I bought a 76’ Tri-hull with a 115HP Inline-6 outboard. The engine had been rebuilt but the boat had been stored for about 10 years. After cleaning entire fuel system and running the boat it would act as if it is losing power after about a minute at WOT (wide open throttle). The fuel ball was hard and so it wasn’t a fuel pick-up problem. I took it to a local boat mechanic and explained my symptoms. He pulled the plugs and the top 2 spark plugs had aluminum specks on them. He said this is due to over heating, and probably a result of a bad water pump impeller, most likely caused by the motor being started out of water. I replaced the water pump impeller but unfortunately had the same result. I then pulled the power head apart and sure enough the top two pistons were melting down. I had the block checked out by a local machine shop and he verified that the engine had been rebuilt as I was told. The block checked out good so I deglazed the cylinders and cleaned the power head, and installed two new pistons in the top two holes. A fun winter project I might add! After reinstalling the power head and mixing a fresh tank of gas 25:1 as per the good book. I took the boat to the water for some break-in time. After warming the engine up and cruising around for a few minutes, I brought the heavy old boat up to about ¾ throttle, and after about a minute or two, it started losing power again. I checked and adjusted my cables to insure full throttle, yet time after time I had the same results. Fearing the worst, I pulled the top spark plug. It was clean, no signs of aluminum. I then pulled the 2nd plug and it had aluminum specks on it. If I waited about 10 minutes, the engine would take rite off and accelerate up to WOT, for about 1 minute and then lose power again. Or if I was impatient, I would dump a little straight 2-stroke oil in the hole and off we would go again. For about 1 minute. I don’t want to write a book here, but does anyone have any ideas of what the problem could be that’s causing the #2 piston to heat up or seize in the cylinder. Could it be a timing issue or carburetion, and if so the later, then why no problems with the first cylinder, it shares the same carburetor?
Thanks for the info, I intend to put a timing light on it first chance I get. When I had it on the water before, you could put your hand all over the power head, and it was cold to the touch. The water pump indicator line has a good steady stream of water rite from start-up. I know that doesn't tell me much other than that the water pump is actually pumping water. Although I replaced the pump impeller before I rebuilt the power head. Would a plugged jet cause the cylinder to seize and /or over heat? Also could the lean condition also cause it to start eating up the aluminum piston?
Thank you for your reply Chris1956. I will pull the carbs off and go through them. Can I assume max timing being no more than 21º BTDC, that this is including total advance ie.. WOT, and not at an idle?
Max timing is 21* BTDC. This is achieved prior to WOT, but the Max Spark Advance screw will hold it at the desired advance.
There are no other spark advances on the motor.
More oil will only smoke more and foul plugs more. With a quality brand modern 2 stroke oil you should not need more than 40:1 at most, and most run 50:1.
Lean mixture will do this as well as an air or vacuum leak behind the carb. Make sure you are getting enough fuel pumped to carbs, it pulses but delivers a fair amount. The fuel system needs to be in good working order on these inlines.
More oil will only smoke more and foul plugs more. With a quality brand modern 2 stroke oil you should not need more than 40:1 at most, and most run 50:1.
Lean mixture will do this as well as an air or vacuum leak behind the carb. Make sure you are getting enough fuel pumped to carbs, it pulses but delivers a fair amount. The fuel system needs to be in good working order on these inlines.
Im with Chris on the oil mixture, after a rebuild heavy on the oil and easy on the throttle, no constant speeds. They are gapless plugs, they clean easily.
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Mercury inlines....nothing else stacks up
The answer to "do i need to change my water pump impeller" is YES
Sorry I missed the second rebuild, yes extra oil is proper for the first tank breakin. It helps mechanical issues but not mixture issues. Of course if that cylinder is not getting gas it is not getting that oil either.
Thanks for the info gentlemen. I took the advise of Chris 1946 and thoroughly cleaned the carbs. I used new gaskets and fuel lines for all 3 carbs. I took a fresh can of fuel mixed at 25:1 and put her on the water. All though it is pretty hard to get started when the motor is cold, it ran quite a bit better. I let it warm up about 5 or so minutes and then slowly increased the throttle up to about 3/4 throttle. The heavy sled planed out good and cruised nice and smooth at about 25MPH for about 5 to 8 minutes. Then it acts as if it looses a couple of cylinders. By this I mean that the power just drops off. After waiting about 10 minutes, I tried it again; and of course I had the same result. I pulled the top 3 spark plugs to see if I had any sign of piston melt down, and there was none. There were good signs of oil on all 3 plugs, notably due to the rich oil mixture, but no aluminum on the plugs. It idles a little rough, but sounds and runs real good when it is cruising out, but then drops off on power and rpm's. Some times if I just keep it WOT it will sound like it is trying to gain in RPM, and it will smooth out again. The fuel bulb is hard during these symptoms and it seems as though if I let it sit awhile, it will take off nicely again for about 5 minutes. Could it be the coil, or would I have these symptoms all the time if that were the case? It has the type 2 Ignition in it, so no points to mess with. Is the switch box a possibility? Again it has good water circulation and a newer water pump, so I don’t think temperature is the problem. One thing that I have noticed is that my tachometer isn't reading correctly at any speed. I assume that that could be a wire, although I’ve checked to make sure they are all tight. How can I test to see if the fuel pump is pumping enough fuel, and is there away that it might be my problem? Also, what is the best way to check the timing with a timing light?
Pumpinf the fuel primer bulb while cruising will tell you if the fuel pump is bad. There is a link and synch procedure(see the stickys) which will help you set the idle pickup and max spark advance. That motor has three screws above the distributor. They are for max spark advance, max throttle and idle stop. There is a brass collar around the midsection of the distributor. Two 5/16" bolts hold it tight. This is the idle pickup timing adjustment.
With the type 2 ignition (pointless), is there an ignition module or is that part of the switch box? Could the switch box or the coil be the cause of the engine dropping off after a matter of minutes?
76' Mercury 1150 Loss of Power
Linear Mode
