Bruce ( Shrinker )

Bewy said
Greg,
Post 67. With that big cam, it is going to need a LOT more than 17* for best/smoothest idle, more like double would be closer.
One way of doing this is to use an an adj vac adv unit connected to manifold vacuum.

This is a most misunderstood subject.

People often tune the carb idle before dialling in the timing at idle, when it should be the other way round.

From D. Vizard:
"The optimum idle advance is typically about 35-40* for a short cammed street engine & [ though not commonly realized ] as much as 50* for a street/strip engine."

And [ scroll down to post #6 ]:
www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

Here's what Bruce said to me when I asked him about this particular issue:

"I assume your talking idle timing as being 20 to 25 for best idle running on your engine. Lean mixture requires more timing advance. Lean burns slow. The other thing that slows burns is exhaust gas contamination of the fresh charge. So think about what 108 lsa does compared to say 114.
The relationship of vaporization to timing is thus. With equal fuel for comparing purposes of 2 engines, one with poorly vaporized mixture the other with very good vaporization, the engine with poor vaporization will require energy that is released from the combustion to bring it to an equal state of vaporization compared to the very good engine. You can think of its in as simple terms as that and you wont be wrong in any conversation. Any energy removed from the combustion to achieve further combustion is wasted and non productive at the crank."

And

If you enrich the mixture at idle it will require less timing and the volume of gas expelled from the chamber will be greater (because there is more fuel in the cylinder ). The greater volume exhausting resists reversion backwards up the exhaust port therefor it actually runs with a cleaner chamber. Funny stuff isn't it.
BUT what you have to do is enrich it correctly with the right processes in the carburetor, sometimes it's not just a matter of winding out the idle screw or changing the IFR. Every engine has a design to it and Chrysler's are a bit different to the other US products, they are not chevs or fords, they are better. They have better combustion and its not necessary to follow the rules of the hotrod industry, as the industry is very Chev orientated.

Lots to think about in those 2 paragraphs.