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1990 20hp Merc - Intermittent Cylinder

P

pl2442

New member
1990 20hp Merc Tiller-

It will run on one cylinder religiously but will intermittently hit on two probably 3-5% of running time.

What could be causing this? Where to Start?
 
If you have a timing light put it on the 2nd cylinder and see if you are getting consistent spark. If not then check coil etc. If you have consistent spark then clean the carbs.
 
I will disagree with the coil. I have never seen coils kick a cylinder in and out. I have seen more switchboxes do that than I can count. That is why you have to test the ignition components.
 
This weekend i did the easy test of checking the coils. Both coils test fine. Even switched the two coils around and still only top cylinder running. Also wires tested fine. It seems both cylinder are getting spark but will test if its good spark tonight.

Some more info. - Top cylinder always runs bottom cylinder does not. Pull top cylinder spark plug and motor dies immediately, pull bottom cylinder nothing happens no tone change motor continues as normal.
 
Ok guys need help with what to test next. Cylinder pressure is good, spark plugs good, spark plug wires are good, coils good, swich box good (bought new 1 installed and still only top cylinder) where do i go next?
 
How do you know both coils are "good"? Talked to a technician at CDI the other day and he told he recently encountered a 3 cylinder Merc where ALL three coils were bad.

Jeff
 
I tested the coils they tested fine then i also switched the two coils and still only top cylinder running. tested the compression with a tester 110 psi both cylinders. moved the bottom cylinder spark plug wire to the top cylinder coil put it on the top cylinder and it fired as normal. replaced the switch box with new no change only top cylinder running.
 
So last night i read to test the trigger (without dva meter) you can swap the brown/white brown/yellow wires to the switch box and the green/white green/yellow to the coils. I jumpered the brown wires and swapped green and the bottom cylinder started running and the top wasnt running. So im assuming the trigger is bad. Anybody have an alternate theory.
 
Ok so replaced the trigger because i could get the spark to follow the trigger wires to the top or bottom cylinder. Well it must not be the trigger because the bottom cylinder doesnt have spark. So the only thing left is the stator. Could the stator do this? Does anyone have an ignition diagram for a two cylinder ive been looking at the adi 3 cylinder and the stator doesnt make since.
 
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