Timing retards off idle, huh?

[K85 Octane]

I run ~13* advance at idle. I can use the buttons with my timing light. When I crack the throttle, the timing retards from it's ~13* for a couple seconds. Depending on how much I crack the throttle, it will either jump back to ~13* or just above. When it retards, it's probably closer to 2-4* advance before jumping back up.

I'm about 35* total at 4k rpm. My vacuum advance can is plugged and so is all my TB vacuum ports. I don't notice a problem running without vacuum advance.


Why does this happen?


MSD Pro-Billet HEI, no ignition box. Just key hot wire and go. No sensors, no nothing

 

[Blue85]

"buttons" meaning it's a dial-back light? If the light is determining the timing, it needs a steady stream of pulses. When the engine speed changes, the calculated speed is changing and you'll read the wrong "degrees" until it's steady state again.

But if it's 13 degrees on the timing tab, I can't think what would retard timing off idle other than the vacuum can. Even if engine speed is slowing as you crack the throttle, the centrifugal weights should be fully seated at idle and off-idle.

 

[paratrooper307]

Agree with Blue, you have to let it stabilize. To ensure your advance is working properly, you need to read it at different engine speeds. It should advance steadily and be "all in" (maximum advance) by around 3500 (this will vary depending on your timing curve). In a purely mechanical setup, you can't really retard timing as the engine goes up. Generally the worst that happens is your advance is sticks and doesn't climb.

A vacuum can is mainly to improve fuel economy during highway cruising. If you drive without one the main effect is crappy highway mileage. A vacuum can adds an additional 16* on an SBC IIRC.

 

[K85 Octane]

I don't have a stop for the vacuum advance and it doesn't have the allen inside the nipple. In my other thread, I mention pinging. I pulled the vacuum advance hose off because of this. Truck still seems to run find on the freeway though I'd like to plug the can back in.

My "all in" doesn't happen till ~4000rpm since I'm running very slow springs. That was, again, due to it pinging in the higher rpms.