Centrifugal Advance Starts Too Soon

[minifridge1138]

Hey everyone,

My base timing is set at 12 degrees.

I'm working to get my centrifugal advance to be all in by around 3000 RPM.

The only springs that get it there start too soon and provide 13 degrees advance at idle. This combined with my base and vacuum produce 40 degrees at idle, and that seems too high.

I have another set of springs that provides 2 degrees at idle, I don't know when they are all in (I stopped revving when I got close to the orange line on the tach).

The only solution I can think of is lighter weights.

If I reduce my base, that will lower my idle but will also reduce my advance when the centrifugal is all in.

Does anyone else have any suggestions?

 

[Hib Halverson]

The first thing you need to understand is that the springs and the weights do not control the maximum advance. They only control the rate at which the spark advances. Maximum advance is controlled by the length of the limit bushing slot and the limit bushing itself. Typically, OE centrifugal advance mechanisms from distributors built after the mid-1960s allow too much centrifugal advance.

The second thing you need to know is while tuning the spark advance curve of a distributor with mechanical advance, you need to disregard vacuum advance. Vacuum advance is a fuel economy and drivability enhancement, not a performance enhancement.

The rest of this discussion assumes both a compression ratio and gasoline octane rating what will allow the suggested spark tuning without detonation.

You need:
12 degs. initial advance
22-26 degs. centrifugal advance all in by 3000-3200 rpm.

Once you have that, then install an adjustable vacuum advance mechanism that allows you to limit vacuum advance to 8-10 degrees.

 

[Muzz]

MY HEI distributor came with an AS-20 vacuum advance unit which can be tuned by using an allen wrench to adjust how much advance it will yield at the rated vacuum. I was able to adjust it easily from lock to lock took about 9 turns. If yours is adjustable that will get you where you need to go I think. Worked for me.

Now, what I want to know is how does advance affect performance? What happens with low vs. higher advance etc.? Where can I learn how the various tuning combinations are SUPPOSED to affect performance?

I think the allen wrench that adjusts my vacuum unit is 1/16".

I ended up using one silver and one gold spring from my kit. Got about 35 degrees total advance. Still tweaking as I learn stuff. I had it up to about 45 degrees without knock but the cam mfg called for about 35 deg total advance.

 

[minifridge1138]

I have a non-adjustable vacuum advance.
I will keep playing with weights and springs. So far I am not getting and pre-detonation, so I have a safe starting point.

If you want to learn more about ignition timing and how it impacts performance, go to this page:

http://lbfun.com/Corvette/Tech/vettetech.html

And scroll down to "Timing and Advance." The 2 articles by John Hinckley (JohnZ on our forum) will explain everything you need to know about ignition timing.

Good Luck!!!

 

[petes74ttop]

Just to say Thank You!

Your right minifridge1138, we owe some thanks to Barry & Linda's Website and to Corvette Enthusiast Magazine, to Hib Halverson and John Z for taking the time to show all of us the correct way to work on our Vettes and to all of the rest of the members that have had the patients to repeat themselves and enlighten all of us on how to do a repair properly. PG.