have a rebuilt 670 srarts and runs well but wene you go to start it after a little riding she wont light up allot of pulling and holding the throttle maby its the coils,starts right up if you leave it alone for a bit
If it is the old rotary valve you need to adjust the float height. If you park this thing facing down hill even just a little bit it will continue to feed fuel to the motor and flood it. You can fix this by parking facing up hill all the time or adjusting the float height so that the bowls are not full to the top.
Check your head gasket it's probably pooched. If it takes a lot to get going when it's warmed up that is because the gasket shrinks and the compression goes down making it hard for the engine to keep running. Best thing to do would be run it until it's warm then check the compression in both cylinders if one is 20-30 psi less it is probably a seal or a gasket. Hope you figure it out.