Mechanical Advance

Julius - I am probably as confused as you are about getting timing all set as I just went through this on mine last spring - although I did not put in HEI. But, I will say from my experience you are about the same as mine. I think you are saying that the mechanical system on your distributor provided 23 degrees of advance (start at initial of 9 and goes up to 32) by 2500 rpm. I'd say that is about the same as mine (at some later point I would be more concerned about how smoothly and evenly the advance is coming in with increase in rpm from idle to 2500 but again - for a later time). I get that when you add in the Vac advance it seems things get a bit too high at first glance...but for us in the high elevation range it seems to be fine if you are not getting any pinging. With my comp cam, I am running even a bit more initial (10 BTDC) and things seem fine so far - I am totaling over 40 when I have full mechanical and vac advance in plus the initial, and it seems ok. I'd say keep running it - especially given the high elevation.

 

One more thing...I have found that my jeep seems to like to idle and run with more initial (12-14 BTDC) but it can be an issue when the vac advance comes in. It has to limit the advance on the vac can for me, but your should be pretty easy. Again, I think our high elevation (and you are higher than me by a couple thousand ft) and possible cam changes allow for more advance in timing than the traditional numbers might suggest. For off road at high elevation (say over 10,000'), I often bump my initial timing up to 14 with no issues, and it runs nicely up to 13,000' - of course it generally isn't at high rpm or generating high vacuum to make the vac can advance a lot.

 

X2

 

I'm going to add an 'excerpt' from the Motts article that I've used as the basis of my 'HEI Tuning and Curve' work - I think there is some 'confusion' going on as to the springs, weights, and vacuum and the impacts:

Getting it to Run Right - Recurving for Mileage and Performance
Just installing an HEI is not all that needs to be done. You have to "dial it in" to make it work like it really can.

The stock HEI's vacuum advance diaphragm is far too sensitive to work with the older carburetor's ported spark vacuum system. It will advance to fast, too much, and too soon, hurting performance, mileage, and possibly burning valves and pistons. Leave the vacuum line off till you make the modification.

You'll need to get an adjustable vacuum advance diaphragm from your parts store. It will run you about $20 or so, but it's well worth it. Parts stores will probably have to order it, speed shops will probably have it in stock. Make sure it's for a GM HEI.

Before installing it, check the instructions to make sure, but all I've seen needs to have the Allen adjustment screw turned all the way counter clockwise to DECREASE sensitivity. You do not want it to work fast, you want it at the least sensitive position.

Install it carefully as per instructions. If you have trouble reaching the mounting screws apply a little vacuum to the diaphragm, it turns the plate out of the way.

Install the limiter cam the new adjustable diaphragm came with. Set it so it only travels about 1/2 the way it did without it. You can make marks on the distributor body before and after the cam is installed to see where to set it. 1/2 way should give you about 14 degrees vacuum advance. Without that limiter it will be worse than it was with the original advance diaphragm.

Clean the damper and the pointer. It will help if it's painted black.

Roll the engine to TDC. Since you don't have timing marks beyond 10 degrees, you need to make some. We are going to make a $100 degree wheel for free.

Use dabs of white, yellow, or silver paint to make it easy to see against the black background. The tip of a small nail or wire makes a good brush..

Put a tiny dab of paint along the notch in the damper. This is your normal timing mark. Put another larger dab right on the 0 mark on the stationary pointer and a tiny one on the 10 degree point.

When the notch and the 0 are lined up exactly, the timing is 0.

Rotate the engine till the scribed line lines up with the 10 on the stationary pointer. Put another dab on the damper exactly where it lines up with "0" on the pointer. Put a "1" next to it. This is now another 10 degree mark.

Move the crankshaft another 10 degrees till your new "1" mark is opposite the "10" on the pointer. Now put another mark on the damper next to the "0" on the pointer. Mark it with a "2" This is now your 20 degree mark.

Again, move the crankshaft 10 degrees more till the "2" mark is opposite the "10" on the pointer. Put another mark on the damper next to the "0" on the pointer. Marking it "3". Right, that's your 30 degree mark.

Again, put the "3" opposite the "10" on pointer, mark the spot next to the pointer's "0" with "4" It should end up like this: 1 - 2 - 3 - 4 -


Be as accurate as you can.

What we did is made the degree marks on the damper rather than the pointer. The reason you heavily marked the 0 on the pointer is that now is your new reference. When the "2" is opposite the 0, it's timed at 20 degrees. We can read all the way up to 40 degrees ("4") now, the original pointer only went to 10.
Disconnect the vacuum advance hose. Start it, take a reading - this is your "Initial Timing." It should be about 10 degrees.

Run it up to cruise speed, about 3000 RPM. Notice the marks moved as you revved it up. Now your "2" should be near the big "0" on the pointer. You can interpolate what it is, like 23 etc.

The difference between the reading at speed and the Initial is how much your mechanical advance moved it. It should have moved it 10 to 15 degrees more than the initial. So if you started at 10 degrees, it should have gone to 20 to 25 degrees. That advance is simply the weights inside the distributor, under the rotor. Kits are available to modify this curve, but stock is better for off-road performance, since the odd-fire loves making low RPM torque.

Now reconnect the vacuum line to the advance. Take another reading at idle to make sure the vacuum isn't affecting it at idle, it should still be at the Initial reading. -- If it moves the timing you have the wrong type of vacuum on the line. It needs ported vacuum, not manifold vacuum. Ported vacuum is near 0 at idle and gets stronger as you accelerate till about 1/2 throttle, then it decreases again as the throttle open more. Manifold vacuum is strongest at idle. It's important you use the right vacuum source, otherwise the system will work backwards!


Now rev it up to cruise speed again.
Take another reading. The difference between what you saw at cruise last time and now is how much the vacuum moved it. It should have moved another 10 or 15 degrees again. So if the cruise reading before was 25, and now it's 35, the vacuum advance moved it 10.

That total reading should not exceed 35 degrees. More than that can and will create problems, like low mileage and overheating. Lots more can destroy things. What happens is the plug ignites the fire long before the piston is at top dead center, creating undue cylinder pressures and possible damage. The term often used is "pinging," but you don't always hear pinging until it's too late.

If the vacuum advance is taking it too far, beyond 35, you can reset the little limiter cam inside the distributor that came with the adjustable vacuum advance to limit it a little more. If it's much below 35, adjust it for more. Initial 8-10 Mechanical + ? (10-15) Vacuum + ? (10-15) Total = 35


That last little bit of detailing can make a world of difference.