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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11052
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Re: 1985 Mercury 115 HP Inline 6 (Tower of Power) "Under

Customer Question

Re: 1985 Mercury 115 HP Inline 6 (Tower of Power)
"Under Load" - with throttle in full power, appears to be running on 4 cylinders, voltmeter goes to 16 volts and motor seems under powered.
"No Load" - with throttle in full power and motor tilted up (generates "rooster tail"), runs on 6 cylinders, voltmeter stays at 13 volts and motor responds with full power.
Is this an electrical problem (e.g. faulty coil) or something else?
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.

Hello. My name is Tan.

We need to do a spark and compression test first. How are you determining the engine is running on 6 cylinders with no load?

Running the engine with no load at full throttle will damage the engine. That's like leaving your car in neutral and pressing the gas pedal to the floor.

Customer: replied 1 year ago.
The no load test was run for less than 5 seconds. Determination of number of cylinders made by listening to the motor (friend who is a motorcycle (not marine) mechanic made the observation.Compression test not conducted, but new spark plugs were installed.
Expert:  Tan replied 1 year ago.

You cannot tell if the engine is running on 3 or 6 cylinders on a 2 stroke motor. I have been doing out board motors for 40 + years and can't tell.

You have to use a timing light and run the boat till it is acting up. The ignition system on this engine has to coils in the stator. A low speed coil and a high speed coil for each switch box. So doing a spark test just cranking will only verify the low speed coil. The engine has to be above 2500 rpms to show if there is a misfire at higher rpms.

Do a compression test first the compression must be 90+ psi on all cylinders with no more than 10% difference between cylinders.

So run the boat under a load and check the spark by connecting the timing light to each cylinder. Check at low speed and high speed. Let me know where you have a misfire.

Also when running the boat an it is acting up choke it. Does it run better?

If no change squeeze the primer bulb. Does it run better?

Post back with findings.

Customer: replied 1 year ago.
Will do. Thanks
Expert:  Tan replied 1 year ago.

Your welcome. Keep me posted to your findings.

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