1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

[Duddiesbuddy]

Losses compression on 3 & 4 intermittantly, I was thinking Reed valves, local boat shop says that it needs an overhaul $4,000.00
Any suggestions???

 

[Droll]

Re: 1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

Losses compression on 3 & 4 intermittantly, I was thinking Reed valves, local boat shop says that it needs an overhaul $4,000.00
Any suggestions???

Remove the intake port covers on starboard side ( remove ignition plate as it's located under it ), with the covers removed you should be able to see directly into the cylinders tru the ports.. use a screwdriver to check the ring tension ( no tension = broken or stuck ring ).

To check the reeds you will have to remove and split the powerhead ( no special tools ( except for a flywheel puller ) or skills needed ), once the carb part of the block are removed you can inspect the reeds....
Reed blocks can be removed and replaced without removing the crankshaft and pistons, but it does require some minor mechanical skills .

Worst case .... a replacement powerhead can be found for a lot less than $4000.-... more like $1000.- +/- some $ .

But since were talking about #3 and #4 ....what propeller did you run on this engine, and what WOT rpm did this setup manage ??

Arne Kjetil

 

[Chris1956]

Re: 1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

Reed valves almost never go bad. Check the rings as was described. It is alos for unusual for compresion to be intermittent. Can you describe that a bit better?

 

[emckelvy]

Re: 1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

There is no such thing as an "intermittant loss of compression". You either have compression or you don't. It doesn't go away and then mysteriously come back!

More likely would be an intermittant loss of function of the compression gage (or maybe the mechanic!). IMHO you need to find another mechanic.

What are the symptoms that would lead the "mechanic" to this sort of conclusion and what evidence was provided to you which would confirm their diagnosis?

BTW they're jerking you around on rebuild cost, $4000 is a ridiculously high price. You can buy a late model motor like that all day in the $2000 dollar range and I know places where you can get a completely rebuilt powerhead for much less than $4K.

HTH..........ed

 

[Duddiesbuddy]

Re: 1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

Thanks for the quick response...

I'm not sure of the prop size, but WOT is about 5500 to 6200 RPM. This motor is one of a pair, pushing a 22' Starcraft Cuddy Cabin. Both motors were running great until a recent trip to Catalina, when the starbboard motor would not pull under load, or achive more than 3000 RPM. During a cylinder balance test while under way, it was found that cylinders 3 was weak, and 4 was non fuctional. A week or so later, the motor was started and it was firing on all 6 cylinders and was able to pull through the full RPM range, perfromed a "finger" compression test and all cylinders had compression, the RPM of all cylinders dropped equally when cylinder balance test was performed. Later the same day had the loss of power, no RPM drop from 4 and little or no drop from 3, pulled the plugs and felt little or no compression from either cylinder. This engine has never been overheated, or abused...

I will try and pull the intake plate off this week and get a look at the piston rings. There doesn't seem to be any damage to the cylinder walls, or pistons, and there hasn't been any debris on the plugs, maybe just a little more fuel than the others. I was thinking sticky reed valves, but it's an overhaul to service them anyway, right?

Thanks for all the advise!

 

[Chris1956]

Re: 1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

To get at the reed valves on that motor you need to pull the powerhead and open the crankcase.

Perform a proper compression and spark test, and post back the results.

 

[arsenalpsu]

Re: 1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

What do you guys thinking about this.......If the 2nd carb is running rich maybe the rings are choked up causing them to seal OK some times and fail others? Maybe a good decarb is in order as the old none-keystone rings tend to get stuck and full of carbon after years of running....just my thoughts.

I'm rebuing my 84, 115 right now and on 2 of my pistons the top ring was broken. BUT they still had compression of 100 & 115, I decided to rebuild due to intermittent knocking. I'd pull the covers as stated above, if she looks ok try a good decarb with seafoam & then check compression again

my 2 cents.....

 

[Duddiesbuddy]

Re: 1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

Pulled the motor apart to find that #3 piston has been scored, but I don't see any reason why. I assume if it were running lean on #3 it would have also damaged #4 since they're are fed by the same carb. This motor has a water pressure fitting screwed into the cylinder head between 3 & 4, but I don't see any cracks/leaks. From surfing the web and looking for parts, I've seen multiple 115's with #3 cylinder damaged. Anyone know why this seems to be a common problem, and how can it be prevented???

Thanks again!

You guys rock

 

[Duddiesbuddy]

Re: 1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

Can you recommend any good parts sources?

 

[Droll]

Re: 1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

From surfing the web and looking for parts, I've seen multiple 115's with #3 cylinder damaged. Anyone know why this seems to be a common problem, and how can it be prevented???

Lugging this engine WILL kill #3, prop it for 5800 at WOT ( you + fuel in the boat ).


Arne Kjetil

 

[xcelerator]

Re: 1985 Mercury 115hp 6cyl 2 stroke - Intermittant compression

These engines love to burn pistons. There is an "idle stabilizer" that controls timing that is linked to coil #3. If this unit fails, it can cause mis-timing on your 3rd cylinder and poof, bad piston. The most common problems are under-revving or loading, water in the gas/dirty carbs. If your timing is right, carbs are clean and in sync, oil/gas is right, spark plugs are fine, all hoses are inspected and tight, and your prop is right, then your engine should run fine. These engines love to blow, believe me I've rebuilt about four of them. I think the older 6 cylinder mercs with the old distributor type ignition are more reliable... talking back to the 60's , the merc 850s, and 1000s, nice sweet motors. The ADI ignition and newer engines are fine, just maintain them, and service them, and they will run.