two plugs not firing?!?

[andreworiordan]

mounted my engine on my boat today for her first voyage and when we started the engine we noticed that 2 out of the 4 cylinders was not firing, the first and last are but not 2 & 3, there is no spark as we checked and the coils appear to be ok as we switched wire and got spark but the engine was running out of sinc, The engiue is a 2004 90hp 4stroke engine Mercury, has anyone experienced this before, if so what was the problem? Thanks guys

[9D280]

"shared coil" is quite common on 4- strokes- it basically halves the number of coils (therefore weight), and they're wired such that the piston on the power stroke shares it's coil with the piston on exhaust, where the spark will make s*d all difference to the engine.

I assume you've done the "swap everything one at a time" trick? My going in guess would be a dodgy connection or a gubbed coil. Which wire did you switch?