I have a 19ft Key West with a 1995 Merc 115. The motor runs great while in gearat idle speed all the way up to WOT… The issue starts when idleing in neutral. The rpms will run around 900, thenthe motor will bump, the RPMs drop to around 200 as it seems it will cut off, then go back to 900. Sometimes, it shuts off. Sometimes, it runs great all together. Yesterday, it was running rough and i seem to remember other times it was doing it, the motor was trimmed up from running. We had come off a run and were putting around and it shut off several times. I trimmed it down almost all the way and it started running great. I trimmed it back up and it still ran fine. The carbs have been cleaned several times since it started doing this a couple years ago. I have also replaced all of the filters, plugs, wires and fuel pump. Others have said it is a fuel issue, I dont know. The carbs were just cleaned, no parts replaced because the floats and needle valves looked good. I wish I could afford to take it somewhere and let them fix it, better yet, wish I could repower with a newer motor, but not in the cards right now.
Any suggestions on what to try? Any chance it has something to do with the motor being trimmed up or just a coincidence?
Did it cut off while in gear or in neutral? Merc manual says it should idle at 625ish in gear. 900 in neutral should be about right. Usually when there’s a carb problem it shows up in gear, not in neutral, but not always.
that is an inline 4 cylinder motor
it only runs on 2 cylinders at idle
it accomplishes this by not having idle circuits on 2 carbs
900 is higher than it should be at idle in neutral
sounds like an electrical issue
That is correct Chris. I was thinking electrical issue too, but was told it was fuel related. When running in a hose, doing the same thing, it idles normal, revs a little, backfires, Dpms drop real low and it climbs back up, only to repeat the process, aometimes cutting off. Electrical wise, plugs, wires, power pack and stator have been replaced. Could it be a coilpack not firing all the time?
Here is a youtube video I took a few weeks ago. I was adjusting to idel a little, and it was too high in this video, so I got it back down to 900 rpm, but it gives you an idea of what it was doing. Basically the same at 900, and dipping down about the same scale. It was doing it yesterday too, and it stopped and ran fine when I trimmed it down a little.
clean the spark plugs and run on the hose for a few minutes unitl it shuts itself off
then pull the plugs and see what they look like. is one dry (running lean) is one wet (running rich)
if its running rich, the fuel pump could be leaking into the cylinder
the reason I said it sounds like it could be electrical is the trim has an affect on it.
do a drop cylinder test and figure out which cylinder is causing the problem.
has the trigger been replaced?
and yes, the coils could be failing when it heats up
Ok, I will try that. I don’t think it is the fuel pump because i rebuilt the fuel pump when it started doing this and there was no change. I do know when it has this issue, it doesn’t it sometimes when the motor first starts and is cold as well as when it has been running. What also get me is it seems to not have an issue when it is in gear without additional throttle, only in idle.
Since it runs on only the top two cylinders during idle, would it be a good idea to switch the top two coils for the bottom two and see if that helps?
We had exact issues last year with a friend’s 150 Johnson! Fuel line was catching AIR at one of the connections! Check all connections on fuel line, his motor was cutting out just like you described. He spent a lot of money and replaced a lot of parts, but it continued to do the same thing. He finally rechecked all his connections and found the problem! He replaced coils, fuel pump, plugs, primer bubble etc. and it was catching AIR the whole time! Hope this helps!
God Bless,
Capt Buddy Bizzell
Being on the water is a blessing, catching fish is a bonus!
Matthew 4:19