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Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10641
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Its a mercury force 70 1995. it will start but runs rough -

Customer Question

its a mercury force 70 1995. it will start but runs rough - seems like its misfiring especially when revved higher
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.
Customer: its a mercury force 70 1995. it will start but runs rough - seems like its misfiring especially when revved higher I pulled the spark plugs and all 3 are discoloured and oily like they have been firing...
JA: Is there anything else the Marine Mechanic should be aware of?
Customer: Fuel is dripping out the front of the carb
Submitted: 6 months ago.
Category: Boat
Expert:  Tan replied 6 months ago.

Hello. My name is Tan.

Fuel dripping out the front of the carb is an indication of a flooding condition. The engine will run rough if it is getting to much fuel. The carburetor that is leaking will have to be removed, disassembled, cleaned and a carb kit installed.

Now do this squeeze the primer bulb does fuel run out the front of the carbs?

Customer: replied 6 months ago.
no
Customer: replied 6 months ago.
I took the carb apart and cleaned it with compressed air
Expert:  Tan replied 6 months ago.

If you have a bad inlet needle that doesn't seat correctly compressed air or cleaning will not fix the leaking carb.

Now we need to do a spark test with a spark gap tester or when engine is running pull each plug wire off. Does engine drop rpms on each cylinder when plug wire is removed? Use insulated [pliers or you will get shocked.

Customer: replied 6 months ago.
the rpms went down as each plug was removed
Expert:  Tan replied 6 months ago.

Ok we need to use a timing light to see the quality of the spark on each cylinder. What you do is connect the timing light to each cylinder and watch the flash of the light. The flash must be steady with no dead spots. If you find a cylinder with a misfire swap that coil with a known good firing cylinder. Does the misfire move or stay?

Post back with findings.

Customer: replied 6 months ago.
I dont have a timing light
Expert:  Tan replied 6 months ago.

Ok you said the engine runs rough and seems like it is misfiring. This requires some tools to trouble these engines. Can you rent or borrow a timing light. You have one of 2 things wrong with the engine ignition or fuel. This is a process of elimination so to eliminate we have to use a timing light to eliminate ignition. The logical cause for a misfire is ignition related.

So unless you know which cylinder is misfiring I don't know what else we can do. We as technician have to have special tools to find engine run issues. So if you can't get a timing light you will have to take the engine to a mechanic at 100 dollars an hour.

What would you like to do?

Customer: replied 6 months ago.
I will take it to a mechanic- thanks for giving me a head start to let them know what it could be
Expert:  Tan replied 6 months ago.

Your welcome.. If you do get a timing light I can help you through the process.

Thank you........Tan

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