1990 Merc 115 Problem

[fjblair]

This past weekend we were putting along out of a no wake zone on the way back from a trip across the lake when the engine would not make power. Could not make more than 2200rpm at WOT no matter what we tried with the throttle. We were a long way from home so decided to not tinker and just make very slow forward progress. All gauges and temps were fine. When we reached the main channel I had a couple of people move to the bow and the boat planed out a bit and rpm's went to around 3000 and speed up to 12 mph or so. I could have thought of that an hour earlier Or was the moving people to the bow and the power increase coincidental with some other explanation?

Once we were back at the dock we found that when out of gear the engine would rev just fine. We didn't try this out in lake because I'm new and didn't know any better. I did not want the motor to quit miles from home.

Any ideas? I bought a new fuel filter(spin on) but haven't had a chance to change and test.


Thanks for the help.

 

[CharlieB]

Re: 1990 Merc 115 Problem

First thing is to pull the spark plugs and look at them, sooty and fouled, replace them.

While they are out test spark to ensure that the ignition is capable of providing a substantle hot sharp crack, like a bolt of blue lightening, 7/16 or greater.

 

[fjblair]

Re: 1990 Merc 115 Problem

First thing is to pull the spark plugs and look at them, sooty and fouled, replace them.

While they are out test spark to ensure that the ignition is capable of providing a substantle hot sharp crack, like a bolt of blue lightening, 7/16 or greater.

Thanks. I can't rule out the plugs but they look good, not wet, no fouling. The engine didn't miss or lug and the temp never came up. Didn't seem rich just fuel starved. No smoking. Not fully tested however. Previous owner suggests maybe the diaphragm. Seems like foul plugs would have grown weaker or shut us down.... at least raise engine temp and that didn't happen.

Appreciate your insight, I just don't know at this point.

 

[mmf]

Re: 1990 Merc 115 Problem

could be the prop hub slipping.........

 

[fjblair]

Re: 1990 Merc 115 Problem

could be the prop hub slipping.........

Wouldn't that cause over revving? I couldn't get rpms up.

 

[fjblair]

Re: 1990 Merc 115 Problem

Update.

Changed fuel filter and the diaphragm/membrane and cannot get above 4000rpm.

 

[bassfisherman14]

Re: 1990 Merc 115 Problem

hello, I have the same problem with my 1977 115 mercury which has a distributor and local mechanic said to clean contacts in distributor with fine grit sandpaper but that didn't solve the problem for me but was thinking i may need a new distributor cap

 

[2dreamx]

Re: 1990 Merc 115 Problem

i am having the same sort of problems. anyone have any ideas what it could be?

 

[StuartT]

Re: 1990 Merc 115 Problem

If you are using a plastic fuel tank, the first place I'd look is to see if the vent is open. Happened to me yesterday. I was out running around on my backup tank and after idling through our no-wake zone, I hit the gas and couldn't get up on the step. Tank was actually starting to get deformed from the vacuum draw. Me bad!

May or may not be your problem, but I thought I'd mention it.

 

[Nauset #37766]

Re: 1990 Merc 115 Problem

I've got a 1991 100hp so I think that your setup should be like mine.
There are four plastic links involved that operate the throttle plates and I thought they were metal until one of mine broke this past spring and the throttle plate for the lower carb was not operating. It ran fine but seemed a bit sluggish as it was only running on three cyls.
Operate the cam roller and make sure that all of the plates are operating.

When I replaced mine, the upper link wasn't broken but it was cracked in the same place where the lower one had broken. Where the self threading screws go into the links. I replaced all four (three different part #)but had a time finding them as some are no longer available from Mercury.
Bill

 

[CharlieB]

Re: 1990 Merc 115 Problem

First thing is to pull the spark plugs and look at them, sooty and fouled, replace them.

While they are out test spark to ensure that the ignition is capable of providing a substantle hot sharp crack, like a bolt of blue lightening, 7/16 or greater.

TEST THE SPARK

Without a HOT BLUE ARC of MINIMUM 7/16 inch the engine's power and RPM will be limited by the ignition intensity.

Weak ignition = low on power and unable to reach max RPM

First things tested by any reputable shop(mine)
Spark test
Compression
If EITHER isn't up to 'snuff, you get a phone call (and your bill starts climbing).

Save yourself serious $$$$$$$

Learn to test it yourself.

 

[fjblair]

Re: 1990 Merc 115 Problem

Seems like it could be a number of different causes. Gonna put her in the shop.

 

[redhatsuxx]

Re: 1990 Merc 115 Problem

did you find the problem i may have similar problem with 1990 90hp merc
dont suppose it was to do with the stator

 

[clevecan]

Re: 1990 Merc 115 Problem

I have a 100 HP as well a 1988 model and I just found that my lower carb linkage was broken as well,it would not go so once replaced ran good but now it seems to be idleing rough and banging out as I throttle it to plain out,
it's a two stroke oil inject. and would sure like to get it running better?