MSD distributor tuning

gliderider06 said
. I feel the heavy silver is limiting my advance. If I take out the heavy for a lighter, it will ping. (Heavy truck)

There's a clue, isn't it. Figure out the timing, rpm and vacuum when it pings. Mark that in red on your timing vs rpm chart, and whether it was moderate load or heavy.

gliderider06 said
I also toy with the idea of using a factory Mopar spring with the elongated end (smog limit spring?)

That's a bunch of nonsense repeated by the comic books and entertainment shows. Nothing to do with smog. The secondary spring is used to slow advance because combustion efficiency improves once the engine is over 1600 rpm or so. On a 318 its probably a little earlier (lower rpm) unless its been hot rodded.

gliderider06 said
This truck needs more timing and responded well to the 25* bushing. I will probably go with the 28* after I get more Mechanical out of it

it seems the truck may be telling you otherwise.

You should look to the factory curve for that engine. Thats the baseline of what the truck wants. And if you can find a pre '79 truck 4 bbl 318 curve, that will be basically pre-smog version. The diffence between 2 bbl and 4 bbl is the 2 bbl is a bit more efficient at idle and off idle and the 4 bbl combustion is more efficient at mid to high rpm. So after 2000 to 3000 rpm the 4 bbl timing will advance much more slowly than the 2 bbl