As mentioned, two gaskets on the float bowls changes the fuel level in the carbs - this is a Bad Idea. You could readjust the float height to compensate for the second gasket, but all you're doing is maladjusting one thing to accommodate another incorrect thing. Float level is directly related to fuel consumption... from your description, your floats are (or have been) too high.

Remove the second gasket; set the floats to the proper height. Confirm that you have appropriate needles in the carbs.

If you still have two manifold gaskets, it's possible that you have false air and that the carb tuning you've been able to do is trying to compensate. I'd encourage you to remove the two gaskets and replace them with one; THEN check the valve adjustment (just to be sure), THEN set the timing to spec, THEN tune the carbs. There's no good reason to use two gaskets when only one is called for.

Additional phenolic spacers might help a little, but they're _not_ a solution to poor mileage. Something else is wrong; and needs to be corrected. Spacers are a fun thing to play with when tuning - but tuning can only be accomplished after everything is set to a solid baseline.

Best,

Cameron
http://www.myspace.com/swedishrelics