Motorcycle Repair: Lousey gas milage, friction discs, fuel consumption

Question
'73 CB500/4. Previous help was excellent. Have the bike running, clutch is a little gooey and when I got it, the clutch was frozen. Unstuck and cleaned the friction discs, but I'm sure I need to outright replace them. The other thing is fuel economy, which is awful. I'm getting close to 20-25 miles a gallon right now, which is unreal. It doesn't SEEM like it's running rich, but I just can't tell. The air filter *is* dirty and old, I'll admit. All the other adjustments should be dead-on, however. (Synch might be a bit off, but not too bad). Any ideaS?

Answer
That bike should get somewhere between 35 and 45 mph depending on how you are using it.  There are a few things that can cause a significant loss in fuel consumption.  First, do a quick compression check.  I think that bike should have about 140-150 psi.  If your bike is somewhere around there and all of the cylinders are within 10-20 psi of each other, the problem is somewhere else.  

The next thing I'd check is the ignition timing.  It needs to be right at idle but also check it with the engine reved up to 3000 rpm or higher.  Make sure the timing is advancing correctly.  

The next thing to check is the carburetors for proper float adjustment and sync.  The synchronization can cause a loss in performance but I wouldn't expect it to increase fuel consumption that much so more likely it's float level.  

Good Luck
Rich