Quick Tech With Marlan Davis
Maintaining idle stability as cam duration increases mandates progressively more carburetor throttle-blade opening (via the idle mixture screws) to compensate for the reduction in cylinder pressure induced by the larger cam’s later-closing intake valve. Eventually, the at-idle throttle-blade angle increases to the point that it uncovers the transition slot, thereby activating the main circuit and rendering the mixture control ineffective. One solution is to raise cylinder pressure by running more base (initial) timing. With cylinder pressure restored, less carburetor throttle-blade opening is needed, so the idle mixture screws should again become effective. At this point, you would want to shorten the centrifugal curve so overall total timing remains optimal for the particular engine combination.
[ If after installing a longer-duration cam you have idle stability problems, try increasing the initial timing and shortening the centifugal advance curve.