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Motorcycle News (UK) - - Garage -

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.

A fail­ing coil could be spoil­ing the fun on your Mid­night Star

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