I was having trouble with cylinder#3, and traced it back to a leaking petcock. Unfortunately, the petcock doesn't seem to match the seca version of my bike (which is the one that the rebuild kit (K & L Part #18-2698) seems to be for):

Front of petcock:

Back of Petcock:

The pieces:

Closeup of the diaphram:

Picked up a '83 Yamaha XJ750 Maxim on Aug 17th, 2006. Bike has low KM, and seems to be in great shape. I guess time will tell...

Here's a few pics of the bike a few days after I got it home: