Re: 2 days ago, only 1 cylinder firing, yesterday BOTH, today just 1 again.

You need to make sure the petcock isn't leaking. It sounds like it is. If so, it will need a new kit put in it. They are available at www.partsnmore.com . Minimum order there though! To check the petcock, with the engine off, pull off the fuel lines to the carbs and turn the petcock to ON. There should not be any fuel coming out of the fuel lines and no fuel in the vacuum lines either. If there is either, you will need to do two things; check your crankcase for a gasoline smell; if you smell gasoline, change the oil after you rebuild the petcock. What can happen is when the petcock fails, it can fill the crankcase full of gasoline and that can cause you to blow the engine due to a lack of lubrication. Never leave the petcock on prime when the bike is setting as these carbs can have a float stick or not close all the way and again you will find your crankcase full of gasoline.