Right , my reason for starting this thread is to (hopefully) provide a resource for information on how to combat those little niggles that can be a problem to KX owners.
Please contribute your ideas cos that way it should grow and help us all out.
Fault
The top exhaust mount (under the tank) on just about ALL KX 500s breaks.
Fix
Get a welder to reinforce it with 1/8" by half an inch flat steel.
The chain seems to rub on the inside of the rear subframe (on mine anyway) so I cut a length of aluminium which is aproximatley half an inch by a sixteenth of an inch thick angle and about 6 or seven inches long (I am writing this at my girfriends house so don't have exact measurements) ans rivet or screw it onto the frame so it wears instead of your expensive subframe.
Destrys bike has this mod cos I have seen a picture in a MX mag and could just make it out on the piccy.
Fault
Aparentley they also can shear woodruff keys which allows the flywheel to slip round and then your timing is out.
This can be caused because the crank end taper is not an exact match for the tapered hole in the flywheel
Kickback anyone
Fix
Remove your flywheel and woodruff key from the crank end taper.
Put a dab of valve grinding paste onto the taper and then put the flyeheel (minus the woodruff key) back on but don't put the nut on.
Lap the two surfaces together by turning the flywheel until they are evenly matt in colour.
Wipe ALL the grinding paste off the taper and use carb cleaner to get it out of the inside of the flywheel cos it's a bugger to get out otherwise.
Reasemble it all and tell the folk with YZ 490s about what you have done cos they have this problem also (acording to Eric Gorrs book anyway).
Thats all I can think of for now but I will post any more if I think of any.
Please add your KX faults and fixes to this thread and any suggestions you come up with