TOPIC:

There's a log. There's a log in the hole in the bottom of the sea.

There's an xs650 on the log....

Actually the hole is in the XS650 piston crown. It's right under the intake valve. It's on the left side. This is the side with the petcock. It's a vacuum petcock. It's the bike I sold to Ed(not our Ed) and said, "Ed, you need to replace the swing arm bushings, change the filter and oil, and fix the petcock".A nice low miles 82 XS650 special for $500.

Did he do it? NO! that's why there's a hole.
Am I pissed?
Not after thinking about it for a bit.
He's learned to pay attention to his bike.
We're all new at something almost every day.
Or pretty darn dull.

Got him building a workbench. Helping him get the engine out of the frame Saturday. His next task is to get the top end apart, measure the bore, make a list of what he needs, add the parts to do the other work he should have done, And I'll help him get it back together when the parts come in.

Oh, I also told him to memorize the manual so we won't have to look things up

Yep, having a look to make sure no piston bits floating around. There is that evidence, white freckles all over the piston crown, that I seem to recall reading somewhere is caused by detonation.

Detonation + hot reducing flame = hole

Never seen one in person before.
Don't want to again.

How did I diagnose?
1)No compression
2)Looked in through plug hle with flashlight.
3)Stuck a stick in and sho nff, that thing looked like a hole for a reason.

Ed said, 'lost power, puffs of white smoke, found oil low, found oil running out left pipe.'

-Duck

The joys of engine repair...have fun!

Forgive my foggy thinking, but how did a bad petcock lead to a burnt piston?

Insufficient fuel making it too lean, the vacuum line letting in more air and leaning it out that way, or something else I'm missing?

Vacuum line leaking air.
Petcock had to be in prime position to deliver fuel.
Don't know that petcock itself was bad, but it was clearly not working.
So, told to fix. He kept riding it...
Lucky. Piston is cheap. Rings and pistin about $45.
Topend gasket set about the same.
Had oil so rest of engine is fine.
What I could see of bore through plug hole looked good.
-Duck

Another reason to sell bikes exclusively to strangers. (who live several states away!)