early 80's 4.5 mercury ignition problem

[oldtruckface]

Im using a cordless drill with the plug grounded and the spark seems intermittent, its there and then its not.Thisis the 1 cyl engine with yellow coil,
Is this the right plug for this motor buhw-2,same as my 90 hp?
Anyone have the ignition test specs so I can determine the fault,thank you