115 running on 3 cylinders

I have a 1987 115 Merc inline 6 that seems to be running on 3 cylinders. 1, 2 and 4 are firing while 3, 5 and 6 are not. Checked compression and all are right at 125, also have good spark. Switching out plugs made no difference. Any ideas? Are there reed valves in these motors? and if so where are they located?

Re: 115 running on 3 cylinders

You need to check spark you probably have lost 1-3-5 or 2-4-6, thats a switch box or 1/2 of the stator. I easy test for which is swap the red and blue wires from one box with the red/white and blue/white from the other box if the spark changes cyls. it's the stator and if it stays the same it's the box.

Re: 115 running on 3 cylinders

I think spark is fine. If I shoot a little carb cleaner in the bottom carb motor seems to rev right up. I also noticed I have great suction on the top carb when motor is running but very little on the other two. Any thoughts would be appreciated.

Re: 115 running on 3 cylinders

Your original post named cyls. that have no connection with one another and it is doubtful more than one reed or labyrinth seal has failed at on time. Without real facts it's difficult to get real answers.

Re: 115 running on 3 cylinders

Please tell me what facts you want me to supply, and I will try to provide them. I'm sorry if my post was confusing or incomplete.

Re: 115 running on 3 cylinders

frb said:

.....1, 2 and 4 are firing while 3, 5 and 6 are not.....

Click to expand...

How did you determine which are 'firing' and which are not? Typically when we refer to 'firing', we are referring to spark related issues, which led Laddies to discussing what type of electrical symptoms would result in those behaviors.

Each switch box controls 3 cylinders, which leads one to expect 1-3-5 or 2-4-6 to be affected in unison if three cylinders stop functioning at once, as Laddies suggested.

Typically fuel problems affect a pair of cylinders or all cylinders at once. Top carb feeds 1-2 , middle 2-3, and lower 5-6 as you'd expect.

Your compression numbers sound good, so you can rule out major issues.

My next step would be to do a spark check on each individual cylinder, to rule out ignition as the source of your problem. Only then would I look for fuel or timing related issues.

Broken or bent reeds show up as a rough idle, and typically they sneeze fuel out of the carb. I don't believe you've mentioned those symptoms.

Don't shoot carb cleaner or starting fluid into the carbs... motor requires lubrication. Use pre-mix fuel if you need to spray something.

Does the motor run at all, just rough running?
If it does run, does it behave differently as you advance the throttle?
Do you have access to a DVA meter to run any ignition tests?

Re: 115 running on 3 cylinders

I determined which cylinders were firing by pulling the wire off the plug while it was running. The motor does run but its rough and will not idle for more than a few seconds. When advancing the throttle the motor seems to smooth out but those three cylinders still show no change when I pull the plug wires. The boat is not in the water and all this is being done at on muffs. I did not notice any sneezing coming from the carbs but did notice small drops of fuel on the bottom of the carb inlets. I checked for spark with an inline tester and spark appeared to be strong but I will check to see how large a gap the spark will jump.

Re: 115 running on 3 cylinders

Sorry forgot to mention no access to a DVA