Reading a burned piston.

[seadoo@19072]

1994 Seadoo gtx. Just completed an engine top end rebuild. (+ ten thousand). by dealership mechanic. Broke it in on the trailer for an hour. Ran great. First test ride, shut down after 20 minutes. Would not start. Turned over easy. Took it back to dealership. 120 lb on prop side, 60 lb on mag side. It was a "no warrenty deal". I pulled the head and found a crater melted in the center of the piston about the size of a dime 1/16" - 1/8" deep with a gutter leading to the valve side of the cylinder. There appears to be rust on the cylinder walls. Is it from water or just heat from spark plug? The plugs are the correct heat. I had rebuilt the carbs earlier. Checked the pop off; around 20 lbs. and adjusted the high speed, full in, back out 1/4 turn. Whats going on? Carb not adjusted right? air leak" .

 

[mikidymac]

Rust would only be from water getting into the cylinders.

Pistons look clean and dry.

You should never break in an engine on the trailer ever as the rings will never seat.

I also would not trust a dealership to bore, hone and piston match cylinders. And I have never heard of a dealer that didn’t have a warranty on their work.

 

[GGuillot]

Did you run a premix tank of 50:1 fuel for break in? I'd say running it on the trailer for an hour didn't help, it's under no load, like Miki says you won't get the rings to seat. After a rebuild, fire it up on the trailer only long enough to set the idle and check the water flow. Shut it off then get it on the water, then slowly break it in, up to half throttle for awhile, 20-30 min intervals. After first tank run it briefly at WOT vary throttle, through second tank and check the plugs, fine tune and so forth. Should keep a premix on the second tank also, after that you're broke in.

I'd double check your oil injection, what was compression shortly after top end rebuild? unfortunately you look like you need to do another top end. Wouldn't hurt to check the carbs again, if you had them rebuilt, I'd follow up with if they were built with OEM kits or not. That makes a difference, depending what you paid for the rebuild that should be easy to determine. Cheap carb rebuild kits can present problems, I just wouldn't trust them.

 

[mikidymac]

After looking at the pictures on a larger screen it looks like they also didn't chamfer the ports after boring either so whoever did the machine work didn't do it properly.

 

[AKnarrowback]

The only time I have seen damage like that was on snowmobiles that had pre-ignition/detonation problems. The culprits I have experienced are a lean mixture (running high altitude jets at lower elevations), a blocked or restricted coolant passage (rare), bad gas, ignition too far advanced, a leaking exhaust manifold (leaking exhaust out) and "oxyfuel" (if the lower 48 still uses that poison....)

Did both pistons show the same type of damage?

The rust I have seen many times on a normal machine. A little water in the hull, seat is on in the warm sun and the humidity level is tropical in the machine. Not many people turn the motor by hand, I was suprised how many DOOs moved slow and squeeked inside after sitting overnight. I always leave my seats off to let all that moisture evaporate out of the hull. Double check for any water leaks anyhow.

 

[mikidymac]

Your cylinders should never look like that just from condensation.

It’s a lot of rust.

And important safety tip... fog your engines people!

 

[Chester]

Two things, never break it in on the trailer and never do a top end without doing the bottom end.

Chester

 

[Sportster-2001-951C-Stock]

What was the compression to begin with, before the damage occurred and was the fuel fresh? Trailer running doesn't generate much heat or cylinder pressure, also low throttle positions mean there's not much oil being delivered unless you added oil to the fuel (I've never seen too much oil hurt a 2-stroke and it's easy to add oil to the fuel tank. Assuming the carbs aren't dry, the oil is carried along with the fuel). Too much oil will foul plugs, I still recall the days we ran 10:1 non-detergent SAE-30 in our egg beaters, LOL.

Detonation from ignition timing advanced too much, stale low quality fuel, too lean A/F mixture will eat holes in the piston crowns. I'd guess the previous engine suffered the same fate for likely the same reason?

General 2-Stroke rules: If bore is too large for piston diameter heat can't transfer from piston crown through the thin oil film into cylinder walls, piston overheats as a result, heat destroys oil film and piston melts. Overheated pistons can cause detonation IMHO, some will argue that but I believe if the piston is hot enough pre-ignition can occur (which isn't the same thing as detonation, see link). Regardless, detonation occurs when the fuel stops burning prematurely (for whatever reason) as if when there isn't enough fuel in the mixture or it flashes from being poor quality or stale fuel.

I'd say most 2-stroke premature engine failures are a result of carburetor and other fuel system issues.

Never ever break in an engine with no load, the rings usually won't seat correctly and the 1st 10 or so minutes are the most critical for ring sealing.

I don't see chamfer around those ports, are the rings broken from snagging on the sharp edges of the ports? See the scoring above and beneath the port indicating oil was wiped off the rings as they passed the sharp edge of the port. Chamferring if done correctly leaves oil film on the face contact of the ring and scoring above/beneath the ports indicates lack of oil due to being wiped away (or possibly incorrectly gaped ring ends).

See this article, I don't recall if he speaks directly for 2-strokes but more so high-strung ICE:
Engine Basics: Detonation and Pre-Ignition by Allen W. Cline