I just changed over from my Rotary Master ignition box to a MSD 6A box to the leading coil only. Started the car with leading plugs hooked up only and everything seems to work. The vacuum has now drop from -426 to -368 and no matter how much I alter the leading timing IGL the idle quality does not change. If I change IGT the idle changes quite abit.

My question is why does the IGL not work at idle anymore? Is this because of the multi-spark ? Can everyone else with MSD 6A change their idle by adjusting IGL only?

 

You said only the leading plugs are hooked up, so no way can changing IGT affect idle unless your coils are hooked up wrong. Please explain better.

If all coils and plugs are hooked up correctly and only changing IGT affect idle, then the leading coils are not firing. One car I worked on that was running poorly had that symptom, the leading coil was fried.

 

You wre 100% correct. Leading coil was not firing, changed the coli and the can ran great.