2001 mercury 40 hp 4 stroke issues. Shop and Mercury stumped.

[SBob]

The shop has no clue as well as tech support at mercury. The basics,
225 compression in all 3 cylinders plus or minus a little but strangely high
Somehow 0% leak down in #1 and 2% the rest
Perfect spark with carbs completely cleaned and inspected
Timing is good, valves didn't even need adjustment
Complete carbon clean and soak over night of all cylinders resulted in next to no carbon release

So the issue is the boat will run then suddenly #1 falls out. Sputtering between 2000 and 3200 rpm. Often it will finally kick in power but then when you pull back it just dies. While the problem is occurring it can't break 3200 rpm. The problem is intermittent but happens more than not. Maintains spark according to timing light and priming while the issue occurs does nothing so fuel supply is there. Shop is stumped as am I. Any thoughts?

 

[Faztbullet]

Re: 2001 mercury 40 hp 4 stroke issues. Shop and Mercury stumped.

How did you narrow it to #1?

 

[SBob]

Re: 2001 mercury 40 hp 4 stroke issues. Shop and Mercury stumped.

While the problem is happening you can pull the wires. If you pull #1 no change. The others make it die.

 

[Faztbullet]

Re: 2001 mercury 40 hp 4 stroke issues. Shop and Mercury stumped.

If it was in my shop the first thing I would check are ignition and fuel system. A running check of spark thru entire rpm range to see if it misses or drops out. If ok as you have posted the fuel system would be inspected and fuel pressures tested at idle and WOT rpm to see if within spec's. If ok engine would be ran till problem arose and fuel in a squirt bottle would be sprayed into #1 carb to see if it picked back up. If the introduction of fuel made problem quit, the carb,isolator plate and o-ring would need to be inspected for cracks or air leak problems to would show up more after engine was ran due to heat expansion. I have see porous casting on carbs that will cause you to pull hair out but not on a newer 4 stroke yet but anything is possible.

 

[SBob]

Re: 2001 mercury 40 hp 4 stroke issues. Shop and Mercury stumped.

They did recommend trying the squirt bottle and I am going to, I got it from the shop still not fixed they cleaned and inspected the carbs thoroughly so it still may be there but kinda doubt it. The spark is great but not sure how strong under wot. While the issue happens a timing light did verify spark was still there. I am going to go through it tomorrow. On a side note, usually when the problem suddenly clears up and the boat starts moving good, any reduction in throttle stalls the boat, oddly enough it starts back up immediately and idles perfect but the issue remains.

 

[jsisley]

Not sure if you are still around 6 years later but I have a 2001 Mercury with the same problem. Did you ever find out what was wrong?