Motorcycle Repair: 1981 Yamaha XS400 Special II, valve rocker, air leak

Question
Hi,
My XS400 has a problem when I rev up the engine it either stays revving at that RPM or will actually keep increasing RPM to the point I have to shut it off.  I have had the carbs apart several times to clean and set float levels, etc.  The valve/rocker arm clearances are good. I don't understand how the engine could be getting more fuel/air when the throttle butterflies are closed and the vaccum controlled slides are not moving.

Answer
This usually means the engine is running with too much air in the idle mixture. Try increasing the amount of fuel going to idle and then reducing the RPMs (via the throttle stop adjustment).

Another possibility is that there's an air leak somewhere in the intake.