77 Mercury 115hp dies after 5 minutes

[TravisPeters]

Hello...first post here so I hope someone out there is more informative than the guys at the boat shop that have been working on this motor!<br /><br />My grandpa has a 1977 Mercury 115 h.p., inline 6 cylinder. Last year the motor would die after 5-10 minutes of running. When we would pull the plugs we found what we thought was carbon, but probably was aluminum all over the #2 cylinder. We would clean the plugs, the boat would run like a dream for 5-10 minutes and again the same thing would happen. Eventually it got to the point where it wouldn't run at top speed at all, but would only run at 25% power, even at full throttle. We took it to a shop and sure enough they said that the #2 cylinder had no compression... $1,300 later after a rebuild we finally get the boat back to the lake. They told us to double the oil for 10 hours, and "baby" it in that time...which we attempted to do. The engine would barely run from all the oil...it was a challenge to get the 10 hours on it, but after we did and went back to the 50:1 oil it ran better....however again, once we ran full speed for 5-10 minutes the engine would slowly slow down...to the point where it would barely run. We took it back to the mechanic...they claimed the plugs were fouled (yeah, but we replaced them at the lake and it still did it...) and they replaced the recitifier...claiming that the boat wasn't getting 12 volts, and that is what was causing the problems. <br /><br />I took it out AGAIN on Sunday and it took off from the dock like a champ...ran for about 7 minutes at full throttle and again...it powered down...eventually dying and wouldn't restart. We used the trolling motor to return to the dock.<br /><br />OK...HELP! Does anyone have any suggestions where to look? I am convinced it's electrical...it seems like as soon as the engine gets good and hot...the problem shows up. I pulled the plugs on Sunday and they looked great, no fouling or anything. The engine sounds wonderful while it runs wide open, and doesn't sound bad while it's running after loosing power...however it sounds like there could be a knock in it...but it's very faint. <br /><br />I know just enough about these motors to be dangerous, but not knowledgable to diagnose something like this. Does anyone have a clue? Could it be a fuel pump...which if it were not shooting fuel it could be the culprit from the original problem which lead to the #2 cylinder being burnt out? <br /><br />Please help me! I didn't want him to sink $1,300 into a motor that is almost 30 years old in the first place...but at 77 years old he couldn't justify $10,000 on a new outboard either. <br /><br />Thanks in advance!

 

[ronmold]

Re: 77 Mercury 115hp dies after 5 minutes

Odd they replaced the rectifier for a ignition problem because the ignition is not connected to the 12v charging system, it is a self-contained system only using the key switch to ground out the blk/yel lead to the switchboxes to kill the motor. The switchbox is likely to be the culprit. The are electronic and prone to intermittent failure - often heat(time) related.<br />If it is original it's well past its time.

 

[andrewkafp]

Re: 77 Mercury 115hp dies after 5 minutes

You need to get Clams Canino on here.. He has moved and has his work cut out at the moment, but would be among the top T.O.P guys. Rodbolt is also pretty cluey on all makes.<br />Good luck.

 

[mirage500]

Re: 77 Mercury 115hp dies after 5 minutes

hot, slowing then dying. I hope its not seizing even though it has plenty of oil. Too tight clearances maybe?

 

[TravisPeters]

Re: 77 Mercury 115hp dies after 5 minutes

No...there is no seizure, although when we first had this problem there was, which ultimately lead to the rebuild. The guys at the shop blasted us for even complaining about the motor after the rebuld, stating that we need to break it in, and we should expect it not to run at 100%. That may be so, but this thing wont run at 25%. I don't think it's too tight, in the 5 minutes that it does run it is awesome...but slowly it's almost like running out of gas, a few sudden drops in h.p., then it seems to almost idle back down to about 25%...

 

[reeldutch]

Re: 77 Mercury 115hp dies after 5 minutes

you might want to check the carbs give them a good cleaning and put new needle and seat in.<br />go from there.<br />you dont want to run your cylinders lean or dry.<br />i just rebuild a 115 v4 evi witch had a lean running condition caused by dirty carbs.<br />on top of that the waterflow deflectors were swollen up and the former owner took the tstats out to let it run cooler.<br />DISSASTER scenario.<br />make shore you are not running lean.<br />make shore you got enough waterpressure.<br />i will soon start a rebuild on a i6.<br />i got it for free so hophely the teardown will go smooth.<br /><br />good luck reeldutch

 

[TravisPeters]

Re: 77 Mercury 115hp dies after 5 minutes

I don't think it's a carb problem, or else the motor would never run good. Like I say, it runs great...but just for 5-10 minutes. There is a good stream of H2O from the pee-hole, so I know it's pumping water and not overheating.

 

[Chris1956]

Re: 77 Mercury 115hp dies after 5 minutes

Travis, The rectifier charges the battery only, and does not have anything to do with the ignition system. You need to check the ignition system when you have failure. It sounds like it is heat-related. First check to see that there is always +12V on the red terminal on the switchbox. The white wire on the switchbox is connected to the ignition key switch. Make sure there is +12V on that terminal when the key is on. If these tests pass, put an inductive timing light on the high voltage wire from the coil to the distributor cap. When cranking the motor, you should see 6 sparks/revolution of the flywheel. If not, the coil or (more likely) the switchbox is bad, due to heat. If this test passes, put the timing light onto the spark plug wires(one at a time) and crank the motor. You should have regular spark on all wires. If not the dist. cap/rotor may be bad. If all these tests pass, perhaps the motor is fouling plugs, due to bad float settings or fuel inlet needles and seats.

 

[ross_larter]

Re: 77 Mercury 115hp dies after 5 minutes

Are you sure your fuel tank is venting ok? try taking the fuel cap off and see it it runs ok.<br /><br />ross.

 

[Bradg]

Re: 77 Mercury 115hp dies after 5 minutes

Make sure the fuel pump is ok. My brother has a 1977<br />85 Merc and he had a similar problem the motor would run great for 10-15 mins then would lose power to almost an idle. Two Shops and $800.00 later still<br />had the same problem(included fuel pump repairs).<br />The third shop found the problem it had wrong fuel pump kit in it $160.00 later and it has not missed a beat.