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Q I am in the dark over my bike’s dodgy spark
My Yamaha XVS950 Midnight Star has started missing on the rear cylinder, but if you pull the plug cap slightly off the rear pot’s spark plug it starts to fire. I’ve renewed the plugs and caps without any joy. When you remove the plug cap completely and hold the HT lead onto the rear plug the spark is a weak orange, but if you move the cable away from the plug it turns bright blue, which seems the wrong way round if it was the coil causing the problem. George Young, email
AAnswered by Charles Marvell, Flitwick Motorcycles The voltage in high tension varies all the time because of the difference in resistance in the combustion chamber. At tickover, there are only small amounts of fuel/air mixture being compressed by the piston, so the amount of voltage needed for a spark to jump the plug gap is 15-20,000 volts. But when you open the throttle wide open, there’s a lot more mixture being compressed in the same area, so the energy needed to make a spark can go up to 40,000 volts at full throttle.
The coil only produces enough voltage to jump a given gap, and when you moved the cap away from the plug, it had to take more of a “run-up” to make that bigger jump, giving a blue spark.
You’ve fitted new plugs with the gaps set, so it can’t be a narrow plug gap, instead it’s probably the coil breaking down. Try swopping the coils to see if the problem moves to the front.