stuck float valve; repair/adjustment of a malfunctioning vacuum petcock may be more complex.
If fuel flows, but bike won't start still, could be contaminated fuel. Checking for this problem, disconnect fuel line at tank; drain fuel bowl COMPLETELY . . . stick a small funnel's nose into the now-disconnected fuel line; FILL funnel with guaranteed righteous fresh, "dry," gasoline (you DID close the carb drain screw, right?); then crank her up. You KNOW you have combustible fuel in the carb now; assume you've checked for spark; whether you have compression should be obvious from cranking . . . if it don't start then, looks like a carb problem, curable, perhaps, by carb disassembly and mechanical cleaning/adjustment (might get by with, "Better living through CHEMISTRY," applying some carb-cleaner fuel additive joy-juice, like Sea Foam, etc. A dose of fuel conditioner, like "HEET" fuel-line anti-freeze in the tank to combat moisture, may be a good idea . . . better still, drain the whole fuel tank and re-fill with fresh gas, to be sure (vacuum must be applied to the stock petcock to perform this routine, draining through the petcock).
Spark, compression, and a combustible mixture are necessary for the engine to run. The first two can be verified; the latter (combustible mixture) may require purging of existing fuel and resupply from a known good source.
A 'way cool device for cases like this is . . . a small bottle of fuel, with a 1/4" i.d. hose attached to the lid; substitute fuel tank for connecting to the carburetor when testing; about $ 10 from Tucker Rocky, or you can use the funnel-and-hose technique previously mentioned . . .
EDIT: SEE POST # 11 BELOW.
Last edited by Damocles; 04-26-2015 at 07:33 AM.