TonyS

Make sure the breather restriction is in place. It should be in the T-piece on the valve cover. If its missing or someone has 'replaced' it, or its damaged inside you will get too much air into the bottom of the carb/manifold (bypassing the fuel). Increasing mixture will compensate, but will make it idle slightly badly and hunt when tickling over traveling slowly.

Also check generally for air leaks, cracked pipes in the breather pipes.