I ran across an article on ignition timing and it stated that you set total timing by disconnecting vacuum advance hose from the dizzy, and plug the hose then rev engine up to 1500-3000 rpm with timing light set at 36 degrees and the initial timing will be as it is. That did not make sense to me. As I have understood it you must set the initial timing first then rev engine 2500-3000rpm to get advanced timing to total 36 degrees. What is the correct way to set total timing to 36degreees? My other question is when disconnecting the vacuum hose from the vacuum advance on the dizzy and plugging it, do you also plug the vacuum advance unit on the dizzy as well?

 

The centrifugal advance generally needs to top out around 36 degrees with traditional old school cylinder heads.

Ignore the traditional factory initial timing setting method - To set the timing, you first disable the vacuum advance by disconnecting the hose and plugging the vacuum leak on the engine side. Rev the engine up while you watch the advance move. If you see it stop advancing even when more RPM is added, you have the centrifugal advance topped out. That needs to be set (while topped out) to 36 degrees. Set it there and lock down the adjusting bolt. You can reatach the vacuum advance hose now.

Wherever the timing falls at idle is just where it falls at that point. You can make changes in the curve from there using different advance springs to change the rate of centrifugal advance. The centrifugal needs to be steady and not start advancing until well over idle, say beginning to rise at say 1200 RPM for example. It needs to be fully advanced at about 300O RPM. You can shorten the advance slot travel to increase initial timing at idle. The total centrifugal needs to stay at 36 while the initial is modified by altering the travel in the advance slot. You can add a copper bushing to the advance mechanism pin or braze the slot closed a bit and it will have the effect of increasing the advance at idle.

Vacuum advance can add about 12-14 degrees for a total centrifugal plus vacuum of around 52 degrees.

 

Above is correct. You do not need to plug the vac advance on the disti side. Once you adjust your timing and reattach the vacuum advance hose, you may need to adjust your idle up or down to your liking.

A golf tee works well to plug the vac adv hose.

 

The key item in the second post above is you cannt do this with the stock advance springs and you make no mention as to whether or not you purchased a spring/weight kit before attempting this.

 

You can do this with stock springs. Initial advance is only important if trying to pass emissions at idle. Too much initial and engine can buck when starting hot Cuz the charge fires soon enough to stop the starter.

 

stock springs do not hit total advance till way past 3,000 rpm, 4k+, which is why you cannt use them for this purpose.

 

True ,kind of like built in factory warranty protection.

 

If you change the cam for higher RPM range, the idle may like more initial advance, and the engine likes the advance to be "all in" at a lower RPM.

The vast majority of factory stock GM distributor curves will have the Initial setting + available Centrifugal advance from the mechanism = 36 degrees already. The need for softer springs is to finish adding all of the centrifugal advance earlier in the RPM of the curve. Reducing the available Centrifugal advance is only used if you need to raise the initial advance at idle to improve idle quality.

Even if you never modify the distributor curve/springs at all, setting the ignition timing at full centrifugal advance to 36 degrees at the top of the curve is still valid.

 

Not always true. I have a NOS 70 BBC GM dizzy that is spot on to the power book curve.

 

Um, it also has a great affect on idle quality and idle cooling ability.

 

I was specifically referring to the springs used on a 1979 HEI set up that the OP is using. The information is from the GM Archive site.

 

Thank you. I did not see that.