Originally Posted by procycle
I've always been skeptical of the old 'warp your valves' claim. I can't see how a little bit of cool air could do any harm. Especially since the exhaust valve heads get a blast of cool air on every intake stroke anyway.
My own opinion of where this old wives tale came from is this: If the motor had a restrictive exhaust and loosing the whole system caused the fuel mixture to become too lean - then you could easily damage the exhaust valves,
We used to run drag boats with zoomies on them which is an exhaust pipe about 8" long coming off the head. Never had a valve problem. The big thing is to let the valves cool before shutting the motor off. When you work a bike (or anything) really hard those valves get super hot. If you just shut the bike off they then get cooled quickly. Doing this multiple times can cause the valves to lose their temper. It is always a good practice to let an engine idle/low rpm to let things cool down at a slower rate before shut down.