I've just put a new piston in a freshly replated bore (don't lose your drain bolt whilst riding - it gets expensive!) and have noticed a couple of things the bike does that it didn't do before. a) It doesn't tick over as reliably. Even when holding it on part throttle it will sometimes just 'pop' and die. Never did this before the rebuild. Sometimes (but not all the time) When trying to start it, it won't kick all the way through the stroke, but stops half way as though I've come up against compression again. Bear in mind that I've had the bike for 18 months so know it quite well, and these things didn't happen before the rebuild. Only thing I can think of is that originally I didn't change to YZ timing until the bike was 6 months old (and therefore well run in). I've set it up with YZ timing with the new piston, so maybe it will settle down when I've done a few meetings?