1999 Mercury 115hp Outboard (2+2 cylinder setup)

[MPKehoe09]

My recently purchased 1998 170cc Osprey has 1999 Mercury 115hp 2-Stroke.

Is anyone familiar with this motor? It's an inline 4 cylinder outboard that has a 2+2 setup.

It runs on 2 cylinders till about 2000 RPM's then engages the other 2 cylinders. Research says it's great for trolling, as the motor is only using 2 cylinders, for better fuel efficiency. However, I'm trying to get use to opening up the throttle to get onto a plane.

When I took the boat out earlier last week, as I would throttle up, after I hit a certain point, it seems as if the throttle "jumped" to a much higher RPM, pretty close to WOT. It didn't seem to be a gradual increase in RPMs/power. Unfortunately, the tachometer isn't functioning correctly, as it says its running at 7500 rpms, which I know is not right. Is this pretty typical of a 2+2 cylinder outboard? Not quiet of a smooth transition from 2 to 4 cylinders as you throttle up?

I have even compression in all 4 cylinders, and I quickly messed around and took alternating plugs out to see if certain cylinders were misfiring. With one plug out, it definitely sounded to be running rougher/harder as I throttled up. I didn't have time to check each cylinder independently, but each cylinder seems to be firing.

Anything you'd recommend testing myself before bringing to my mechanic to look at? Or is this just the nature of a 2+2 outboard?

[fitz73222]

Hi MP,
I have twin 2+2 115's on my 22-2. To own them is to love them or not. If you like Harley's, you'll love these motors. A tinkering mans dream you might say. The transition between 2 and 4 cylinders is nothing more than than "bip" at 1800-2000 rpm. Now if you think about it the engine is making 60 ish hp on two and then jumps to 115 on 4 so the power increase is fairly dramatic but reasonable. If you think the engine is transitioning at a higher rpm the first place I'd look is on the port side of the engine where the throttle cam contacts the accelerator pump. First look to make sure the diaphragm is not leaking through the plastic pump pin, if it is replace the diaphragm and pin assembly. Next, make sure the pump is set at the right height. Engine off, open the throttle wide open. See if you can place a .030 feeler gage between the pump body and the throttle cam. If too much or not enough clearance. Loosen the screws that hold the pump body and put the feeler gage between the body and throttle cam. Adjust the pump body to where the feeler strip slides with a small amount of tension. Not the plunger pin. This will set the correct accelerator pump height which will help with the transition from 2 to 4 cylinders. I've never seen engines that will get down and idle like these will. Set them at 650 +\- 25 rpm and they will crawl like a 10 hp. On my floscan it shows .9 gallons per hour at idle which is like crazy low fuel consumption to troll with. This is 4 stroke world. I've spent many hours playing with mine so I'd be glad to help. Your engine is a much improved version over my 1995's, better ignition system and simplified fuel delivery for the 2+2 system. They made these from '89-'05 and actually worked pretty well once you get them set right, fuel mixture on the two top carbs and idle timing and the do purr.