Is there fuel in the carbs? open the drain screw on the bottom and see if fuel comes out. You could also remove the air filter and spray some starter fluid, wd-40 etc in there and see if the engine fires, if it does you know its fuel if it doesnt then its probably not getting a spark. You said its getting a spark but it may be to weak to start the engine. I would first check the carb for fuel, then try starter fluid in the air box, then service the ht lead (remove the boot, cut 1/4 inch off the wire, strip it to get a good connection then replace the boot.) if its still not starting change the plug.