@Mattax I have a blue box oem style electronic ignition. I timed it using the method of adding time until the idle stopped increasing. I ended up at 14*. Engine runs fine no detonation or pinging. Just changed from 12 to 14 heat range plugs.
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Sorry to say but that is one of those methods someone makes up and thinks it works but it doesn't.
* Too much timing will only ping
under load. Part throttle load like accelerating up an incline, or full throttle in upper gears. When its borderline, it may only show up when the engine is fully heat heat soaked - like after an hour of driving the mountains or interstates.
* Way too much timing under light load and low throttle may show up as the opposite of smooth running; described as trailer hitching or fish biting.
* Yes the engine will run faster and even pull more vacuum with more advance, in neutral. But there's no load other than the internal friction. So we can not set base timing on that - especially with a truck that we wish to drive at walking speed. With an automatic, this shows up as soon as the selector is placed in drive. RPMs drop because even the tiny increase in load needed to turn the torquflite is too much and it shows the timing and fuel mix is not producing any power to speak of.
Those spark plugs don't look like they've been excessively rich for whatever they've been through. Get them in good light and check close with a magnefier for shiny metal specks in the porcelain. Those bright specks of metal would be indicators detonation. I suspect they are fine.
They also make me doubt the accuracy of that 10.5 AFR. Idle mixes can be trickier to read. Exhaust sometimes come back in - especially with some emissions era cams. Compression and heat in the chamber is low. The advanced timing puts less heat into the chamber walls also reducing the initial heat needed to vaporize the fuel in the chamber and burn it well. CO meter would be better in this case but they'e not common and I don't think you need to spend the money on one.
14 of course will accumulate less carbon than than 12s.
With a stock engine, use the factory specs as a baseline.
What distributor? Where did it come from?
Timing curves vary considerably.
In general a 318 is relatively efficient starting at low rpm. In '68 Plymouth spec'd non-emissions initial timing at 5* BTC and emissions version at TDC; 625 rpm.
In either case, it should be around 14* at 1400 rpm. You can give or take a couple degrees, even maybe as much as 5 * more advance depending fuel etc, but that's a reasonable baseline unless the truck engines were different.
A factory electronic distributor
for a car is almost certainly going to have a longer, quicker initial curve to hit that 14* by 1400 rpm (because the initial is set at 0*). But again a truck may be different.
I'd have no problem testing out with an initial of 10* around 600 - 650 rpm if you want to experiment. Maybe obvious, but set the initial with the hose to the vacuum advance disconnected and plugged. (Golf tee is handy).
