Selecting power valve method Holley 4160

An AFR gauge helps but is unnecessary as mattax said. Change one thing at a time and quantify the results. For primary jetting, lean it out two jet sizes at a time and go for a cruise. When it surges under cruise go back rich one jet size and go for a cruise again. If it surges go one more size rich and retest. When it stops surging in cruise, you leave the primary jetting alone.