60 HP Mercury (1984) Loss of RPM at WOT

[DenverDan]

Hello to all from the High Country,

I have a 17ft. Bass Boat with a 1984, 60 HP Mercury (2-stroke, 3 cylinder).
The local Mercury Dealer recommended I use a BLACK MAX 10" pitch for the best performance/rpms. I have put new spark plugs, replaced the fuel pump gaskets, overhauled the carbs & added the new plastic floats. Replaced the impeller & the rectifier. Performed idle, shift & throttle cable adjustments. Checked the timing, which was right on the nose. Performed compression checks, (top-100, mid-115, bot-113). Also, I have added a new 19gal Moeller permanent gas tank which I use stabilizer in my fuel to prevent moisture buildup & only keep about 5-6 gallons for use for each trip. In the past I had the rpms up 5500 & it would take off like a rocket!!! Now they are maxing out around 4700-4800 with a steady increase, running around 30-31 MPH without extra persons, trolling battery, tackle, etc. basically minimum weight. When in Neutral the idle is set to 1200-1300 rpms/in forward 850rpms which (750-850) is recommended.
I used Mercury's DECARB spray which didn't seem to change my symptoms.
I originally purchased the Mercury Outboard Repair Manual (SELOC) through iboats. I have fixed & learned a lot using the manual but now I am leaning toward performing a "Cylinder Block Disassembly" , which I prefer not to do, due to the low compression on the top cylinder. The manual states a variation of more than 15 psi indicates the lower compression cylinder is defective. So my question to you being the "EXPERTS" is would new "RINGS" resolve my issue? Or should I take another approach by checking the carbs again for any fuel clogs, etc? Or if I should continue to run it as is will this cause wear on the powerhead due to the lack of RPMs at WOT? I am open to any advice at this point.

Thanks for your time.

P.S. I only use it 5-6 times a year for fishing & run it about 15-30minutes max at each interval.

 

[NathanY]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

How low is the compression? Exactly what are the numbers? Have you done a leakdown test? I am going throught the exact same thing on a 90hp motor. I opted to rebuild, tore it down and it goes to the machine shop Saturday. You have a good motor to learn on. Go ahead and rebuild it.

 

[jebeebe]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

I rebuilt that same engine and the top cyl. was ugly. It had no compression. Broken rings (most were clear gone. Out the exaust I presume) and what was left was welded in the groves. I took the block to the machine shop and the guy honed it and said it would go standard. He said that most of the ugly was built up on the surface and only had a tiny scratch and wouldn't cause a problem. I put it back together with all new pistons and rings and all cyl honed and it purrs like a kitten. Starts good and idles, turns 6,000 by my tach and I concider it a success. I wasn't confident about the outcome as I had never done an outboard before. Had about 500.00 in it when I got done. That includes fuel pump, water pump,starter selenoid, starter gear and all new hoses. I would go for it. Good luck.................Jerry

 

[Lone Duck]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

Did it run fine before you added new tank? If so check if the air vent is plugged.

 

[Joe_Navy]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

Squart a very small amount of oil in each cylinder. Then check your compression agian. The oil will seal any gaps your rings may have. If your compression test indicates higher readings. You have isolated the problem.

 

[DenverDan]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

How low is the compression? Exactly what are the numbers? Have you done a leakdown test? I am going throught the exact same thing on a 90hp motor. I opted to rebuild, tore it down and it goes to the machine shop Saturday. You have a good motor to learn on. Go ahead and rebuild it.

The exact numbers are:

Top - 100
Mid - 115
Bot - 113

You might have overlooked them in my auto-biography!!!

No, I have not performed a leakdown test. It's on the agenda...

 

[DenverDan]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

I rebuilt that same engine and the top cyl. was ugly. It had no compression. Broken rings (most were clear gone. Out the exaust I presume) and what was left was welded in the groves. I took the block to the machine shop and the guy honed it and said it would go standard. He said that most of the ugly was built up on the surface and only had a tiny scratch and wouldn't cause a problem. I put it back together with all new pistons and rings and all cyl honed and it purrs like a kitten. Starts good and idles, turns 6,000 by my tach and I concider it a success. I wasn't confident about the outcome as I had never done an outboard before. Had about 500.00 in it when I got done. That includes fuel pump, water pump,starter selenoid, starter gear and all new hoses. I would go for it. Good luck.................Jerry

Thanks Jerry, I have all the means to perform the rebuild and I even have a backup motor 115 HP Mercury 1984 model sitting at my folks home in Georgia. I have made plans to pick it up by next summer. The 60 HP has been a real challenge for my first outboard I've worked on, and I'm no mechanic! I am sure before this project is over I will be fully trained.

Thanks for your thoughts,

Dan

 

[bgbass.1]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

Those # are not bad it should run. check likage to make sure its opening all the way. Or it could be as simple as a dirty fuel filter or dirty carb.

 

[DenverDan]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

Did it run fine before you added new tank? If so check if the air vent is plugged.

Yes, I do recall it seemed to be running fine. I could have picked up some debris from the new tank. I did check one of the vents on the carb and it looked ok but the other one's vent was too tight. I will most likely just go ahead & pull both carbs back out for a full inspection. To me I would think a partial clog would definitely cause issues. I just wish the compression wasn't so out of wack!!!

I'll let you know if I find anything...Thanks, Dan

 

[DenverDan]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

Those # are not bad it should run. check likage to make sure its opening all the way. Or it could be as simple as a dirty fuel filter or dirty carb.

I will buy a new fuel filter & inspect the carbs and let you know what I find. Also, I originally adjusted the the carbs to open all the way, which is still intact.

For a broke-in 60-mercury, what would you say should be the normally compression rate, 120psi?

 

[DenverDan]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

Squart a very small amount of oil in each cylinder. Then check your compression agian. The oil will seal any gaps your rings may have. If your compression test indicates higher readings. You have isolated the problem.

I will definitely try it & get back to you. Thank You for the tip!!!

 

[Joe_Navy]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

Another good thing to remember about compression:

How do you really know what the compression should be? There really isn't a spec for it.

Multiply the compression ratio by atmospheric pressure(14.7 at sea level)

ex. 10:1 X 14.7 = 140.7 psi

 

[bgbass.1]

Re: 60 HP Mercury (1984) Loss of RPM at WOT

If your compression is 100 or better and each cyl. is within 10% of each other your ok.Also check when warm and try a different tester have found sometimes tester off.