79 merc 115hp......compression issue????

[spaceghost]

I just bought a 79, 115, for my boat, seller advised motor ran great when he took it off, ok, Im thinking just install , fill up Go.... So thats what I did . After getting it started. (alittle ruff), it ran about 25yds perfect, then all the sudden shut down, started smoking, I think it got hot, water pump worked it shot out the back end fine. after getting it to shore I noticed my #2 plug was fried totally gone, #1 plug dry< all other plugs normal. So I thinking i had carb problems on the #1 carb, not getting the top two cylinders enough gas or oil??? 85psi on top 2 cyl. 125 on bottom 4cyl...what now can it be saved???? thanks for any advise.....

 

[briannh1234]

Re: 79 merc 115hp......compression issue????

85psi is low. Anything can be saved but is it worth it? (When comparing against a new motor it's always worth it.) If your looking at the motor from behind your boat, on the right side you will find 3 transfer port covers. Remove the top two, and it will give you a small view of what is going on in there. If piston 1 and 2 are toast you should be able to see the damage.<br /><br />There is a remote posibility that carbon got the rings stuck on 1 and 2, but not the rest??. Do you feel lucky?<br /><br />Carb and fuel issues are the most common problem with these motors. Once you have it running correctly you may want to use seafoam on a regular basis to keep the carbs from gumming up on you.<br /><br />Good luck.

 

[spaceghost]

Re: 79 merc 115hp......compression issue????

thanks for the advice , Ill look into it. so maybe the motor is salvagable......???

 

[spaceghost]

Re: 79 merc 115hp......compression issue????

anybody else have any helpful advise? this motor has only one cover one the right side, head gasket cover one piece. currently letting the motor set with the 2 top cyl. full of mercury marine, as to let is set and soak , maybe let the piston and rings loosen up abit and get some compression back????? whatta think???

 

[Mnemonic]

Re: 79 merc 115hp......compression issue????

How long has the motor been sitting? Piston ring stickage or rustage is a definite possibility.. <br /><br />Spray some fogging oil on the sylinder walls (Use a liberal amount) turn the crank by hand and make sure you get the entire insides of the low compression cyl's. Let it sit for a day or so.... <br /><br />Then pour a tablespoon of 2 cycle oil in the low cylinders and retest the compression. If it is back up... congrats, run the motor and do a decarb.<br /><br />

 

[Bob H.]

Re: 79 merc 115hp......compression issue????

Engine is likely toast. If the engine had been sitting a while, a few thermal cycles can usually restore some compression - but not enough to bring it up to your other cylinders (Strike 1). Does it rotate freely by hand without making noises? From you discription of the plugs the carb is clogged and it ran lean (Strike 2). Unless you have a good tow plan you may want to open it up and check for carnage. 40psi difference between cylinders = rebuild time.

 

[briannh1234]

Re: 79 merc 115hp......compression issue????

spaceghost -<br /><br />Check the other right side - the same side the starter motor is on. Should be 3 covers. Might have fuel pumps mounted to the bottom 2. Don't play with the big cover or the head gasket yet. (You will find the head gasket is just a water jacket cover - these things have no heads.)<br /><br />Try some marvel mystery oil. Good luck.

 

[spaceghost]

Re: 79 merc 115hp......compression issue????

thanks Ill check it out........