Sometimes when starting, particularly if the old girl has turned over a few times and is a bit reluctant to fire up, I get a loud one-off clunk. Scares me to death! But when the bike starts and runs beautifully, with no strange noises at all. Until the next time it happens - maybe next week, maybe next month.
The sound seems to come from the upper engine, RH side.
Back in 2008, Classic Motorcycle Mechanics published a very unhappy article claiming all sorts of woes for our prized XZ550 motors, including cam chains that "thrashed themselves to oblivion" Certainly the bike they describe as a lemon bears no resemblance to my experience. But I am curious about that clunk! Anyone had a similar random clunk? What was the diagnosis and cure?
https://trademe.tmcdn.co.nz/photoserver/plusw/1027044674.jpg
Does it happen just as you let off the starter button? If so it is the low voltage no fire situation. The starter drops the voltage below 10 and no fire. Let off the starter and you can get a spark, bang! More or less harmless but disturbing.
Has your starter clutch been staked to the flywheel ? Is your starter clutch in good condition? A worn bearing surface where the starter clutch rollers engage the starter gear, or worn rollers, can cause a slipping and grabbing starter clutch.
Have there been more clunks lately? Next time try to focus on where a noise comes from, it might be helpful, or not. :)
A starter clutch can make a heck of a racket at times when it isn't firing just right. Kind of like it's trying to rotate backwards with the starter still engaged.