1980 Merc 115 6 Cyl, low compression in lower cyl

Hi - I have a 1980 Merc 115 that was giving me all kinds of problems last summer. When it ran, it ran great. However towards the end of the summer, it would throttle fine at the dock but get it out on the water and increase throttle to mid-way and the engine would die. It would then be a really b*tch to start again.

I took it in to a local mechanic who took a look at it. He first suggest of course a carb clean/rebuild. Anyway he says that the compression on all cylinders is about 120 but the lower cylinder is at 50.

So...what to do? Is rebuilding one of these worth the cost? Can something like this just be repaired? Would you just go out and start looking for a replacement motor?

Any thoughts would be really appreciated!

Re: 1980 Merc 115 6 Cyl, low compression in lower cyl

I thik I would confirm thelow compression on the bottom cylinder. Then remove the lower transfer cover (starboard side of motor) and inspect the cylinder and rings. The cylinder will normally have scratches, if the compression is that low. If the cylinder is smooth, perhaps the rings are stuck and a good decarbonize can free them up.

Re: 1980 Merc 115 6 Cyl, low compression in lower cyl

hey man i have the 85 version of that engine,i have had the exact same issues with mine,first of all your problem may be comming from a few things, the power packs may be bad or getting ready to go bad, i replaced my power packs and the engine ran much better,not great but better,i wasnt happy,so i then replaced the impeller,and the water pump,cleaned out the carbs,put a new gas pump on it,and the engine finally runs like brand new,
there is a place in connecticut where you can buy 2 power packs for that engine for $100, he will mail them to you, hes got lots of them,search online and you will find the place im refering to,unfortunately my computer got stolen and i no longer have the contact number but thats how I found it originally, after you get the power packs get a water pump and impeller ( under $100) install them,
then run the boat on the water,I guarentee she runs like a dream, if shes still sluggish,have those carbs cleaned,and use mid grade 89 octane gas, dont use regular gas,and use stabile gas stabilizer with each fill up,believe me it makes a huge difference.....

also with these tower of powers,the oil must be mixed exactly to attain optimum performance level,make sure you are using a 50:1 mixture,go to west marine and get a measuring tube,only costs $4 and it will help mix precise oil to gas mixtures.

Re: 1980 Merc 115 6 Cyl, low compression in lower cyl

Merc's do not have Power Packs. The boxes that produce the power for ignition are called "Switchboxes".

Re: 1980 Merc 115 6 Cyl, low compression in lower cyl

Thanks so much for taking the time to reply. Your replies have been really helpful. I'm going to try replacing the switchboxes and we'll see where we are at!

Re: 1980 Merc 115 6 Cyl, low compression in lower cyl

What about the bad compression??? I would confirm that 50 before spending money on a switchbox. The switchbox is not the reason it's so low, in case that's what you were thinking or what was implied above.

Re: 1980 Merc 115 6 Cyl, low compression in lower cyl

Don't spend a nickle on anything. Like chris1956 says........

Check the cylinder walls
Check the rings
Check the piston.

These are the things you must have for decent compression.