Fish-Hawk.Net

My motor is 4 cylinder inline 115 HP Merc.
Recently it couldn't run as fast as it ran before. It's still OK for me to fish.
One mechanic simply heard it ran, and said one cylinder mignt not work for some reasons. He said it could be due to spark plug or other.
Anyone here can give me any idea what problem looks like?
It's an old motor. It maybe not worthy to be rebuilt.

Thanx alot.

Its way too soon to even think about abandoning your motor. Any good outboard mechanic should be able to tell you with an hour's labour or less whats wrong with your motor given what you are describing. What you are describing could be as simple as a dirty carb. You should get it looked into before you go fishing again by the way - if that cyclinder isnt burning the fuel its drawing you will just end up pushing the unburned fuel out into the lake from that cylinder.

Really thanx for your reply.
I plan to wait it until season ends. However, I worry that it will get worse and worse, even makes other cylinders not working.
Recently I had difficulty starting motor. I thought it's battery problem. Accoding to what I see here, it could be another problem like carb.

An easy way to check to see which one isn't firin' is to pull all of them out. The most fouled one would most likely be your culprit.Don't forget to tag each wire with masking tape with cyl.numbers 1-4 top to bottom,don't need no mo problems! You can also check spark by letting THAT annoying guy down the lake hold on to the wire with wet hands,if his eyes light up and he craps himself,you've got spark,move to cyl. number two!!
As an alterative "damn" put a "good" plug in the wire making sure to ground the plug to the motor,you a good spark will be white or a blueish,an acceptable spark is orange. BUT!! I STILL like my earlier suggestion,but wouldn't recomend it,I know what your thinking....!!

ERNEST T.BASS

Ernie,
You are absolutely right everyone has a neighbour that should test this engine's plug wires!!...Larry

Change all Spark Plugs, and RPM is still low.
If compression of one cylinder is low, it wil cause speed drops?

Hi,
We had the same problem with a 2 cylinder 60 hp evinrude this spring.
Motor only wanted to go half speed
A 10 minute job to compression check it. Any backyard mechanic can do it.
Low compression coulld be caused by a damaged head gasket (easy to fix) or your cylinder is shot.
Our compression was too low, so took off the head ourselves.
Our cylinder was shot. Chunks out of the piston head, groves on cylinder walls.
Estimates are $1,000 to fix just the one piston. It may cost more.
If you do not repair it and the chunks start to grind, then the piston breaks and the piston rod goes through the head. Pieces of metal fly all over the place.
Had that happen to me a long time ago.
Fortunately I still had the top on the motor so no metal hit me and it was only a short tow back to the launch. Then I had to replace the motor.

do compression check as suggested on each cyclinder, and at the same time check the condition of the spark plug compared to rest.

compression results should be withing 5-10% of each other, 100+ psi is good.

I had a mechanic check my motor recently. Seems the motor is in good situation. Compression is good, spark plugs are good..
However, RPM is still 1000 lower than normal.
Any other problems could be ???

I leave my boat on water in a marina. Maybe the lower unit has water inside??

I had the same problem 2 years ago. It was a bad reed.

FlyingDutchman

How to fix it? Need to rebuild Carb?

Thanx for your advices.

You could have a weak ignition coil or powerpack. Switch the wires on your coils and see if the problem appears on the other plug. If it does replace that coil.