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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10895
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a 1991 mercury 40 hp oil injecteted engige that whe

Customer Question

I have a 1991 mercury 40 hp oil injecteted engige that whe it is cold it runs great after it warms up it bogs down and misses doesn,t quit just not running smooth just redid the carbs 2 weeks ago and also use sea foam any ideas
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.

Hello. My name is Tan.

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

First we need to run the engine till it acts activate the choke. Does it run better? If no change squeeze the primer bulb when the engine acts up. Does it run better? If no change then we need to use a timing light and check spark on all cylinders when the engine is acting up.

Connect the timing light to the top spark plug wire run engine till it acts up watching the flash of the timing light. The flash should be steady with no dead spots. Repeat this test on all cylinders. If you find a misfire swap that coil with a known good firing cylinder. If the misfire moves then the coil is bad. If the misfire stays then you have another ignition component failing. This will require testing with a peak reading volt meter.

If the engine run better when you choke it you have a carburetor issue. Trash in the carb. If the engine runs better when squeezing the primer bulb then you have a failing fuel pump or a fuel restriction. To determine which run the engine on another fuel tank. If the engine runs the same you have a failing fuel pump. If it runs better then you have a restriction or air leak in the boat side fuel system.

One of these checks should lead you to the cause of the problem or point you in the right direction.

Post back with findings and questions.

Follow up questions are always welcome on this subject.

The question still stays open even after you have positively rated.

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