Quote:
Originally Posted by Swordsman
I find this one extremely unlikely. Octane isn't increased in fuel grades for more power, it's increased to slow combustion so you don't get pre-detonation (aka spark knock or pinging) with more aggressive cam timing. Longer octane chains are slower to break down as they burn.
I suspect there was a valve clearance issue, maybe when it pulls the valve closed. If it got really tight, it might have popped the valve head off since they're not exactly quality materials. Isn't that the exhaust side? I know the exhaust valve on my Ural used to get tighter over time while my intake valve got looser. I assume it was from the heat.
Looks like there could be some wear around the break point also, but I have no idea how that would occur because I don't know how scooter valves are put together. Maybe that's just from it banging around inside the chamber before it battle-axed the piston. 
~SM |
TOTALLY agree!! and this is one of THE biggest misconceptions about high octane fuel! Folks believe higher octane = more power cause it burns faster. Yes Sir...you are correct..it burns slower as to prevent detonation in high compression engines. In my Scoots it actually decreases performance.. Folks say.. "But my manual says to use 91 octane or better" Here is where the confusion comes in.. Ya see..in European and Asian countries they use a different formula. They use RON only..which translates 91 octane in their gas to 87 octane in the U.S. where we use the (RON+MON)/2 formula. READ the little decal on your local gas station pump...it even says octane is based on (RON+MON)/2..
Here is a good link..a good read that should clear up the confusion on octane ratings that are posted so often on here. I have a 2003 Tank Urban Sporty 50 that has seen NOTHING but 87 for all of it's 21,200 miles so far. Whole engine is all original still...but is about due for rings..which is not bad for that much mileage. ok..the link..read and heed..
http://www.pencilgeek.org/2009/05/oc...nversions.html
In regards to your valve to piston collision...I have seen this a few times in my Scooter parts/repair shop. It is caused by a stretched timing chain and or a weak or defective chain tensioner. What happens is that the chain jumps or slips a few teeth on the Cam or Crankshaft gear...this causes it to jump valve timing so that the valve is open as the piston comes up. What appears as pitting or melting of the piston is actually just scarring from the broken metal pieces being scattered around... Just my 2 cents... I only work on Scooters for a living..so I may be wrong...
Glenn