[TW_in_BC]

This happened to me again over the weekend...has happened a few times and the build up is always the same:

Riding along...then make some sort of last minute course-correction (taking a different trail, etc)...lug the engine a bit...a few pings...dead.

Then, when I go to crank her over...it's as if there isn't even a piston. Cranks over VERY fast (almost as if there is no compression at all) but won't start.

Sit down for a few minutes and let her recover...start her up...away I go.



As I said, this has happened once or twice before and I don't know WHY she sounds the way she does when I try to re-start.



It's a mystery.

 

[srs713]

Whoa... sticking valve? Not sure if you want to do this with a hot engine, but next time ya might pull the plug & test for compression. With something other than your thumb of course.



I'd guess a sticking exhaust valve would keep the piston from inhaling enough fuel/air mix from the intake valve. And then the stuff would just be pushed out the exhaust valve and there would be nothing compressed for the plug to ignite.



If it's a bent valve, it could be catching part way open when hot, then letting loose when it cools. It might just keep moving while the engine is running fast, but have a chance to stick when slowed down due to lugging the engine.

 

[TW_in_BC]

Well, I pretty much dismissed any notion that there is something wrong internally, or it would show up all the time, no?

I rode it hard for at least another two hours after this incident without so much as a hiccup or miss. If it was a bent valve, it would be bent permanently, wouldn't it?

As for the "cool down" period...it was maybe 5 minutes. Just long enough to remove an extra T-shirt and to take three photos.

When I went to start it up again, it still cranked over faster than normal, but started up quickly.



Vapour lock? (I did overfill the tank in anticipation of a LONG ride, and I did remove the gas cap to verify a fuel supply (rule #1))