It can only be one of the things here...
Compression (should be above 100)
Timing (Check your flywheel key)
Spark (put new plugs in and check gap and check for spark)
Fuel (Clean properly mixed fuel, make sure plug is wet, try starting it with the gas cap loose too)
Try a little starting fluid too, pop off the air filter give each carb a squirt and see if itll turn over a little.