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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11073
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a 1985 mercury 90hp 6 cylinder on a pontoon. It seems

Customer Question

I have a 1985 mercury 90hp 6 cylinder on a pontoon. It seems to be down a cylinder under a load. At full throttle it will reach about 3900 rpm and sometimes pick the cylinder up for a short time then drop it again. At half throttle it seems to do it more often. Idle is rough. The longer it runs the worse it gets. I have rebuilt the carbs, cleaned the fuel tanks and has fresh gas, all new fuel lines, rebuild the fuel pump, replaced spark plugs twice (ngk)s, i have done a spark test on all cylinders and they seem to all be consistent with a strong spark while cranking not under a load. Any ideas thank you Scott
Submitted: 2 years ago.
Category: Boat
Expert:  Tan replied 2 years ago.

Hello Scott. My name is Tan.

This may take more than one reply to find your issue.

Testing the ignition without a load won't show anything if the engine isn't actually acting up.

Your symptoms are an ignition issue most likely a coil, switch box or trigger.

To find the misfire this will require a timing light and a fried to run the boat while you check spark on each cylinder. Connect the timing light to the top cylinder run the boat till it starts acting up. Watch the flash of the timing light you should have a steady flash with no dead spots. If the flash is steady go to the nest cylinder. Do this test on all cylinders. If you find a misfiring cylinder move that cylinders ignition coil to a known good firing cylinder. If the misfire moves then the coil is bad replace the coil. If the misfire stays we have another ignition component failing. This will require a peak reading volt meter to test the trigger or switch boxes. The other option is to swap the switch boxes around. If the misfire moves then the switch box is bad. If the misfire stays then the trigger is bad.

Do these checks and post back with findings and questions.

Expert:  Tan replied 2 years ago.

Please don't forget to positively rate me as that is the only way I get paid from helping you.

Please post back if you need further assistance.

Thank you...........Tan