2.6L rebuild time, dam - help wanted please!

[cracker]

Just took out my 1992 2 stroke 225hp yamaha, trailing a 23P laser2 propeller, before the 23P it was running a 19P laser, which would easily smash the rev limiter and did so every so often.
i was told this motor had been rebuild and had around 44hrs on it when i brought it, iv done around 12-15 with it.
and im not going to lie it gets ran hard. the motor did have perfect compression when i brought it i personally checked that.
its running yamalube oil and 98 or 100octance petrol ( australian fuel - the best grade you can buy here)

now it was running at 5000rpm doing a comfortable 52mph, it had water pressure as we have a gauge.
it then sounded "different" and began to run poorly eg, 5 cylinders
i managed to restart then engine and return to the ramp - around 10-15mins

i removed the plugs and notice one had metal on it - ****.
iv pulled the head and its lost a ring on the bottom portside.
the piston is damaged and the bore is scratched badly the head also has some damage.
the other bore are "okay" some small marks on them

so whats the next question is why did this happen?
it was doing 1200rpm less then it has in the past and had been running great for about an hour.
where should i look for possible causes?
any advice would be great, this isnt a race motor or anything thats been modified its all standard.

where should i look for a rebuild kit and what should it cost me to have rebuilt?
what can be done while its apart ? porting wise?
any help would be great... im thinking along the lines of ring gap too small or it was running too hot?

cheers tom

[martinc]

Hear the same story, all to many times. Based on everything being some what correct ( Fuel flow, engine temp,mechanical condition and ignition) then it went into detonation. 5000 rpm at WOT is not enough. engine failure will happen. thanks Martin

[DoktorC]

Ya...that motor needs to spin closer to 6k WOT. The knock sensor can only pull so much timing...obviously not enough to save this one.

[Smoking Joe]

Bugger. To go from a 19P to a 23P prop is a big jump. Did you confirm that the 23P prop allowed the engine to spin up to it's recommended max rpm or was 5000rpm WOT?

[cracker]

so not enough rpm killed it? 5500-5600rpm was w.o.t
6200 being hard cut rev limiter and it was doing around 4800-5000rpm at 75%-80% throttle when failure happen, i thought it would have been quite happy to cruise about at 80% at around 5000rpm isnt the operating range like 4800-5800 or something.
we have used a 21P in the past and was still massively undersized, the 23P came up cheap and was in good condition - worthy of a trail.

w.o.t was seeing 66.4mph at 5600rpm, i wasnt getting w.o.t when the failure happen it was running around 5000rpm for quite some time good 10-15mins, the boat was trimmed up and it was cruising around 50mph...

[cracker]

few pictures... the plug doesnt show any sign of damage due to pinging, nor does the piston as far as i can tell.

[cracker]

i thought the operating range for these motors was 4500-5500rpm, hence getting a prop to suit 5600rpm @ w.o.t was bang on.
and running it at 5000rpm seemed well... nice it wasnt labouring or struggling at all its only on a 18foot ski boat not pushing a 25foot cruiser or anything

[DoktorC]

i thought the operating range for these motors was 4500-5500rpm, hence getting a prop to suit 5600rpm @ w.o.t was bang on.
and running it at 5000rpm seemed well... nice it wasnt labouring or struggling at all its only on a 18foot ski boat not pushing a 25foot cruiser or anything

Sorry...I misunderstood. I thought the 5000rpm was your MAX rpm. That is not a detonation burn down...something has gone through the motor...kinda looks like the ring locating pin...

[cracker]

Sorry...I misunderstood. I thought the 5000rpm was your MAX rpm. That is not a detonation burn down...something has gone through the motor...kinda looks like the ring locating pin...

yeah sorry poorly worded on my part.

any idea what would/could cause this? iv only ever really played with 4 strokes... and having owned pretty much all boosted 4strokes well aware of the effect/look of things that have detonated, melted plugs, blown gaskets, ring lands missing, and genrally i would have thought the edges of the impacts on the piston crown would be burnt/melted smooth.

what would the next step be? is there likley to be damage elsewhere? the bore is very rough/ scratched... as i said iv always ran the best fuel in it, and brought the brand name ome yamaha oil...

[cracker]

i thought the operating range for these motors was 4500-5500rpm, hence getting a prop to suit 5600rpm @ w.o.t was bang on.
and running it at 5000rpm seemed well... nice it wasnt labouring or struggling at all its only on a 18foot ski boat not push a 25foot cruiser or anything

[martinc]

Ditto!!! Thanks Martin

[cracker]

Ditto!!! Thanks Martin

ditto meaning ring locating pin

[DoktorC]

yeah sorry poorly worded on my part.

any idea what would/could cause this? iv only ever really played with 4 strokes... and having owned pretty much all boosted 4strokes well aware of the effect/look of things that have detonated, melted plugs, blown gaskets, ring lands missing, and genrally i would have thought the edges of the impacts on the piston crown would be burnt/melted smooth.

what would the next step be? is there likley to be damage elsewhere? the bore is very rough/ scratched... as i said iv always ran the best fuel in it, and brought the brand name ome yamaha oil...

I'm not sure on what causes the locating pins to fall out...bad luck...I dunno. It depends on how deep the scratches are you may need just a hone and a new piston/rings or you may need to get it bored oversized. It needs to come apart either way to clean out the mess left behind from the honing/boring so depending on the hours/wear you may want to replace all six or at least re-ring it. Those motors are pretty bullet proof...

[cracker]

right so shes coming apart.
only issue is how the hell does the crank case come apart! and how does the flywheel come off!

its a .25mm oversize piston in one pot too, looks like the same thing has happened before going by its respective head... worrying.

[Lockjaw]

I have had two motors do the same, thing, not running WOT either. It sucks.