Distributor springs
The early smog stuff focused on reducing CO and HC. Idle air fuel mixtures were leaned out to 14.2 and initial timing as well as engine deceleration timing was reduced. This makes no sense in terms of efficiency (less timing advance with leaner mixture) but put more heat into the cylinders and exhaust ensuring the burn of excess HC. By '73 NOx was also being addressed so not sure what that might have looked like in terms of timing curve.
For best efficiency, assuming no smog equipment other than PCV, the timing curves from '66-7 are good starting points. Of course there was no 340 in '67, so that means we have to look at the '68 timing with the understanding that timing at idle was less advanced to meet the emissions goals of that year.
Here's the Plymouth timing curve for '68 340
One thing to watch out for when experimenting with initial timing. In neutral an engine will run lean, with lots of advance, and love it, because its only overcoming internal friction. As soon as its slipped into gear, its got no power, loses rpm and wants to die. I'd target experimenting in the range of 10 - 15* initial for a stock 340, with advance begining between 600 and 700 rpm. Keep in mind this timing at idle pairs a bit richer mixture for best power efficiency. if you access to a CO meter or WBO2, something between 12.9 and 13.9 once warmed up is probably where it will end up.
Taking that and using the '67 273 4bbl timing for some guidance, move the initial to 10* BTDC. Then looking at some general performance target for high rpm WOT timing, 32 to 36* is probably the range for that. Shifting the stock timing to accomplish that and the result is the dashed blue timing curve.
Do you have the stock advance info from the '73 service manual? Just curious about what they were doing, that's all.
The early smog stuff focused on reducing CO and HC. Idle air fuel mixtures were leaned out to 14.2 and initial timing as well as engine deceleration timing was reduced. This makes no sense in terms of efficiency (less timing advance with leaner mixture) but put more heat into the cylinders and exhaust ensuring the burn of excess HC. By '73 NOx was also being addressed so not sure what that might have looked like in terms of timing curve.
For best efficiency, assuming no smog equipment other than PCV, the timing curves from '66-7 are good starting points. Of course there was no 340 in '67, so that means we have to look at the '68 timing with the understanding that timing at idle was less advanced to meet the emissions goals of that year.
Here's the Plymouth timing curve for '68 340
One thing to watch out for when experimenting with initial timing. In neutral an engine will run lean, with lots of advance, and love it, because its only overcoming internal friction. As soon as its slipped into gear, its got no power, loses rpm and wants to die. I'd target experimenting in the range of 10 - 15* initial for a stock 340, with advance begining between 600 and 700 rpm. Keep in mind this timing at idle pairs a bit richer mixture for best power efficiency. if you access to a CO meter or WBO2, something between 12.9 and 13.9 once warmed up is probably where it will end up.
Taking that and using the '67 273 4bbl timing for some guidance, move the initial to 10* BTDC. Then looking at some general performance target for high rpm WOT timing, 32 to 36* is probably the range for that. Shifting the stock timing to accomplish that and the result is the dashed blue timing curve.