I would agree with the secondary air valve being the problem. The best way to tune it is to have both the allen wrench and screwdriver in hand on the right hand side of the carb. Use a pencil to make a mark where the screw is starting out, loosen the allen screw (on the underside) then tighten the screw just a little bit (an 1/8th of a turn makes a huge difference) and take it for a drive. Mine was too tight and we loosened it. That little change was worth about a .15 at the track. Little test was to bang the screw driver on the carb to see it the flap moves a little. The heavier car will need the carb to open a little quicker. Like you said, it is an art.
