Leaking intake valve

[Take Warning15]

I've got a 75 with BS 38 carbs. I've got a bad intake valve on the right side only. Left side runs and has compression. Looking into the head with the manifold off I can see lots of carbon buildup around the valve.

Two questions:

1. Why might this happen? Running too rich on that side?
2. Thoughts on lapping the valve with the engine in the frame? If I can get to it in the frame, would it be effective enough or should I expect to need a machine shop?

thank you

 

[5twins]

Re-lapping the valve in so it seals again would probably fix it but I don't think you could do that with the engine still together and/or still in the frame. You use lapping compound on the valve and seat when you lap it. This is a grinding paste. You have to thoroughly clean it all off after lapping. You also need to inspect the seat area on the valve and in the head to see if you've lapped enough to remove all the pitting. There's a good chance that the seat got all covered with carbon and that's why it isn't sealing up anymore. That would need to be cleaned off, then the valve lapped back in. I can't see you adequately accomplishing any of this with the motor still together.

 

[Take Warning15]

This is what it looks like inside. Any thoughts on why the valve would contact the piston? Would having cam chain off a tooth cause this?

 

[TwoManyXS1Bs]

Over-revving can do that, but usually results in severe boogering.

Valve sticking in the guide more likely...