Weak cylinder on Merc 225hp two-stroke?

I have a 1994 Merc Offshore 3.0L 225hp carbureted two-stroke with about 625 hours on it.

When I brought it in for annual service recently, the shop manager noted that the #6 spark plug did not look as clean as the others. He said it was "crusting up," with "whitish debris" on the plug. He recommended pulling the plug every now and then (say, after every three trips out) to keep an eye on it.

He told me at the time that the psi on all the cylinders looked good. However, I see on the receipt that the pressure on this #6 cylinder is a bit lower than all the others:

1) 115
2) 113
3) 118
4) 115
5) 118
6) 105

Any thoughts on this? Is there a probable cause for the observed symptoms (gasket? rings?). If this continues to get worse, how would it show up in performance? Would a failure likely to be immediately disabling, or gradual?

Re: Weak cylinder on Merc 225hp two-stroke?

The whitish deposit could be combustion products, from the piston. That hole gets first grief with cooling issues, and it might be running a bit lean.

You shouldn't have any deposits on yer plug but carbon and oil.

At the very least I'd bore scope it and make sure the piston isn't scuffing off onto the cylinder wall. Then I think I'd do some WOT power off's and check the piston tops for fuel pattern, and the plugs.

If you run part throttle a lot, check it at that speed also.

To answer yer question, odds are it's beginning piston failure from overheat, and the end game is catastrophic.

hope it helps
John

Re: Weak cylinder on Merc 225hp two-stroke?

it may be worth having the carbs overhauled and cleaned , if that pot is running lean it would cause the problem you have , I am suprised your dealer did not sugest this , to just "keep an eye" on it may be fatal , if you have a part blocked jet it will not fix its self.

Re: Weak cylinder on Merc 225hp two-stroke?

It's still within the 10% deviation range,therefore it's still good. However, you should follow the advice of the other members as far as maintenance is concerned

Re: Weak cylinder on Merc 225hp two-stroke?

Thanks for all the comments, they're really helpful.

I talked again to the shop manager this morning. He said the "crusting up" he was seeing was not the type of discoloration you'd see from running lean. Rather, he thought it looked more like what you'd see from water intrusion. Sometime back, we flushed the cooling system a couple of times when the boat was in the water with a hose connected to the static port on the powerhead and the outboard tilted up, not realizing that water could get in through the exhaust port and enter the cylinders that way. I gather his thinking is that the crusting up he's seeing is an artifact of that incident, and hopefully is working itself out by now.

Re: Weak cylinder on Merc 225hp two-stroke?

In my experience, water intrusion makes the plug (and piston top) squeaky clean.

I am not going to argue with anyone, so this is my last post. What you described would alarm me if I owned the engine.

John

Re: Weak cylinder on Merc 225hp two-stroke?

j_martin said:

In my experience, water intrusion makes the plug (and piston top) squeaky clean.

I am not going to argue with anyone, so this is my last post. What you described would alarm me if I owned the engine.

John

Click to expand...

Agree unless your running in salt water.. you didn't say..

Re: Weak cylinder on Merc 225hp two-stroke?

It's in saltwater.

I should add that I really appreciate the comments -- certainly wasn't meaning to question any advice, only passing on the shop manager's response when I asked him about this. I'll give careful thought to next steps.

Re: Weak cylinder on Merc 225hp two-stroke?

I agree , getting a splash of water into that pot will not effect the deposists on the plug it would flash off in an instant the first time it fired , infact some old enignes where fitted with water injection to stop pre ignition when runing on high loads (not outboard engines) . It may well be the idle jet which is blocked, this would not cause the massive failure that you get with a high jet blocking but when on idle it would slowly run that pot dry.

rgds
bob