He was advised to crack the secondaries open slightly because he was pulling vacuum on the spark advance port at idle. That's an indication that the T ports on the primary side are too far exposed, so to make up for it, he was advised to pull the carburetor, get the primary T ports square and crack the secondaries very slightly. It's a very, very common practice people have done for decades, along with drilling the throttle blades. I don't like drilling the throttle blades, because that's irreversible. Playing around with the T port exposure is not.
