Asiab3 and I were trading PMS back and forth, but they are useful so here they are: (I don't want the damned dashpot if I can avoid it)
Bob wrote:
"Drove it around the neighborhood (they must think I'm nuts) again, perky and doesn't die at stop signs. BUT after running at high RPMS, during the the transition to idle, the RPMS drop to about 850 and then slowly recover to 1100. When the engine is cold it just dies"
Asiab3 replies:
How are you denoting lean or rich idle mixture? I have witnessed, with a half-assed explanation, that a rich idle mixture will cause a stumble and/or reduction in idle speeds under certain driving operations like:
- Shifting into N from 3rd as if you're suddenly stopped by a yellow light on a main city street, then applying the brakes from a high RPM (but in N.)
- Applying the brakes downhill in neutral while the RPMs decrease
Do you have a throttle positioner or dashpot? I should have sent one damn; I have like 7 spares. Do you have the bracket on your throttle arm for one?
The dashpot slows the throttle arm closing, and does a whole bunch of emissions stuff. Since the distributor was retarded for emissions originally, the dashpot actually works in concert with the odd slow-down you're experiencing. When the throttle plate snaps shut, the engine is still sucking air+fuel. The vacuum (negative) pressure sucks WAY harder when the throttle is instantly closed but the engine is still sucking hard due to RPM. This means the engine sucks a plethora of fuel that it doesn't need for the first few seconds after throttle shutting. Add a significant retard of timing, and the fuel that gets sucked in with the extra vacuum just can't burn effectively. Enter the dashpot. It slows the final closing of the throttle arm, even when the pedal snaps back up under your foot. This eases the transition of vacuum, and prevents a significant amount of extra fuel from richening your mixture (and killing the earth, dude.)
Some '71 California cars even had the dashpot AND throttle positioner to REALLY save the planet: