Scored Piston Causing Engine To Misfire (See Pictures)

The top cylinder was not firing (at idle) on my 1983 15hp Johnson although at high rpm's both cylinders would kick in. I had 1/2" of spark from both and plenty of fuel. The compressioin tested at 115 lbs each. I rebuilt the carb, replaced coils and powerpack, rebuilt fuel pump then replaced spark plugs and wires. There was still no effect when the top plug wire was pulled at idle whereas the engine would die immediately when lower wire was pulled.

Next I pulled the powerhead and checked reed valves. One pair didn't seat perfectly but it didn't look bad. I finally called a Johnson mechanic for his opinion and when I told him what was wrong (and what I repaired) without hesitation he asked if I had inspected the pistons. I hadn't pulled the side plates and told him, "no". He said, "you've got a damaged piston"! I pointed out that there was 115 lbs of compression and he said that didn't matter and that the 'skirt' was likely damaged. When I pulled the side plates I could see the pistons and just like he said, the upper piston has some serious scoring. Koodos to the mechanic.

Could someone explain why a scored skirt of a piston affects the idling of a motor?

If the cylinder walls haven't been damaged should I simply replace the piston and rings with a standard set or should I hone the cylinder first?

If cylinder is scored, can I just have the damaged cylinder bored or do I need to oversize both cylinders? In other words, do you need both pistons to have matching sizes or can they vary slightly?

The first two pictures show the upper (scored) piston and the third pic is the piston rod (notice the strange discoloration at the midpoint).

Re: Scored Piston Causing Engine To Misfire (See Pictures)

No, not Torx. Was Torx even invented in 1983? Don't know.

You are looking at a 12 point bolt head...most likely a 5/16, 12 point bolt head for the rod cap bolts.

Re: Scored Piston Causing Engine To Misfire (See Pictures)

If one cylinder is damage on a two cylinder motor, the idle will be rough because the good one is pulling the bad one. Low cmp means less explosion then less power,

I had a scored piston on a three cylinder once and it was not that bad at idle.

No need to oversize all cylinder,

Re: Scored Piston Causing Engine To Misfire (See Pictures)

You may be able to hone the cylinder if ti is not scoerd badly keeping the same size pistons. you machine shop should be able to determine this for you when you have it honed. If you hone yourself I recomend a ball type hone. you will then need to measure if you need larger rings or pistins.

Re: Scored Piston Causing Engine To Misfire (See Pictures)

Low crankcase pressure that cylinder. Will not push fuel thru ports into the combustion chamber.

Re: Scored Piston Causing Engine To Misfire (See Pictures)

Scored piston like shown may affect the crank case compression which is just as important as the cylinder compression. If cylinder wall is not scored, de-glaze and measure for size and out of round. The color difference on the rod is from the hardening process of the rod, not to worry about. Change the center main bearing as well as that even works as a seal between crank cases.

Re: Scored Piston Causing Engine To Misfire (See Pictures)

Thanks to all for great input.

The cylinder walls looked pretty good but they should be honed and then measured.

The pistons are .030's and the powerhead was probably previously rebuilt. Is there a source for larger pistons? And what about the rings ... should I just get standard rings or something more suited to my situation?

Re: Scored Piston Causing Engine To Misfire (See Pictures)

marineengine.com shows .030 rings available ....no larger pistons available through them....have you inspected the crank journals and bearings?...asking because crank damage can be a deal breaker cost wise on a rebuild

Re: Scored Piston Causing Engine To Misfire (See Pictures)

'Haffiman' said to change the main bearing which makes sense so I'll check the journals at that time.

A Johnson mechanic inspected powerhead-block and said nothing else looked damaged (on the surface). He said that what likely happened was that a piece of carbon caused all my problems. I pointed out that the cylinder head was extremely clean and carbon-free and he said that doesn't matter. I should have pushed for more details but ...

... now that I think about what he said, where could a chunk of carbon come from that ended up in my crankcase and ultimately in my cylinder? Both my water tube grommets were 75% folded over and I suspect my waterflow was not 100% and engine got hotter than usual (but not to the point of damaging the block or head). This may have helped carbon to form and/or dislodge?

And, more importantly, is there a preventive maintenance application to prevent this happening again (e.g. periodic fogging, gas additive, etc.)?

Re: Scored Piston Causing Engine To Misfire (See Pictures)

Probably an overheating cause more than a carbon flake cause. Might even be a rpm cause, engine not reaching proper rpm at full power. Ever checked the rpm at full power??

Re: Scored Piston Causing Engine To Misfire (See Pictures)

Haff ... could you elaborate! Two questions regarding your reply:

Is there a simple test to check the rpm's at full throttle; and,

Why would improper rpm's cause scoring to occur?

To your point, while operating different engines for miscellaneous applications (not necessarily only outboards), sometimes you have a throttle (say, 3/4 open) and have reached maximum speed. In other words, there is no increase in rpm's (from 3/4 to full throttle) even though the throttle plate had not opened completely. Does this indicate a fouled or mis-calibrated carb, a tired or mismatched power supply, or something else? And, are you implying that this could lead to a breakdown in the combustion area of the head leading to a scored piston?

If you take a closer look at the middle photo (above) and this new photo (below), it appears that the scoring extends above the rings indicating that the breakdown probably began in the firing stroke and the particle(s) blew past the rings ultimately scoring the lower part of the piston. I like the theory that my moderate overheating caused either carbon to form, or for carbon to bake and dislodge, ultimately ending up in my crankcase!

Re: Scored Piston Causing Engine To Misfire (See Pictures)

As for the RPM check, use a tach and measure. As you do not have a charging system, you would need a mechanical (vibration or rotation) or an electronic either by using a timing light with built-in tach or one reading a 'reflecting' mark on the flywheel, often used on diesels.

The scoring groves is only partly of interest, they seems to be from alu melted to the cyl walls and not from broken rings or bearings. The piston(s) seems to be quite 'glazed' even on the non-pressure sides which indicates they have been squeezed. That is quite normal from overheating. However if it is overheating of the piston that made it expand or engine overheat is difficult to say without pics from all sides. If piston skirt on pressure side is severely glazed and blued out from pressure, it might be you have been running at too low rpm and full throttle. (might need to look inside the piston to find a blue spot). If piston skirt is glazed all around, it might be over reving. Happens often when people are reving un-loaded two-stroke engines!! I do not see any burned piston crowns or exhaust edge indicating lean condition.

Re: Scored Piston Causing Engine To Misfire (See Pictures)

Actually it does have electric start and a charging system so I suppose I should hook up my dwell/tach?

Haff ... great explanation relating to 'glazing, squeezing, rpm's' etc. Hopefully others (too) will benefit from your input.

I will admit however, I'm having trouble understanding the relationship between "running at too low rpm and full throttle" ... it just seems that these are mutually exclusive terms and from a layman's perspective, difficult to comprehend. In a car or a truck with a standard transmission, one could be in too high of a gear and trying to pull a load or go up a hill thus having low rpm and full throttle (foot to the floor) or, visa versa thus over-reving an engine. I can see how this could happen in a boat/motor scenario too and wondered if my land-based explanation is a general, non-technical way to illustrate your points.

Ultimately, you did an excellent job relating improper throttle control vs. internal engine damage!

Re: Scored Piston Causing Engine To Misfire (See Pictures)

Hi Bobgritz,

You mentioned that the cylinder walls look ok. Most probably you can use a new set of 0.30 oversize pistons.
Measure the existing cylinder bores. A machine shop should be able to tell how much the de-glazing (honing) would remove from it, and if the bore still would be within the tolerance for the new oversize pistons.

I am wondering if during the previous rebuild may be they used some "salvaged" pistons? (It certainly had new rings.) The scoring on the pistons look serious enough that the cylinder would also be more heavily scored.

Good luck, Peter

Re: Scored Piston Causing Engine To Misfire (See Pictures)

The piston looks worse than the cylinders (see pix below). The markings in the photos are not even noticeable to a 'finger inspection'. I ordered new .030 pistons and rings and will use a flex-hone (ball-type) tool.

I assume that the piston, being much softer metal, took the brunt of the damage. Perhaps the instigator was a rogue piece of carbon which was not able to score the hardened sleeve/cylinder?

Re: Scored Piston Causing Engine To Misfire (See Pictures)

Hi Bobgritz,

If you can not even feel the scoring on the cylinder with your finger, it should clean up nicely!
Just do not overdo the honing, keep measuring for proper clearance!

I would revisit the reed valves. You mentioned that one of them did not seal that well. Check against the specs, for other models I have seen tolerances how close the leaf plates should lie against the leaf plate base. (Just checked, for a '01 25HP Mercury, it is 0.007", or 0.178mm.) Also check if the base plate is flat, and the screws are torqued to to the spec (not overtorqued), and the leafs are not distorted.
Your engine is a two-port design, with reed valves. Compression in the crankcase is primarily the function of the proper sealing of the crank case chamber. At one end, it is the reed valves. At the other side it is the piston rings that seal the crankcase chamber (that seemed to be fine for you, based on cylinder compression). Also check/renew the crankshaft seals, at both ends and between the cylinders.
Piston skirt only plays a limited role in the sealing of the crank case chamber at a two-port design. Piston skirt certainly does the sealing in a three-port design, where the carb in on the side of the cylinder and the bottom part of the piston, the skirt, covers and uncovers the intake port.

Good luck, Peter

Re: Scored Piston Causing Engine To Misfire (See Pictures)

The only way carbon flake may have been involved in this incident is that you find a heavy build-up of carbon INSIDE the piston like on this pic: