Ok, so I'm trying to tune my engine to idle better, and I'm seeing
something very strange with the timing.
Basically, while looking down on the harmonic balancer, while using a
timing light, I see that I have it set at around 4 degrees BTDC at
idle. When I rev the engine, the timing mark moves in the direction
the engine is spinning...as if it's retarding rather than advancing! I
KNOW this is wrong, because I've time this engine before. I know that
the timing mark should move the opposite direction that the engine is
spinning (engine spinning clockwise, advancing the timing should move
This isn't exactly a complicated setup... 350ci Chevy with a HEI
distributor, vacuum advance. What the Hell could be causing this? The
vacuum advance module was replaced with a brand-new one.
What's even stranger is, the same thing happens regardless of whether I
use ported or manifold vacuum as the source to the advance module.
Shouldn't one produce exactly the opposite effect of the other?
FWIW, the only thing that's been changed was, I jumpered the 1st and
3rd pins of the 5-pin module together to bypass the ESC (cheapo
ECM-like device that retards spark based on input from a knock sensor).
This mod was/is GM approved and was released in a service bulletin