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Mercury 115 4 cyl. Problems??????

J

jcs07

New member
Hey Guys I am new to the forum. I am having a few problems with a 95 Mercury 115 4 cyl. that I am hoping to get some ideas on. The motor will run rough at idle, run rougher in gear at low idle and to 1800 to 2000 rpm and then will smooth out a bit, it is weak out of the whole and when at WOT it surges. Now for what has been done to solve the problem. I took the motor to a boat mechanic. He ran a compression and spark test said everything was great with both spark and compression. He cleaned and rebuilt the carbs and fuel pump. He replaced the fuel lines from the tank to the motor, replaced the fuel filter, replaced the a.s valve, replaced the plugs and plug wires and checked for air leaks. I get the boat back still the same problem, so I notice the rpm gauge running about a 1000 rpm higher than normal, I then replace the regulator and while I was at it I decided ( for preventive maintenance purposes ) to go ahead and replace the ignition components as in the stator, trigger coil, ignition coils, and switch box. I still have the same issue after all of this. The idle/air adjustment has been redone over and over according to the service manual, still the same results. Anyone have any ideas? Oh yea from time to time, say once in a blue moon the motor will take a notion to run like a dream been a while though.:confused:
 
I believe this motor runs on the top 2 cylinders at idle and on all 4 above 1900 RPM.------------Bottom 2 carburetors do not supply enough fuel to run / fire at idle.
 
I have the same engine...2002 model that I troll with. My experience w/it idling poorly occurs when the weather is hot and humid and I use off branded fuels . As soon as the sun goes down and the air temp drops it idles perfectly for hours. If I use Shell, Sunoco or BP 89 octane or higher fuels it idles perfectly all of the time. Try a different fuel. If that does not work then
He cleaned and rebuilt the carbs
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you might want to revisit the cleaning of the carbs because it sounds like the idle circuits are not 100% clean. Find someone that uses ultrasonic cleaning.
 
guyjg said:
I have the same engine...2002 model that I troll with. My experience w/it idling poorly occurs when the weather is hot and humid and I use off branded fuels . As soon as the sun goes down and the air temp drops it idles perfectly for hours. If I use Shell, Sunoco or BP 89 octane or higher fuels it idles perfectly all of the time. Try a different fuel. If that does not work then
you might want to revisit the cleaning of the carbs because it sounds like the idle circuits are not 100% clean. Find someone that uses ultrasonic cleaning.
Click to expand...

Thanks for the advice. I have always heard that higher octane fuels were not good for outboards but if the shoe fits I will give it a shot and see what happens. thanks again.
 
I have always heard that higher octane fuels were not good for outboards
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My Merc. factory manual states the recommended fuel is 87 octane minimum. The higher the octane the slower the fuel burns allowing time for the energy pulse to build to the maximum before being blown out the exhaust port. Try a 1/4 tank of 89 octane. If there is little or no improvement add a 1/4 tank of 91 octane. I'll reiterate that not all gasolines contain the same ingredients. Shell is my 1st choice for all of my vehicles. They'll all run on any brand of fuel. They just perform better w/Shell...just my personal opinion based on 40 years of experience checking MPG.
 
I would say carbs. Those carbs are a pain to get tuned just right and if they are not adjusted correctly it will cause similar issues. Also a quick not if you are mainly having these problems on the low end after letting it idle you may be building up fumes under the hood!
 
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