Well, it fianlly happened to me. One of my 350s has suffered a major melt down. I haven't taken it apart yet, so don't have pictures yet. Was running perfectly normal, seemed to suddenly be a bit down on power and then the old "high idle" came on, then just quit. When I got it home stuck the compression gage and it had 0 (zero) on the gage. I put my thumb over the plug hole and felt a tiny bit of "push". Pulled the exhaust off and ... I can't believe it! I can't even see the top ring ... it's gone and what I can see of the top of the piston, it's completely melted!

I haven't leak tested yet, can't find my stuff, so will go get parts to make another. This is the one that I put a new Honda piston on about 30 hours ago or so. It has run flawlessly up to this point, and the plug was always slightly chocolate brown ... just a tad rich. I just don't get it.

Any ideas?

Old gas? Detonation?

You may have hit it. Was using last season's gas, but ... that had already run through it. We had ridden for about 2 hours and then topped it off with fresh gas. But it was regular (85 octane) as my friends had "helped" me out with gas. Rode for about another hour before it "melted".

The old fuel was not a good choice,i wont even use it if it's two- three weeks old.I use fresh fuel no matter what anymore and only premium.
a case gasket go bad ? sounds like you had a bad air leak all of the sudden and it went quickly on you.

85 octane, what altitude were you riding?

85, really?

I've never used anything less than 90 and we ride almost exclusively at sea level to 500 feet.

Any gas older than 3 months we mix 50/50 with fresh.

When I was in Denver a few years ago 85 octane was the norm at all the
gas stations (mile high city) so if he was riding at 5000 feet he was probably
ok, 89 octane is the norm here at my local stations (500 feet) some places
you can find 87 octane but not too often.

In Colorado, you can find 85, 87, and 91. The compression on any Engine is quite a bit lower here than at lower altitudes. Most gas companies try to sell the higher octane, I think they must make more money on it. I have found that up here my machines will run on most anything, at Little Sahara, they are a bit fussier, and at Galmis, I get locally the highest octane I can get, and add octane boost.

Last year at Glamis, I melted down a 3 cylinder 4-stroke, but the problem was over boost creating a lean condition, not octane rating.

According to my GPS, the altitude is 4179 ft. I have always used regular or mid grade and never had any problems before. It certainly failed quickly. I will have my camera back by this weekend, will post pics and should have leakdown tester built by then also. Will share what I find.

If I don't find a leak, I will almost certainly become a fan of "super", as nothing else makes any sense.

You will get away with a leaner condition with a higher octane fuel. I run a 50/50 mix of 114 octane and 93 octane and run 175psi of cranking compression. It helps to eliminate detnation due to incorrect adjustments from a part coming loose/leaking before it can blow up the motor (( Internal Combustion Engine ? )). It also adds peace of mind when you running WOT (Wide Open Throttle) for an extended period of time and you can't hear the detnation. You should invest in an EGT. You will find power and reliability when you tune with it.

Been pretty busy, sorry it took so long to come back to this topic. Put the leakdown tester on, pumped it to 10 lbs and after an hour it was down to 6 lbs (and I know my tester oozes a little bit) which certainly doesn't seem drastic. Is this thinking way out of line?

So, that's good news and bad news. I would have rather found a big leak than wonder again what caused the melt down. Anyway, here's some pics through the exhaust port. Will post more when I dis-assemble the top end (I'm in no hurry, don't have $$$$ for rebuild right now anyway).

Now what?

Guess I don't know the trick to multiple pic posts, here's more:

And more:

one more

Your leak down rate should be good enough to rule out an air leak, detonation
would be my guess.

Well, here's hoping I can figure out how to post multiple pic's. At any rate, here's what it looked like inside. Not as bad as I thought, but looks like a bottom end job is probably in order ... lots of aluminum "balls" wandering around. The cylinder doesn't look too bad, probably just needs the aluminum removed (read that muratic acid does the job ... where do you get some?).

Good news, the rings are still there and intact, so major "hard" stuff went into the bottom end.

I can only think I must have gotten some bad gas and this is from detonation. Now I wonder what my other 350 and 250 look like inside.

Looks like detonation to me, you can get the acid at any hardware store,
be careful with the stuff do it outside if you can, buy the smallest bottle you
can get.

When you get it running again verify the timing with a timing light.

You ever get this thing running again?

Naw. Due to a meltdown in funds, it's gonna have to wait until probably the end of summer or so.

At least I have the others to "play" with.........