1988 Mercury 70 hp 3 cylinder bogging down

I'm stumped! Calf Rope!

1988 Mercury 70 hp 3 cylinder oil injected

Thursday (4.9.2015): New plugs, new fuel line, new vacuum hose, new gas, can of Sea Foam, new battery and she cranked right up. Ran her out on the lake for about an hour and she did just fine. (at idol, medium and full throttle)

Yesterday: Wouldn't crank, pulled plugs, checked spark, no spark. #-o Checked battery connections, key switch (no kill switch on it), wiring harness, shorted starter with a wrench at the motor, she cranked. #-o #-o Key switch wants to work sometimes and sometimes it doesn't. (push to choke)

Took her out for a spin, she bogged down when I accelerated. Acts like she's only running on 2 cylinders. Pulled plugs again; 2 are perfect, 1 looks burnt. WHAT???? They are brand new!!!

I shook rattle and rolled all wires. Made sure it had oil. (it does have the sensor for low oil). T&T hasn't worked in years, so ruled out anything to do with that first thing. Put a new rectifier on her last summer. It still looks brand new.

No gauges on this ole' girl so don't know the rpm's.

Comments? Suggestions? All are welcomed and appreciated.

M
Tyler, Texas

Maybe try switching the plugs around and see if the burnt plug is ok in another cylinder and if the cylinder it was in is still dead. Could be a chunk of carbon broke free and killed the plug. Maybe try an old plug as well.

JL8Jeff said:

Maybe try switching the plugs around and see if the burnt plug is ok in another cylinder and if the cylinder it was in is still dead. Could be a chunk of carbon broke free and killed the plug. Maybe try an old plug as well.

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Thank you for your comments. Already tried switching the plugs around. The plug still works. If you think of something else, please let me know. Scratching my head for sure on this one. #-o

M

Was it the bottom cylinder that was burned? If you tilt the motor up, sometimes the fuel/oil in the carbs can run out into the air cleaner box and then get recirculated or pulled into the bottom carb/cylinder. I had a 90's 90 hp Mercury and I had to do some adjustments to get all the carbs/timing set up right so you could be out of adjustment. Seems weird if it just ran fine all day but I've noticed the last couple of years this lousy gas will take out fuel pump diaphragms a lot faster. I lost 2 fuel pumps in the same 99 Johnson 40 hp in less than 5 years so don't rule that out.

JL8Jeff said:

Was it the bottom cylinder that was burned? If you tilt the motor up, sometimes the fuel/oil in the carbs can run out into the air cleaner box and then get recirculated or pulled into the bottom carb/cylinder. I had a 90's 90 hp Mercury and I had to do some adjustments to get all the carbs/timing set up right so you could be out of adjustment. Seems weird if it just ran fine all day but I've noticed the last couple of years this lousy gas will take out fuel pump diaphragms a lot faster. I lost 2 fuel pumps in the same 99 Johnson 40 hp in less than 5 years so don't rule that out.

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Thanks for your comments.
The top cylinder is the one causing the headaches. I think it may be a coil, but not for sure.

I hear ya on the ethinol gas. Gums up fuel lines and carbs, fast. :x We just replaced the fuel line and bulb to eliminate that problem.

Thank you again for your input. I can use all the advice I can get.
M
Tyler, Texas

UPDATE:
Listed her Monday night for sale. She sold yesterday at 9:00 am, to the first couple that looked at her. We allowed for carb kits, coils and a key switch in the deal. Sad to see her go after 15 years of bringing such joy to our family, but glad she is going to a man that knows how to fix her, right. [-o<

Thanks for the comments!
Michelle
Tyler, Texas

BTW: He says it's probably the top coil, and the key switch has a loose wire. Says he will replace all 3 coils, all 3 carb kits and have them synchronized, and a new key switch. =D>