1978 Mercury tower of power 115 bogs under load

ak.man1

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So I was cruising at 4500 rpms at about 30 mph in my 20 aluminum and all the sudden the engine starts cutting out and then bogs down. I throttle back to 2000 rpms and stil sounds rough but running. WOT and it acts and sounds bogged. Put it in neutral runs and revs great. Idles a bit rough and wants to die when put into gear but I have been working on getting the idle mixing screws just right. Anyone have any idea why this would happen all the sudden? I bought a fuel pump rebuild kit and new fuels lines around the engine and will be installing this weekend. Tank and lines outside are all brand new.

Any ideas would be appreciated. Thanks
 

CaptnKingfisher

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Spark is an easy and free thing to check. Id start there. I'd also inspect the prop maybe take it off and see how much fishing line you might have caught up in there. If the tanks and lines are all brand new and presumably new gas/oil, then I doubt anything got sucked into your carb jet unless you have foam falling apart under the cowling, then maybe I'd recommend pulling your carbs and looking for dissolved foam goo in your orifices.

Just random ideas from a novice.. but I always look at easy things first. Check the prop first, check spark on all 4 to jump manual specification, check compression, then report back!
 

ak.man1

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Thank you for the response! I will tinker this weekend and report whatI find. I plan to give it a seafoam treatment as well just in case it's carbon build up from trolling for hours the other day.
 

Chris1956

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Take a compression test. You can lose compression in a cylinder (usually #3 or #1) and she will still run, but roughly and power will be down.
 

ak.man1

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Ok. Here is an update. Rebuilt fuel pump. All new fuel lines. Did a few cycles of seafoam. Still running rough. Compression test was 125-135. Pulled plugs and did a spark test and passed. But I did find 2 plugs that had build up on them that looked like grout? These were brand new plugs with less than 5 hrs on them. Through some spare plugs in and ran on the hose much better. Plan on lake test tomorrow. Anyone seen a new plug do this? Post won't let me download a pic.
 

Chris1956

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Was deposit soft or hard? If hard, it is likely aluminum and very bad. if soft, likely motor needs decarbon, so seafoam treatment is good idea.
 

tlewick1

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Feb 29, 2016
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Which two plugs were they? You said it was an inline? If it was the upper plugs, you could have a clog around the powerhead where water is supposed to run around and cool it. They could be overheating, and the grout-like crud could be aluminum melting. Look inside the spark plug holes and see if the pistons look okay or melted.
 

Chris1956

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if you remove the transfer covers on the starboard side, you can see the piston skirt on the intake side and the top of the piston. If the piston top looks misshapen, it has been melting, and likely causing your deposits.

Unfortunately, the exhaust side of the piston suffers the most damage, and it is hard to see w/o a teardown.
 

ak.man1

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Thank you for the advice guys. I could not get a good look through the plug hole so I took off the transfer covers. I do have the same residue in both #2 and #4 pistons. It is coming from the very top of the pistons. This residue is on small portions of the dome. Lightly scraping above the dome releases the residue. All other cylinders have some carbon but not the aluminum flakes. So any ideas why all the sudden at 4500 rpms these 2 cylinders would do this? Motor was peeing and did not seem to be hot when I touched the block looking for issues. Are these pistons shot or if I figure out what is the problem can they still run ok? I appreciate all of your time sending the responses. This is a real drag as I just got this boat and 1st time I took it out the starter motor through the nut and spring, second time it trolled fine for hours then wouldn't idle. Adjusted my idle mix screws and seemed to run fine until 30 mins into our 3rd trip. Cant a guy just take his kids fishing? Well I guess a 1978 motor is bound to give me some grief. Ha ha
 

racerone

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Have you replaced the waterpump impeller ?---That is the first thing to do on a new aquisition !!----A cylinder running hot will lead to oil / lubrication problems.-It may be too late now.--Possible it was pre-ignition too and that causes hot pistons.---Clean the carburetors before running it again.
 

ak.man1

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I had a receipt from last falls (2016) service that had new water pump installed do I didn't check it.. But I will pull the lower unit and check the impeller. What do I check for pre-ignition? Am I to take the carbs off to clean them or was the seafoam treatment sufficient?
Thank you
 

Chris1956

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Yes, it sounds like pre-ignition. Unusual on even numbered cylinders, since they tend to run a bit richer than odd numbered cylinders. I would check max spark advance. It should be 21*BTDC. Also check cooling water pressure and carbs for clogged main jet or float set improperly.

If everything checks out, maybe you got inferior fuel or are using insufficient oil in fuel mix.
 

merc850

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A 115 is not a fishing motor and long periods of slow speed running can cause carbon build-up that leads to pre-ignition and clogging of the exhaust tube in the LU.
Do you have a tach? if not the engine may be overreving or under performing - 5000 to 5500 is the full throttle range.
 

ak.man1

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My water pressure runs low about 5psi at 4500 rpms but block not hot to the touch. Prop is a 19 pitch but should probably come down as I have not been able to get to 5000 rpms. Only had this boat out briefly a couple times so very hard to know it's character.I don't know much about carbs and checking tolerances. But I can learn.

The local mechanic here said I am wasting my time and money figuring this out. Once the pistons start detonating the engine is shot and it's a 78 so too expensive to rebuild. Is this your guys opinion?
 

ak.man1

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Also I can't seem to find a gasket match for the transfer covers that I took off. Can I use a permetex gasket maker?
 

Chris1956

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I do not think gasket maker is a good choice. Try crrowley marine to get the correct part number. You should be able to order it there, or elsewhere.
 

ak.man1

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Ok Chris1956. Do you have any thoughts with my post #15? Thank you for all your advice.
 

racerone

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A shop will say it is not worth fixing / makes no ECONOMIC sense.-----But if you can do the work yourself it is a different matter altogether.-----Keep in mind that you may need 6 new pistons and shop work for the boring / honing at say $ 50 a hole.----It also depends on the demand and availability of good used motors in your area !
 

Chris1956

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I agree with Racerone. A rebuild could cost $900 plus. You could buy a newer V6 for that kind of money, or thereabouts.

In my case, I broke a rod in my 1977 Merc 1500 inline 6, forcing me to buy a newer motor. I ended up with a '93 Merc 135HPV6, for $1K. It is a much better motor IMHO.
 
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