Ok, I have been battling this issue for a while, so I figured I would put it out here to see if anyone could come up with a solution to my problem. I have a 95 Mercury 115. It runs great in gear all the way up to WOT. The problem is at idle in neutral. It will idle fine, then spit and sputter a few times, then rev up, back fire and back to ideling fine again. It will do this in a cycle over and over again, and eventually, at random, cut off. I have to idle up the throttle to keep it running.
I have pulled the carbs and cleaned them. Everything looked fine inside. I have replaced all of the filters, spark plugs, installed a fuel/water seperator. When I got the boat, it was full of olf fuel, so I drained the tank and started with fresh fuel. I changed the bulb and fuel line coming from the transom to the motor. I inspected the fuel line coming from the tank to the transom and it looks fine. I also rebuilt the fuel pump. To eliminate the chanch they the problem was in the fuel line or tank in the boat, I put my portable tank from my small boat and got the same results.
Does anyone have any suggestions as to what it could be
? I was told by others that it was a fuel problem, and not spark. I am running out of ideas as to what the problem could be. To add to the mystery, the motor ran fine for months after I got it, and just started running bad one weekend, which is why I cleaned the carbs.
Key West 19’6" CC 115hp Merc
14’ Fast Craft w/ 60hp Merc
that motor has 4 carbs
it only runs on 2 at low rpms
the other 2 don’t even have an idle circuit in them
if one of the carbs isn’t getting the proper amount of fuel…
also, you would need to check spark at idle while running
need to eliminate an electrical issue
Correct, I believe the lower 2 carbs come in to play around 2000 rpm. All of the fuel lines are held on with zip ties. Could it be possible that one or more are not tight enough and could be allowing air to enter the fuel system? I did notice that the fuel filter is full bwfore I start the motor, but drops to about half full while running. The fuel level will constantly rise and fall inside the filter, but it never stays full.
Key West 19’6" CC 115hp Merc
14’ Fast Craft w/ 60hp Merc
I had a 135 Evinrude that did exactly the same thing you have described.Turned out it was low compression in one of the cylinders.Hopefully not, but if you have access to a compression tester you can eliminate that as a possible cause.
what rpm is it idleing at
if they are the correct zip ties, they are designed for that application
Yamaha uses them from the factory
the only way you could be getting air in the system would be if the leak is on the tank side of the fuel pump
also, when you first start the motor after pumping the primer bulb, the motor is running off the fuel in the carb bowl, so an air leak wouldn’t be an issue until the bowls got low on fuel.
those kinds of issue are usually more noticeable at higher rpms.
the fuel filter fluctuation is normal
I guess that could be a possibility. The float in the oil tank was bad, so the previous owner unplugged it. I believe it was run low on oil one time… It has a slight tapping sound coming from it, almost sounds like a lifter tapping…
Key West 19’6" CC 115hp Merc
14’ Fast Craft w/ 60hp Merc
If it sounds like a tap or " ticking " sound I would look at the coil/ plug wires etc. as a possible problem. They can partially short which will cause a rough idle but will sometimes overcome at higher rpm and run fine. Try running it with the cowling off in the dark and see if it is grounding somewhere. You can also try a piece of PVC pressed to the coil and listen if you see no arching.
If plug wires could be the cause, then it is a good possibility that could be the problem. The wires are original and are not in the best shape. I guess I need to bite the bullet and buy a set…
Key West 19’6" CC 115hp Merc
14’ Fast Craft w/ 60hp Merc
Yeah, there is another member on here that has a parts motor like mine and has a spare set of carbs. He is going to let me use them to see if it clears out the problem…
Key West 19’6" CC 115hp Merc
14’ Fast Craft w/ 60hp Merc
Took off of work yesterday for a Direct TV install, so figured it would be a good time to pull the carbs. Bought a gallon of the Chem soak and disassembled the carbs, soaked them for about an hour each, washed and blew out every hole I could find with the air compressor. Reassembled the carbs and put them back on. The motor fired up without having to be choked and purred like a kitten. I guess when I had help cleaning them before, we weren’t thorough enough and left some trash in one of the carbs… Seems to be fixed now. Thanks to Chris V and all others who helped with this issue!!!
Key West 19’6" CC 115hp Merc
14’ Fast Craft w/ 60hp Merc