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Ask Dominick Your Own Question
Dominick, Marine Mechanic
Category: Boat
Satisfied Customers: 1009
Experience:  25 Years Experience as Mechanic, 20 Yrs Boat Yard Owner/Mechanic, Factory Certified
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I have 1 know problem that has never been repaired, though I

Customer Question

I have 1 know problem that has never been repaired, though I have tried 2 different shops to figure it out. I still have that problem. When riding at full power, on plane, it will suddenly loose 50% power and drop off plane but keeps running. Then just as suddenly and without doing anything it will gain full power and run as it should. It may do this 10 times during a day of boating. It's been that way for many years now. However that is not the trouble I am having right now. It will not start. I really need a mechanic that knows my motor, not just a boat mechanic that knows a little about all motors. I can through parts at it until I hit the correct one but I want someone that knows the Johnson GT 150 inside and out. Mine is a 1986. Do you have such a person in mind in the Cincinnati area? Thank you.
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.

Hello Calvin. My name is Tan.

Have you checked the spark on the engine when it acts up with a timing light? We need to try to determine if you have a spark issue or fuel issue. The symptoms tell me an ignition issue. The only way to know is to run the engine and check spark when it is acting up. What you do is connect the timing light to #1 cylinder run the boat till it acts up look at the flash of the timing light. You should have a steady flash of the light. If no misfire move to the next cylinder. Continue till you have checked all cylinders.

If you have a misfire on only one cylinder move that ignition coil from that cylinder to a known good firing cylinder. Does the misfire move?

Can you do these tests?

Customer: replied 1 year ago.
I went on the lake with a mechanic and some home made tester he hooked up to 3 spark plugs at the same time and when it lost power he was sure it was the power pack so he replaced it. Worked fine for a month.
Expert:  Tan replied 1 year ago.

Ok but is it acting the same way it did before? Do we know that you have a power pack issue again. You could have a stator or timer base issue that will tale out 3 cylinders at a time. So we still need to test to find out if it is a pack or something else. What I would do it take the boat out run it till it acted up. If we have 3 cylinders on the same time go out I would then swap the power packs. If the misfire moves then we know the pack is bad. If the miss stays on the same side then we have either a bad timer base or bad stator. What you would do next is test the out put of the timer base and the stator with a DVA adapter or peak reading volt meter.

Do you have the means to do these tests?

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