Understanding timing issue 72 LT-1

[Gary S.]

While I have had a dial back timing light for a while, I've never really used the dial back feature. Today I hooked it up and I am getting information that confuses me. I could use some help if you can.

Here are the background details - 1972 LT-1, 40+K miles and all original drive train. In 2004 the original distributor and carb were rebuilt at Camaro Hi-Performance, by Mr. Jerry MacNeish, whom some of you probably know. The specs on the distributor (1112101) from the report are:
1. Dwell 30-32
2. Centrifical advance curve total degrees 26*
3. Total advance in at 3000 rpm
4. Set total timing at 36* vacuum disconnected (and plugged) at 3000 rpm
Testing performed on Sun #404.

Today I set initial timing at 10 BTDC with vacuum plugged, the placard says 4* but it seems to run better at 10*. I ran the engine up to 3000 rpm, dialed the "Craftsman Professional Timing Analyzer" timing light back to 36 and then moved the distributor to zero. At 900 rpm with vacuum still disconnected my timing is now at 16*. Btw, I rechecked dwell at 29-30 and the distributor has probably less than 5000 miles on it.

So, do I have springs that are too light or ...? I did remove the distributor cap, verified that the internal components are clean and free of dirt and grease.

I am not understanding the problem here. Am I missing something very elementary?

Gary Schisler

[Edward D.]

Re: Understanding timing issue 72 LT-1

Gary,
Total mechanical timing of 36 deg less the measured advance curve of 26 deg would indicate an initial of 10 deg. Thats what MacNeish setup for you, and thats a very nice street setup for a performance small block. Forget what the placard says, especially if the distributor curve has been setup on a Sun machine. Yes, you want to plug the vacuum advance when checking timing, see other posts about correct vacuum advance, cans, etc. I suspect your mechanical advance probably starts before 900 RPM. Try remeasuring with idle at 600 to 750. I'll bet it isn't 16 deg anymore. Dwell angle is only adjustable with old fashioned contact points and some MSD boxes, not at all on modern replacements like the Pertronix. At any rate it should be between 28-30 deg but won't affect the mechanical timing anyways. Don't change any springs or flyweights, like I said your distributor is likely setup correctly. Using your dial back light, set total timing @ 3000 or more RPM to 36 to 38 deg and you will be fine.

[Gary S.]

Re: Understanding timing issue 72 LT-1

...Don't change any springs or flyweights, like I said your distributor is likely setup correctly. Using your dial back light, set total timing @ 3000 or more RPM to 36 to 38 deg and you will be fine.

That last sentence is the one that causes me the most confusion. After numerous searches here and online as well as reading the various articles from Duke and others, I am slowly beginning to understand this system. So, set 36 degrees at 3000, tighten the distributor down, re-install vacuum line and then enjoy.

The confusion I have is, if the distributor was set up as advertised with initial timing at 10, then at 3000 rpm & 36 degrees and the distributor "zeroed" on the dial back feature, how is it then 16 degrees at idle? To be honest, I haven't driven it yet with this setting so I really don't know how it drives. I will try your lower idle but it doesn't like idling below 800 at all.

Gary
ps, thank you for the comments.

[Duke W.]

Re: Understanding timing issue 72 LT-1

As you found out it can be difficult to set initial timing on high overlap cam engines because the centrifugal may start at lower RPM than the engine will idle stably long enough to get a reading. That's why you should use the "total WOT advance method". Forget about initial timing.

Verify that max centrifugal is all in at 3000 as you say. Then set the dial at 38-39 (what I recommend. The range is 36-40). Now rev the engine a few hundred revs above 3000, turn the distributor to get the balancer notch to zero on the timing tab, then blip the throttle up to at least 5000 and verify that the balancer notch stays at zero. Of course, the above should be done with the VAC signal line disconnected and plugged.

Now, if you want to check initial timing, drop revs down to about 1200 and set the dial at zero. Slowly bring down revs and the balancer notch should walk down the tab until it's aligned with 10, but the engine may stall before you get there because the centrifugal may start below normal idle speed. If the dist. was professionally setup the vendor should have given you the centrifugal specs that show at least the start point engine speed and the maximum centrifugal and the speed where it's all in. There is no initial on a distributor machine. That is only set on the engine.

Duke

[Gary S.]

Re: Understanding timing issue 72 LT-1

Duke,
That was about as clear and simple as I could ask for. THANK YOU. I followed your steps to the letter, buttoned it up and went down the street. Between the solid lifter valve adjustment (thank you for that paper too) and the timing, the car runs better than it has in a L O N G time. Maybe even better than ever.
Edward,
Thank you for your help and advice. I have many days of fun driving ahead of me because of the advice from you two.

Gary