1986 Mercury 115 2-stroke: RPM. fluctuate at WOT

[EricPontoon]

I am a first time boat owner and recently purchased a 24 foot Godfrey Sweetwater Pontoon boat with a 1986 Mercury 115 2-stroke (inline 6 cylinder). I took the boat out on the water for the first time this past weekend and had two issues, which may or may not be related. A little background on the motor. The engine had a fuel mixer which has since been bypassed/removed with the previous owner electing to use mixed fuel instead. The previous owner let the boat sit all last season. There was almost no gas in the tank at all. I filled up tank with premium, and the 50-1 mercury brand two stroke oil. I also added a few ounces of the "Quickcare" fuel treatment. I used about 10 gallons of this gas already, so I feel confident that old gas cannot be an issue.

Issue 1: The first issue, is that the boat stalls when shifting into gear from neutral and sometimes when driving at minimum speed. I found that if I rev up the engine and then drop it into gear, it will not stall. Obviously this can't be good for the engine. Also, makes it super tough to trailer. After reading other topics related to this issue, I think that I may be able to fix this issue by increasing the idle speed. However, if this issue is somehow related to the second issue, I am not sure.

Issue 2: When I drive the boat at WOT I am getting right around 5,000 RPM. However, sometimes with no change in throttle, the boat will slow down to about 3,750-4,000 RPM. Then after a moment the engine will rev back up and the boat will speed back up. The engine does not stall when this happens, just a momentary slow down before speeding back up. It does not appear that this happens at mid range engine usage. (although I am not sure I drove for any extended time at mid-speed). What might be causing this engine fluctuation at WOT? Fuel Pump?

 

[Chris1956]

I don't think the issues are related. They are both common enough for Mercs of that kind. The high speed fluctuation issue is likely a fuel pump issue. I recommend you rebuild it.

The poor idle could be that it is set too low. You may turn it up a bit, using the idle stop screw on the throttle arm. Normal idle RPM would be 850-900 in neutral. If that doesn't solve it, you will need to clean and rebuild the carbs.

 

[racerone]

Open the low speed jets 1/8 th turn at a time to see if stalling goes away.----------You have installed a new waterpump impeller I hope !

 

[EricPontoon]

I don't think the issues are related. They are both common enough for Mercs of that kind. The high speed fluctuation issue is likely a fuel pump issue. I recommend you rebuild it.

The poor idle could be that it is set too low. You may turn it up a bit, using the idle stop screw on the throttle arm. Normal idle RPM would be 850-900 in neutral. If that doesn't solve it, you will need to clean and rebuild the carbs.

Chris1956,

Thanks for the advice! Is the fuel pump rebuild a reasonable task for a rookie? Or should I have a professional do this? Do they sell kits with all the replacement parts (gaskets and diaphragm) that I would need?

Eric

 

[EricPontoon]

Open the low speed jets 1/8 th turn at a time to see if stalling goes away.----------You have installed a new waterpump impeller I hope !

racerone,

I am not sure what is meant by low speed jets. Is this the same as the throttle speed adjustment / idle stop screw that Chris1956 described?

I have not installed a new impeller yet. Previous owner said he had one installed every year-every 2 years. Engine is peeing a steady stream and did not get hot, so I think that the impeller is fine for now based on those facts.

 

[racerone]

There is a mixture screw on each carburetor.---Look for it on front of carburetor.--Classic for these Mercury in line motors to idle very nicely .----Then they stall in gear because they are set too lean for acceleration in gear !!

 

[Watermann]

These motors are super simple to rebuild the fuel pump. The only mystery will be where the 2 thin cork gaskets go and they go under the inlet valve and outlet valve which is held down by 2 screws inside the pump housing. The valves are just gently pried out after removing the 2 screws and the retainer. Just be sure to have everything clean, I used a can of carb cleaner and an air nozzle on my compressor.