1986 Merc 115 burns top plugs

[frozenokie]

After installing new plugs again, and running at WOT for about 10-15 minutes, the top two plugs run too lean, the middle two are fine, and the lower two are really wet. I can run anywhere from idle to about 3500 rpm all day long. But, as soon as I hammer down, I can hear when the motor leans out the top cylinders after about 30 seconds. NOT FOULED, but burned.

Everything has been replaced earlier in the year (fuel pump kit, carb kits, fuel lines, impeller and pump). Fuel premixed at a little more than 50:1 for safe running. I've been working on this motor for 7 months now and this is the last problem I seem to have. Other than this - it runs perfectly with no hesitations or stumbles.

I had some advice: re-adjust the top carb float drop to be a little lower to allow more fuel and correct the burned plugs and re-adjust the lower carb fuel drop to be a little higher to correct the wet plugs. Is this what I need to do? Could the fuel floats be sticking on their posts? What's the best way to assure that the floats do not stick?

I currently have all three carbs set to the shop manual settings.

 

[Laddies]

Re: 1986 Merc 115 burns top plugs

Could be a top seal leaking air but they usually drizzle oil, the inly things that could cause it would be lean' heat or timing. I would chech water flow first, pump housing air leak under the housing letting exhaust in the water pickup or obstructed water passage.

 

[frozenokie]

Re: 1986 Merc 115 burns top plugs

Hey thanks Laddies. I've gone round and round with this problem.

Water flow is good if not great. All water pump parts are new including the impeller which was installed by a mechanic before I bought the boat. Water coming out has never been too hot to the touch - maybe luke warm at the most.

Timing was set to the shop manual which is 20 deg BTDC w/idle stabilizer disconnected to achieve 18 BTDC with the throttle wide open and 6 deg BTDC with the throttle shut. Both settings achieved at cranking speed and not with motor running. The motor starts, runs, and accelerates perfectly so could it be out of time and still run this good? Is there another way to determine (maybe with the motor running) if the timing settings performed are incorrect?

Drizzling oil??? Where do I look for this? In with the water from the pee hole?

 

[Laddies]

Re: 1986 Merc 115 burns top plugs

When a upper seal is leaking the oil drizzles out from underneath the flywheel. sometimes a water system that is leaking exhaust won't show as when it's idleing the pump intake is below water but when the boat is on plane the water drops in the exhaust cavity and the gasket under the pump will let exhaust into the system. Also we always remove the idle stabilizers as the cause a lot of problems.

 

[CharlieB]

Re: 1986 Merc 115 burns top plugs

If the top carb leans out and they have all been set to factory float levels, then you have a weak fuel supply, not able to pump fuel high enough to fill the top carb when under load. HP takes fuel, lots of it.

You are fortunate not to have melted a piston yet!

Verify fuel pressure at speed, also install a clear section of hose immediately after the pump to visually inspect for air bubbles indicating an air leaking into the fuel line between the pump and the tank.

Check pump output flow into a can, cranking, you should have a strong pulsed 5/16 diameter flow, no weak dribbles or you will toast pistons.

Pull the tank dip tube , check for gunk restricting flow.

 

[frozenokie]

Re: 1986 Merc 115 burns top plugs

Laddies & CharlieB - Thanks for the info and I will be checking into all mentioned after the 100 degree heat subsides

I'll be looking into the fuel delivery first as a starting place

As stated, I'm plenty rich on the bottom two cyls and perfect on the middle two. Is this where you're going with the "fuel not sufficent" in the top carb theory?

 

[BM]

Re: 1986 Merc 115 burns top plugs

Another alternative is to fit one size larger main jets to the top carbs. This is a regular "safeguard" move for these engines anyway.

I rebuilt a customers 86 model inline 6 90hp and after a couple of hours he had burned the same piston again. 2nd time around the mill, I knocked his WOT timing back 2 degrees and fitted one size up main jets. Test ran it myself for 2 hours (up to WOT) and it didn't miss a beat.

Of course his elaborate fuel system (very non standard) would not have been helping and his prop was a couple inches higher in pitch than I would have liked too. But you can only tell people what they need to do, can't make them do it

Cheers

 

[Chris1956]

Re: 1986 Merc 115 burns top plugs

Froze, You really need to address the cause of the issue, or your motor will be toast. Have someone pump the primer bulb while running and see if that improves the appearance of the top cylinders. Make sure there are no air leaks around the carb base, top end cap or crankcase. Spray some mixed fuel around the mating areas of these areas and see if the motor changes speed or sound while running.

Make sure the inlet needles are the correct ones. There were some updates and some that were pretty close, but longer or shorter than the correct ones.