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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10903
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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115 hp Merc outboard 1982 speeds up and slows down at set

Customer Question

115 hp Merc outboard 1982 speeds up and slows down at set throttle. At full speed it sometimes slows down to half speed, reducing throttle may help sometimes. Plugs are new, not wet upon inspection. Starts and idles fine, running half speed for several miles and all of a sudden the rpms increase, reduced throttle and then after a short while the rpms decrease requiring a throttle increase.
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.

Hello. My name is Tan.

this may take more than one reply o solve your issue.

These are symptoms of one of 2 things. Loss of spark on one or more cylinders or a fuel starvation issue.

There are a couple of checks we can do to determine the cause of the issue. The fist test is to run the boat to where it starts acting up and squeezing the fuel primer bulb. If the engine runs better we have a fuel restriction on the boat side fuel system or a failing fuel pump. You can do one of 2 things to determine if this is a boat side fuel system issue. Swap the fuel supply line on the engines. We know the other engine is running ok so we know it is getting good fuel supply. If the engine run the same you have a failing furl pump. If it runs better then we know the boat side fuel supply is the problem.

Now the other test is to determine if we have a misfire causing the engine to go up and down. This will require the use of a timing light. Connect the timing light to the top cylinder run the boat till it starts acting up then watch the flash of the timing light. You should have a steady flash of the light with no dead spots. Do this test on each cylinder. If you find a misfiring cylinder swap that coil with a known good firing coil. If the misfire moves the coil is bad. If the misfire stays then we have another ignition component failing. This will require a peak reading volt meter to check voltage out put of each ignition component when engine is acting up.

Do these checks and post back with findings.

These tests should lead you to the cause of your issue.

Thank you.......Tan

Post back with follow up questions if you need more info.

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