1984 Merc 115 compression test results-

[ab59]

I bought this motor to use as parts for another I was in the middle of buying but the guy just flaked so I'm trying to salvage this motor. Motor is complete the PO tore it down and used the crank out of another older 115 but the rest is the original parts with the exception of 3 new pistons .. I built a stand out of an auto engine stand and set it up to test , readings as follows---
#1- 118 #2-112 #3- 119 #4- 105*** #5121----#6 I haven't been able to get the @#$%%$#$$&^& test hose screwed into the block because of the lower cowling , will have to rig something up tomorrow. So then , my question would be about the 105 psi reading in #4 . This is outside the 10% recommendation from Mercury so is there something that I can do to remedy this situation ?? Should I worry about it at all ? I could use some knowledge from those who know .
The reason the PO gave for selling the motor after having done a lot of work to it was that he said that he got it to start and idle and run fairly smoothly but when he mounted it one his boat and went to throttle it up the motor would not develop enough power to plain his boat so he took it off and was done working on it .
I plan on giving it a good once over to see if I can find the problem without breaking the bank .
thanks , A.B.

 

[Faztbullet]

#5 and #6 likely had water intrusion from bad lower seals...runs ok on muffs but pollutes cylinders when in water. Need to pull transfer covers and inspect cylinders

 

[racerone]

Remove bypass cover and inspect #6 piston & rings.-----Also remove middle bypass cover to look at # 4 piston & rings.

 

[ab59]

Thanks for your replies -- This time I'm not going to bail off into a repair without a manual. What would be a good repair manual to get for this motor ? I have an original Mercury manual but only goes to 1978 . Looked on fleabay and all I really see are clymers . What should I look for that is worth buying.
thanks-Bruce

 

[racerone]

There really is not much difference between a 1978 and 1984 model as far as the mechanical bits.----Possible difference in electrical / ignition parts.

 

[ab59]

Ok , well , if you think it won't come back and bite me I'll use the good manual that I've got then. Here is a you tube video that I just made of me starting and running the motor a little while ago today.

https://youtu.be/Q79xxlvt5jU It starts and runs so I guess that is good news anyway.

 

[Chris1956]

A 1978 Merc 115HP will have battery distributor ignition. A 1984 Merc 115 will have ADI ignition.

If you Mercury manual has both 4 and 6 cylinder models, you can use the 115HP part of the manual for all BUT electrical components.

Use the 85HP part of the manual for the ignition and other electrical parts of the motor, as it will close to the same. The '84 115HP will have dual switchboxes, and the 850 one switchbox, but the theory and timing specs should be the same.

 

[ab59]

Thanks for you reply Chris 1956 -- Great , since this will be the first time that I have gotten into an outboard motor I need all the help I can get and a good manual is essential . I have removed and rebuilt the carburetors on my old 1974 85 hp Merc. but that's the extent of my experience. I appreciate the help .
thanks , Bruce

 

[ab59]

Just to note , after running the motor for a short time yesterday and re taking a compression test today I get these readings , much better that before.
#1@ 125--#2@120 #3@130 #4@125 #5@ 130 and #6@ 120+ . I am looking up how to remove the bypass or transfer covers to inspect the pistons and rings next.
Faztbullet--- when you say lower seals , you are talking about the lower unit right ? This has a jet on it currently but I don't know if it did when the PO put it on his boat and had the problems with power. Will I find water in these cylinders if the issue is water Intrusion ?
thanks----

 

[Faztbullet]

Water climbs the drive shaft .....sealsin bottom of block on crankshaft

 

[ab59]

Thanks for your reply Faztbullet-- So , that would mean I will have to pull the the motor from the lower cowling to replace those seals ? The PO did say that he used a crank from the other parts motor he had so maybe he used the old seal and it's not doing the job any more. If so I also read about some kind of gasket in that areal as well that seems to have a habit of leaking water into the lower cylinder as well and the temp fix is to fill the plug well with rv silicone until you pull the motor and replace the gasket. Not sure about where exactly that is either.
thanks,, Bruce