Senior Tech Team
Join Date: Apr 2012
Location: Yerington, Nevada
Re: HEI rotor marks off center
Everybody calm down. If that HEI has a vacuum advance on it, the pattern is totally normal. When the mechanical advance moves, the firing tip on the rotor stays where it is, NO movement. The weights and scroll plate on the mechanical advance move the timing, but not the rotor tip.
BUT, with a vacuum advance, the V/A moves THE MAGNETIC PICKUP IN AN ARC, from one side of center at rest, and to the other side when fully vacuum'd up.
This is why when some dyno hero rips the vacuum advance hose, or, the advance itself off a distributor, and puts a fixed stop pin in its place, the rotor firing tip aims to one side of the center of the cap terminal, and doesn't move. So, MSD makes an "adjustable" rotor, so the rotor tip can be centered on the wire terminals inside the cap if some moron disables the vacuum advance
So, when the rotor moves, it makes a long pattern on the plug wire terminal in the cap, normal, no problem. The reason the wire terminals inside the cap are so wide is that there are more than 30 vacuum advance curves in large, coil in cap HEI's, so, some move the rotor tip a small amount if travel, others move the rotor a great deal. Instead of making 30 different caps, why not just lengthen the wire terminal in the cap? That is just what we did, and the idea came from the point distributor cap for the odd fire Buick V6, which fired not 120, 120, 120, 120, but 90, 150, 90, 150, with a set of long wire terminals in its points distributor caps.