Are they new or used bikes, most of the time it's something stupid wrong with them. Try taking the plug out and see what it looks like, then check the plug for a spark, then put alittle gas down the hole, plug back, see it it fires, if it does, the carb is probably dirty inside the float bowl and the main jet is clogged. Or the reeds on the reed plate behind the carb and manifold might not be sealing.