How do I know if the valve seals need replacing? From what type of smoke can I detect?
A 'leak-down' test is the best bet, shows where in the system the compression is leaking and if it's around the valve stems than for sure the seals need replacing. Can't judge it by the smoke accurately as oil could be slipping past the rings also.
A leak down test won't tell you anything about the valve stem seal. It will about the actual valve face sealing in the valve seat, but the valve is closed when performing a leak down test. The best way to diagnose worn valve stem seals is if you get a lot of oil smoke onstart up, but it goes away in a few minutes. A real giveaway on bikes is after the bike has set on the side stand overnight and when you fire it up, you get more oil smoke out of the left side muffler than on the right side muffler. That's because the oil in the valve area runs to the left side when parked on the side stand.
