Mercury 40hp 4cyl no spark on #2

[Davidpkl]

I have a 97 mercury 40hp 2 stroke 4cyl that I had a no spark so I tested stator and trigger resistance and they both were in spec so I ordered a new switch box and that fixied it all 4 cylinder were firing great took boat out to lake and first trip it ran great for a few hours and then started running bad check for spark and now I don't have spark on #2 but stator and trigger still test good I also switched coils and it stayed the same so i know coils are also good so do I get a bad switch box or am I missing something on the trigger and stator as I have only done resistance test on them do to not having my dva adapter.

[Davidpkl]

I can't seam to figure out what the issue is now some times it will spark on all 4 and run good and then a little wile later it will not spark on #2 and run bad. What would cause a intermittent no spark on #2 when everything seams to test withen spec?

[Steve Pope]

You could have a dud switch box or a fault in the stop circuit wiring or key switch Disconnect the stop circuit wire( black with yellow trace) from switch box and recheck for spark. If #2 still missing switch box bad. Also, voltage on stop circuit can damage switch box. Test for voltage with key in run position. Should not exceed 2 volts, typically only about . 030 of a volt. If high volt then faulty key switch.

[Davidpkl]

I tried taking off the kill wires and same issue and tested for voltage and it was 0.34. Could a bad key switch cause only 1 cylinder to not fire? As some times the motor will run great firing in all 4 but most of the time it will only fire on 3cyl

[flatbackcanoe]

Plug wires. Could be the boot.

[Davidpkl]

No plug wires are all good also.

[herculesboat]

I tried taking off the kill wires and same issue and tested for voltage and it was 0.34. Could a bad key switch cause only 1 cylinder to not fire? As some times the motor will run great firing in all 4 but most of the time it will only fire on 3cyl

I agree about it!!