Try using NGK BP 7ES spark plugs. Do not use resistor type, if you also have spark plug lead or caps that have built in resistors this will make the bike extremely hard to start. Yes I know if you take the spark plugs out and crank the engine over you can see the spark but it is not until you put screw them back in that the effect of having Plugs that have resistors built in and leads that are resistor type and maybe spark plug caps that are resistor type are going to have a big effect on the spark that does come out of the plug when it is cranked over by starter motor. Try this it may work.