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No Spark on 1998 15 hp Merc

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shogunbern

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"I have a 1998 15 HP mercury.I

"I have a 1998 15 HP mercury.It will run full throttle for about a minute. Once I let off it will only idle. If I turn the throttle it seems to bog down, and stall, like its choked, or no fuel. Well it is not fuel, we rebuilt carb ( with internal fuel pump), and its getting plenty of fuel. I have replaced the coils, plugs,plug wire and switchbox.With either switchbox I still get the same spark effect. The bottom cylinder is getting very weak almost zero spark. Top cylinder is great.Both cylinders have a compression of about 95.I have replaced the stator. The only thing I haven't replaced is the trigger. Would a bad trigger give low spark to the bottom cylinder, and normal spark to the top one? Any ideas on what type of test I can use to determine the cause and/ or what readings I should get for this motor on my tester."
 
"If the trigger was shot you w

"If the trigger was shot you wouldn't get spark at either cylinder. It's just a single bobbin (winding) that produces an AC voltage that is "chopped" into positve AC and negative AC "pulses" (peak voltages at max pos/max neg for a very short duration).

That "pulse" is what tells the SCR in the switchbox which cylinder to fire - positive pulse = 1, negative pulse = 2.

On some models the trigger can be the culprit if you have bad/weak/no spark at one cylinder, but not Merc's "twins" with the Thunderbolt 4 ignition.

Anyhow, it really sounds like (since you have changed everything else) that the wire from the switchbox to the coil could be bad or partially grounding out on you.

I can't remember which is which (white on green and green on white for the cylinders) but an easy check would be swap them around and if your number 1 goes "weak", you know you have a toasted wire....

To your other question - aside from basic resistance tests (to check for integrity of the windings) you would require a DVA meter or a DVA adaptor (for a standard volt meter) to get usable readings from the stator and trigger - no tests can be done on the switchbox...."
 
"Graham,
Thank you for the r

"Graham,
Thank you for the reply. Like I said we replaced everything. When the stator was replaced it was an aftermarket version with 3 wires insread of 2. The black wire was the odd one, and we assummed it was a ground ( So we grounded it). My buddy diconnected that wire today and everthing so far has worked out fine.Plenty of spark, and he ran the boat for about 45 minutes. You were right, a grounded wire.This is great site/forum keep up the good work. I own 3 mercs, and my buddy works on any and all outboards.} I am sure you will be hearing from us !"
 
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