the air door on the secondaries has no direct connection to the secondary fuel delivery system. What puts the fuel into the secondary airstream is the flow thru the venturi effect created by the secondary air door. What i think is happening is that you are getting too much air thru the secondaries faster than the fuel can start flowing into it causing a lean spot. So I would tighten up the tension spring maybe a quarter turn at a time and test until you are happy with it.
They make a special tool to adjust this, if you don't have one it can still be done but the odds are you will loosen the lock nut and then the spring tension will unwind on you and you;ll be starting from scratch.
