[padgett]

(sigh) OK are three things going on here. (maybe should be in a FAQ).

Everything has one purpose and one purpose only: maximum chamber pressure at 5-10 degrees ATDC (exact point depends on engine geometry.

To accomplish this ignition needs to take place a fixed time before that depending on the charge density which determines the flame front propagation rate.

Most post WWII cars use three things to accomplish this:

1) initial - the the basic ignition time at idle to achieve the above say 10 degrees.
2) centrifugal - as the engine speed advances the combustion time does not change much so needs more degrees advance to make peak pressure at the same point
3) vacuum: as manifold vacuum increases the charge density decreases and so does the burn rate. It needs more time to burn

All are kludges with a lot of variables (as octane increases the flame propagation rate decreases because it does not burn as well so 130 octane needs more advance than 87 PON).

Also emissions control called for more perfect combustion but that led to increased heat (as CO goes down, NOX goes up so more kludging - EGR.).

Finally the FED said emissions controls had to be guaranteed for 50k miles so the manufacturers found a point were some wear would not affect and MPG would be maximised. Drivability suffered & some strange things were done (ported vacuum advance, TCS switches,...).

Now Pontiac played some games in 1973 and cars on highways would take a sudden surge forward and MPG would increase (don't forget in 73 before catalysts, the CAFE was about 13.5 MPG) This led to the EPA decree in April that pretty much killed the SD455 in A and G bodies.

However, every engine has a "sweet spot" that varies with RPM, charge density, and octane. Some mechanics were better at finding that point than others. Delco Remy also had a vacuum advance that was "more" but not certified.

Delco Remy also published the 1D manuals that for every distributor part number listed the centrifugal and vacuum advance as curves. A Sun DT-506 is a very handy device for serious tuning of pre-computer cars.

Have rebuilt mine once and probably needs it again.

[67gtospud]

Originally Posted by steve25

Does stabbing the gas and switching off the ignition just as the rpm starts to drop down make the run condition on worse or better?

Stabbing the throttle does not make it stop dieseling.

After resetting the carbs and idle that seemed to help tremendously with a part throttle cruise to light acceleration stumble I had.

[67gtospud]

I finally got my new springs in. With the 2 heaviest springs timing is all in right around 2500-2600 rpm. I haven't test drove it yet but I'm wondering if I should maybe lighten the weights a little to allow the timing to come in a little later anyways? Thoughts? If anything this would allow more options to change the curve later on anyways

[Rocky Rotella]

Can you take a picture of the center cam on your distributor shaft? It may shaped for quick advancement and that could limit the possibilities.

[padgett]

Thought the good set came with several sets of weights. Also for 10:1 compression and premium fuel you wanted max to be 34-38 degrees total in the 3200-3400 range range.

[shermanator2]

Originally Posted by johnta1

So more than likely you would never have full centrifugal advance and full vacuum advance.

How about backing off the throttle at 4000 RPM? May not matter much in a car that only goes in a straight line, but in a corner carver it does happen.

[67gtospud]

First picture is my current distributor, second pic are spare distributors I have.

[Rocky Rotella]

Notice the difference in the shape of the center cam between that in your car and the spares? It looks like yours is very aggressive. It might be time to swap the shafts around to see if you can arrive at curve that you desire. Unfortunately, that's best accomplished on a distributor machine and not necessarily in the car.

What's the number of the distributor presently in the car? It would be interesting to look it up in the Delco manual to see how aggressive it really is.

[lust4speed]

Originally Posted by 67gtospud

I finally got my new springs in. With the 2 heaviest springs timing is all in right around 2500-2600 rpm. I haven't test drove it yet but I'm wondering if I should maybe lighten the weights a little to allow the timing to come in a little later anyways? Thoughts? If anything this would allow more options to change the curve later on anyways

The heaviest weights should produce the slowest curve and your results have full advance at an early RPM level. Not sure what's going on there?

How much advance are you seeing?

[shermanator2]

Originally Posted by lust4speed

The heaviest weights should produce the slowest curve and your results have full advance at an early RPM level. Not sure what's going on there?

No. Heavier weights bring the advance in sooner.

[padgett]

+1 heavier weights advance faster (at a lower rpm) than light. Strong springs advance slower than weak.

A good dial retard timing light is a wonderful thing to have.

[JSchmitz]

Originally Posted by 67gtospud

Would I be better off with my existing weights or the moroso weights if I use their springs?

Never had ANY success with aftermarket weights and cams from anyone! Use the factory stuff with aftermarket springs.

[lust4speed]

I really shouldn't type when I'm tired. OP was talking about springs and please substitute "springs" for "weights" in my comments. I meant to say that his using the two heaviest springs would produce the slowest curve and timing would come in later than the lighter springs in his kit.