I rebuilt my carb a while back (1954 Merc - stock) and my mileage is only about 10 mpg.  I often smell raw gas, especially on down grades, and my exhaust tips are getting carboned.  I pulled the plugs and they are good and running clean.  I read some time ago that the power valve actuator cover seal could break due to backfiring and that the result could be poor mileage but that it would run okay.  I had a lot of backfiring when I inadvertently installed a distributor with the wrong amount of teeth!  (Took a while to figure that one out).  Does anyone have direct knowledge of this phenomenon?  Also, is there something else I should look for?  The engine runs well, have to pump it quite a lot after having sat for many days and is slow to start on very hot days when the engine is warmed up.