what could cause this???

[Nessism]

Since it was just one cylinder it doesn't seem likely that it was the oil pump (although I'd check for debris of course). That piston is clearly melted which means it got killer hot. The only thing that comes to mind is that the cylinder was running lean while the others pulled it along...straight into hell. Great paper weight by the way. Take the carbs apart and look for an obstruction in the main jet circuit would be my recommendation.

[49er]

Since it was just one cylinder it doesn't seem likely that it was the oil pump (although I'd check for debris of course). That piston is clearly melted which means it got killer hot. The only thing that comes to mind is that the cylinder was running lean while the others pulled it along...straight into hell. Great paper weight by the way. Take the carbs apart and look for an obstruction in the main jet circuit would be my recommendation.

I agree. Check your plugs on all cylinders. You will get an indication as to whether it was the engine or just that cylinder that was running lean. I suspect that its just that pot. Looking at the pics, there has been some serious heat across the whole piston. The top ring has expanded that much that it laps by around 1-2 mm. Even the oil control top ring has overlapped due to excessive expansion.
If there was an oil feed problem, there should be some signs of that on the other pistons/cylinders as well. Check the condition of the wrist pin and gudgeon holes on the melted piston. These get lubrication from oil misted up from the crank/sump area during normal crankshaft rotation. If the gudgeon and pins look ok on all the other pistons, you can dismiss the lack of oil theory.

[NATEO]

I agree. Check your plugs on all cylinders. You will get an indication as to whether it was the engine or just that cylinder that was running lean. I suspect that its just that pot. Looking at the pics, there has been some serious heat across the whole piston. The top ring has expanded that much that it laps by around 1-2 mm. Even the oil control top ring has overlapped due to excessive expansion.
If there was an oil feed problem, there should be some signs of that on the other pistons/cylinders as well. Check the condition of the wrist pin and gudgeon holes on the melted piston. These get lubrication from oil misted up from the crank/sump area during normal crankshaft rotation. If the gudgeon and pins look ok on all the other pistons, you can dismiss the lack of oil theory.

the rest of the rings look fine. There is no excessive wear or warping.. the cylinder walls look ok too. I will check plugs tonight.

[Jethro]

Could there have been an exhaust leak on that cyl? Bad valve timing combined with a nasty exhast leak could cause that, maybe, but your head would probably be trashed too. Take the cams out of that head before you reinstall it and check the cam seats for scoring or worse.

I'd say it's definitely an intake or exhaust valve burn becasue if it was a general mixture problem, the piston would probably burn in the middle. What side did the piston burn, intake or exhasut?

[NATEO]

Could there have been an exhaust leak on that cyl? Bad valve timing combined with a nasty exhast leak could cause that, maybe, but your head would probably be trashed too. Take the cams out of that head before you reinstall it and check the cam seats for scoring or worse.

I'd say it's definitely an intake or exhaust valve burn becasue if it was a general mixture problem, the piston would probably burn in the middle. What side did the piston burn, intake or exhasut?

that was on the exhaust side. the cams look ok. there is wear on them, but nothing excessive...

[t3rmin]

Doesn't look like it could be anything but heat from friction, which would equal an oiling problem, as already said.

[moto_dan]

Curious.....Which piston was it? 1,2,3 or 4?

[NATEO]

#4 piston. I doubt it would be an exhaust leak. the pipe for that cylinder sealed well. the #2 and 3 pipes didnt very well, so I would have expected issues from those, not #4.

what makes the most sense in my mind (I know, I know... stop thinking lol) is that when I had the head off this spring, I was cleaining the top of the pistons with a screwdriver and I probably knicked it, creating a hot spot... the rest of the cylinders are perfectly fine... oh well... Im gonna check out the oil pan, screen and stuff, then slap the new block and pistons in...

thanks for your help, everyone... and that piston will definitely be a paperweight!

[slugsride]

The NOS needs to go to all the cylinders, with a boost in fuel supply for cooling and lube.:twisted: But seriously that picture is exactly what I visualize when I read one of Kevin Cameron's articles on detonation a while back. Hot spot-preignition-detonation-top ring failure-flame front travels down side of piston-melted mess.

[NATEO]

The NOS needs to go to all the cylinders, with a boost in fuel supply for cooling and lube.:twisted: But seriously that picture is exactly what I visualize when I read one of Kevin Cameron's articles on detonation a while back. Hot spot-preignition-detonation-top ring failure-flame front travels down side of piston-melted mess.

so basically, me and my damn screwdriver... I would rather that than plugged oil line or something... I can learn to not do stuff like that. plugged oil lines are different

[slugsride]

so basically, me and my damn screwdriver... I would rather that than plugged oil line or something... I can learn to not do stuff like that. plugged oil lines are different

Hmmmm. At very high RPM that by itself might do it, but for most of us something else would have to start it down that path like lean mixture or oil problem. The nick may have moved the focus of the detonation away from the piston crown where you normally see it (as earlfor said earlier). Since it's just one cylinder check the under piston oil sprayer on that one, but I think something went very wrong with the #4 carb or intake boots.

[t3rmin]

Since it's just one cylinder check the under piston oil sprayer on that one.

My money's on that.

[NATEO]

Hmmmm. At very high RPM that by itself might do it, but for most of us something else would have to start it down that path like lean mixture or oil problem. The nick may have moved the focus of the detonation away from the piston crown where you normally see it (as earlfor said earlier). Since it's just one cylinder check the under piston oil sprayer on that one, but I think something went very wrong with the #4 carb or intake boots.

I was running it at a very high RPM when I think it died. (I stopped and then I had trouble. it ran, but not strong, and it was burning oil- the rest of the engine was pulling it along, I guess) the bike was running lean last summer, and while I adjusted floats, it could have very well been a lean condition combined with a hot spot, and very high RPM (I dont have a tach, and I may have run it too high.)

[focus frenzy]

the remaining piston crown is black, a lean condition would lieve the crown a light grey to white, also lean conditions will normaly melt a hole in the thinest point in a piston and with GS's that is dead center.

detonation is the spontanious uncontroled ignition of the AF mixture and is commonly caused by several factors, wrong grade of gas, excessive combustion chamber temperature, carbon build up, fault or defect in piston allowing a hot spot to form, incorrect timing advance.

if you gouged the piston while scraping it with a screw driver (bad idea) that could have givin it a place to form a hot spot, combine that with running hard and poof toasted piston.

a coworker of mine has a PWC and melted a piston in a similar but not nearly as severe fashion and his crown was a very light grey pointing to a existing lean condition I had warned him about (he had been complaining about stumbling and a surge) your piston crown looks black from that picture.

[NATEO]

the remaining piston crown is black, a lean condition would lieve the crown a light grey to white, also lean conditions will normaly melt a hole in the thinest point in a piston and with GS's that is dead center.

detonation is the spontanious uncontroled ignition of the AF mixture and is commonly caused by several factors, wrong grade of gas, excessive combustion chamber temperature, carbon build up, fault or defect in piston allowing a hot spot to form, incorrect timing advance.

if you gouged the piston while scraping it with a screw driver (bad idea) that could have givin it a place to form a hot spot, combine that with running hard and poof toasted piston.

a coworker of mine has a PWC and melted a piston in a similar but not nearly as severe fashion and his crown was a very light grey pointing to a existing lean condition I had warned him about (he had been complaining about stumbling and a surge) your piston crown looks black from that picture.

what is the recommended grad of gas for these bikes? last summer I was using premium, but I read over the winter that I dont need to, that regular is fine for the older engines...