Does your B20 have a vacuum retard unit on the distributor?--many B20's came with that system. It pulls advance back at idle and at cruising speed. The distrbutor rotates counter clockwise so the vacuum unit should pull in that direction too. See if you have that type and if it works--it should be connected to manifold vacuum. Every little bit helps.
One other point--maybe something you can't do anything about----various production models used variations of the Weber we know as the 32/36, etc. Some of them (Renault for one) had electric idle jets--shut the key--no more fuel--I used that system to great effect using two Renault/Weber 30/30's on a B20. The great number of vacuum ports on the production carbs also allowed me to tailor a simple water injection system (which kept the top end perfectly clean). -- Dave