Erratic high rpm ignition timing
 

On my 66 502, with an MSD pro billet distributor, the car runs well. But the ignition timing (as seen with a timing light against the balancer) gets "jumpy" above about 3500 rpm. From what I can see it moves about +/- ten degrees at higher rpm.

Obviously, this is not good. What should I do, other than replacing the rotor and cap (which I doubt will help). I have the distributor out now, and it appears in good shape -- relatively new, no play in the shaft that I can discern.

Gerry

Re: Erratic high rpm ignition timing (Hank)
 

Which MSD box are you using?

Re: Erratic high rpm ignition timing (Monty)
 

It's painted black but I can just make out MSD 6AL on the side. Seems to have a heckuva lot of wires going into it. At least 6 wires from harness to MSD box. Hope I don't have to mess with that.

Re: Erratic high rpm ignition timing (Hank)
 

Is there any chance that your timing light pickup is being triggered by another spark plug wire (#3 maybe)? One thing I've noticed with the fat wires (8.5mm+) and wire coverings used these days is that the spark plug wire becomes too big in diameter to fit inside the timing lights pickup and it produces irratic timing light function.

As you probably know, MSD iginitions produce a multi-spark disharge that typically lasts around 20* of duration. This can sometimes cause erratic timing light function, but this is usually the case at lower rpms. Most MSD-type ignitions switch over to a powerful single spark at around 3000-3500 rpms, because there isn't enough time to produce multi sparks over such a wide duration as engine speed increases. This transition might be what you are picking up at that rpm with your timing light.

How does the car run? Does it feel like it's missing, hesitating, etc at that rpm?

What retard module(rpm) is in your 6AL? Are you using any other ignition products, such as a stutter box, 2 step, etc?

Re: Erratic high rpm ignition timing (Hank)
 

Are you running a roller cam?if so then do you have a cam button and is the endplay properly adjusted this will cause all kinds of timing problems.