engine slows when advance timing

[fried okra]

Thx, Fullraceflathead.....it looks the same (or similar) as others I've seen on line, the tab is offset for example, wonder how one would tell if it's one of the bad ones?

Bought it from one of the major suppliers recently.

You got 3 deuces on the SD455 in the GTO?

[P.S.]

Originally Posted by fried okra

Thx, Fullraceflathead.....it looks the same (or similar) as others I've seen on line, the tab is offset for example, wonder how one would tell if it's one of the bad ones?

Bought it from one of the major suppliers recently.

You got 3 deuces on the SD455 in the GTO?

Other possibility is the distributor upper plate. Some of the repro plates position the points in a location so that when the points just open (to cause the coil to produce the spark), the rotor is too far forward advanced in its rotation. That causes the motor to run fine with the timing lever up, but as you advance the timing, the rotor ends up so far ahead of the button in the distributor cap that the spark has to jump further and further in order to make it to the plug. That has happened on a few distributors that club member have brought to my shop.

The fix is to move the points mounting location and re-drill the repro upper plate. Or, better yet, replace the upper plate with a better quality repro or an original.