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1989 Mariner 90hp 2 Stroke performance issue

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mechaneer31

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I have a 1989 Mariner 90hp 2 Stroke 3 cylinder outboard attached to a 16.5 foot Campion boat.

Issue: When on the water the motor will not perform correctly. With WOT boat speed maxes at 6-8 mph. Will not plane. Can throw water pretty far behind the boat though. When I bought the boat this issue has been there from the get-go. Did not know when I bought it cause running on muffs to make sure motor worked was not enough to know.

Current maintenance items being performed is replacing water impeller, refreshing lower unit gear oil.

Current fixes: Poppet valve is spitting water out from the valve last I had it on muffs trying to get it going for the season so I am replacing poppet valve, thermostat and temp sending unit wire.

Prop on boat is 13"d, 19P.
I talked to a local prop shop and they are recommending to verify the motor alignment with bottom of boat which I will do when the lower unit goes back on.

Mechanically, is there anything else I should be looking for or trying during my next lake trial to figure out what could be causing this problem?
 
Thank you Racerone. Spark test did find that there is no spark in cylinder 1. Test of spark plug and spark plug wire showed they were fine. Diagnosis to coil pack and beyond now ensues.

Also, I found that the outboard did not have a thermostat installed either so that will be fixed also.

Based on my research and collective understanding through the guidance here I am fighting a dead cylinder and possibly an over-cooled motor. My understanding is that either condition would make the motor struggle but I imagine both combined are pretty rough for it.

I did check the motor alignment to the bottom of the boat and that was satisfactory.
 
Looks like 3 coils all same part number mounted in a rubber mount. Just swap 2 around & see if problem follows. Problem follows coil - bad coil. Problem stays with #1 cyl, replace switchbox.

BTW, my money is on switchbox being bad.
 
So I did the multimeter checks in the Seloc Manual for my motor. As far as I understand, what it is telling me is that the stator and trigger can be verified completely with multimeter but the coil and the switchbox cannot. For all of the checks the Seloc manual had, the readings came back fine so that does point me, I believe, to the coil or switchbox. I'm thinking you are probably on the right track. I will do the coil swap and check that next and see what I find.

Thanks for the help, this is an interesting learning process of figuring out how these motors work.
 
If there is enough wire (the small wires from swithbox) you can just swap the wires and not the whole coil for troubleshooting. You're just checking for spark. Can be done on the trailer, no need to run it.
 
I went out and looked at it some more. I see what you are talking about by switching the wires that go from switchbox to the coil. I will tackle replacing the poppet valve and installing thermostat and check on that tomorrow and see what I find out.
 
Alright, so I swapped the wires at switch box from coil 1 (which is no spark) with coil 2 (which has spark). Boat backfires instantly and will not start or run. Just tries to turn over. Boat will start and run when wires are put back in the original configuration.

Couldn’t get to poppet or thermostat today so that’s future me work to do.

So if I’m understanding the system correctly, this would isolate the switchbox as being bad?
 
You swap the wires to check for spark with a spark board not to try & run it.

Here are my last 2 sentences from above:
You're just checking for spark. Can be done on the trailer, no need to run it.

Trying to run it, you have 1 cylinder 120 off time, 1 correctly timed & 1 dead.

So, pull all 3 plugs out & check each for spark while cranking the motor. If top coil now has spark & middle is dead, then bad switch box. If top coil is still dead, bad coil.

As of now, no you have not isolated the switch box as bad because you have spark 120 degrees off time on # 1 or #2 but you haven't determined which one.
 
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