[Jack21]

Same problem.

351W, MSD dist, Accel adjustable vacuum advance. Misses under high vacuum conditions.

The brass tip on the rotor is about 1/4" wide. It sits right under the brass tip in the cap under static timing conditions (14 deg BTDC). The spark has to jump this gap to fire the plug.

With the engine running, the centrifugal advance moves the rotor tip away from the cap tip 20 deg crank, 10 deg dist. The vacuum advance moves the rotor tip an additional 16 deg crank, 8 deg dist away from the cap tip. Add them up, and if the rotor tip is too far away from the cap tip, the spark won't jump the gap and the plug misfires.

Still working on this, but here's what I'm doing.

Made a new rotor tip with some brass stock with a 1/2" wide foot on it. That solved some of it. Cranked the vacuum advance back some, and that helped, but defeats the purpose of the vacuum advance part throttle cruise economy. Next, I'll try rotating the distributor one gear tooth retarded so the center of the rotor tip advance motion ARC is centered on the cap tip. Will let you know how that works out.

So why don't OEM setups cause this problem on the family sedan, mid '70's - 80's vintage? The centrifugal advance isn't all in until around 4000RPM. At cruise, the advance is mostly vacuum. With your foot in it, the vacuum is gone, and the advance is mostly centrifugal. You don't have conditions where all the vacuum, and all the centrifugal is in at cruise RPM. And this one tooth off misphasing may have some merit also.