Need help with 115 Merc/compression

qmile92

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Jul 23, 2009
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2
Well, I found a potential good deal on a boat today but im a little concerned with the motor. Not sure what year the motor is but it is a 115hp inline six and is on a 1997 boat. I thought the inlines were out of production by 1997, what was the last year? The owner said that he thinks its the origional motor but is not 100% sure, boat has been sitting for about three months and was rarely used. The motor has no spark but that is alright with me because I know its fixable and its happened to me before. Heres the more serious issue, even though the motor wouldnt start I decided to do a compression test on it when I went to look at it, heres what I found. All cylinders were sitting at around 115-120 PSI with the exception of the #2 cylinder, it kept reading 80-85 PSI. This was a shot in the dark but we thought it might be due to the cylinder walls being a little dry, so we put a little 2-stroke oil in the #2 cylinder and the compression came up to 110 PSI. Anxious to see if it still holds that compression tomorrow afternoon. Really confused as to it being a good deal or not. What to do? Is it good? Is it bad? Any input would be greatly appreciated. Thank you!
 

CharlieB

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Joined
Apr 10, 2007
Messages
5,617
Re: Need help with 115 Merc/compression

You're right, that is NOT a 97 engine, it was old long before that boat was built.

However, the stacked 6's have a long history of being fine running motors, as long as they are taken care of.

Low compression on #2 needs attention, you shouldn't have had to oil it any more than any other cyl in the motor. As long as it isn't run that oil will remain there for quite a while and falsely give you a decent #, but do not rely on that to mean that it is sound. Plan on at least replacing piston rings, if not the piston also.

Merc used a number of different ignition systems on those 6's over the years of model/production changes. Troubleshooting a 'no spark' condition depends on which ignition it has.

Does it have a distributor or two ignition modules each firing 3 coils/cyl's?

Depending on the price and your willingness to get busy, it could be a pleasant adventure, then again it could be a nightmare, hard to assess with lilmited information.
 

qmile92

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Jul 23, 2009
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Re: Need help with 115 Merc/compression

Yeah sounds like too much candy for a nickle...oh well the search goes on. Not sure about the ignition system, had large electrical distibution blocks on the right side and I dont think it had a distributor. Sounds like a good project for the next owner. Thanks so much for the help.
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: Need help with 115 Merc/compression

Do your homework, read the free outboard ignition troubleshooting guide from CDI Electronics, test the ignition, price a gasket set and rings.

That old 115 will pull the ***** off my newer 4 cyl 115.
 
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