Mercury 115 rough?

Hi All,

Looking at buying a 16ft fiberglass ski boat with a 2002 Mercury 115 two stroke on it (115EXPLTO). The engine starts fine and idles well in the cold, looks to be in pretty good shape externally.

Two things worry me:

1. When you open the throttle quite far you get to a point where it shudders and chugs a bit, backing off keeps it smooth. The owner reckons it's the rev limiter cutting in? It's going fairly quick at that point but the engine isn't screaming.

2. When you take off from an idle, at low speeds there is a bit of a (different) shuddering from the boat - but this smoothes right out once you push the throttle forwards. It kind of seems like the gears haven't fully engaged. I'm used to a distinct clunk from other outboards. The throttle control feels like it could need some lubrication or something.

In the mid throttle ranges it works just fine and the boat performs really well. Any thoughts?

Thanks very much,

Thomas

1. Test for spark. Results?
2. Test compression. Results?
3. Carbs may need cleaning. Youtube video on procedure. Approx. cost of parts $75
4. Throttle linkage may need adjustment.
5. Fuel pump may need rebuilding. Youtube video on procedure. Approx. cost of parts $15
6. Fuel line may need replacing.

If you plan on doing the work then buy mercury shop manual to guide you. Ebay.

These are things to get started.

Try one thing at a time and see if there is a difference in the performance.

I don't have the boat at hand to to test unfortunately - pretty sure spark is good because it performs well in the mid throttle range. I've read in places of having high-speed only ignition issues. Unfortunately the tacho isn't working. Carbs/fuel/linkages sound like things I can tackle to eliminate, I just don't want to be stuck with something real nasty!

Thanks

Thomas

Get some more history on boat&engine. Possible old fuel onboard,new fuel mixed with old, Service records, Fuel lines are 14 years old,ethanol fuels play havoc with them along with jets in carbs.

So I've got the the boat now and done some investigating. Compression is even across all the cylinders and the thing starts super easily and idles is very smooth at speed.


New tacho is on it's way so in the mean time I used a multimeter with frequency counter...I believe I should see 6 pulses / rev. Now that would mean pulses per second I read (Hz) I need to multiply by 10 to get revolutions per minute. Figures below based on this assumption...


I got around 1000 rpm idle (out of gear). The engine appeared to transition from 2->4 around 2500 rpm (stopped being a little rough and power picked up) and topped out at 5700 rpm...there's a bit more throttle available but it hits a brick wall there.


To me that seems like the idle and 2-4 transition point are high but the top end makes sense. Now, flat out we were doing around 60 km/h (36 mph) measured by GPS. The prop is labelled 16P...thinking it might not be quite right.


Another question is re. the fuel lines...I've replaced the line from fuel/water separator to the engine but I seem to have a vapour lock in the line from the tank to the separator and judging from the feeling of the fuel bulb, in there too. Is that normal?




Thanks,


Thomas

From what you describe it doesn't sound "bad" to me.

What I would do - run a bottle of seafoam through the system (just follow the instructions on the bottle). You may have a few cooties in the fuel system/carbs that has "not" become a full blown clog yet and the seafoam "may" take care of it.

36 mph sounds almost right. A 16 foot fiberglass will be heavier than an aluminum and 36'ish is what we typically see on a 16 foot aluminum (full console fish/ski model) with a 90 horse - so you are absolutely in the ball park.

The new tach will tell you if the 16 pitch is a little light - depending on how you will use and load the boat (passengers, gear etc), again, I wouldn't consider a 16 "too little" necessarily (again, when I was rigging we would often pair a 90 with a 17 pitch on the aluminum's I noted above).

A 115 compared with a 90 while about a 27% horsepower increase would only typically give you about 8-9% more top end speed. So again, 36 mph doesn't sound "way low".

And no, there shouldn't be any vapour lock in the fuel line. Is the tank built in?

If so try running on a portable tank and see if anything changes - if it does check the vent, pick-up and anti-syphon valve on the on-board tank - could be an issue there...

Get the new tach because your 2-4 transition should be at 1800 rpm and max WOT for that motor is 5250. That is not a motor that likes to spin much above that.
Your speed also seems off Because you should easily be in the 40s with that motor. I had an starcraft SS 170 with the 125 version of your motor and pushed over 50. I would bet you are under propped.