I got around to marking the balancer today. With 16* initial, at 1700 rpm I get full mechanical that puts it at 36* WITHOUT vacuum advance. With the vacuum, I get 10* more. I seem to remember that total should not be more than 36*. Is that right? And, if so, where do cut back, the mechanical or the vacuum?

The engine perks up with more timing. At 10* btdc I had 13hg vacuum. At 16* I have 16hg, 18* I have 17hg.

 

I don't think you'll have an issue with 46° total since you'll only hit that at highway speeds when you're cruising. Also, since you now have overdrive, you'll probably never hit it unless you're cruising in 4th with your 1:1.

Otherwise, your mechanical and vacuum advance are probably going to be working at different times. Mechanical when you're getting the RPMs up, and vacuum when you're engine vacuum is high and low load. Those two things are usually not working at the same time since high RPMs are usually under load when you're accelerating. Otherwise, more advance at cruise can help with mpg.

For what it's worth, my DUI, which PerformanceDistributors custom curved for my engine:

Base: 12° (idle 750)
Mechanical: 22° (all in at 3000)
Vacuum: 14° (starts at 9hg and is all in by 14hg)

So, I have a total of 48°

The only time I'll really hit that is when I'm above 3000 RPMs and have greater than 14hg of engine vacuum, otherwise, one is working when the other isn't.

 

Thanks, AB. The distributor advance system is another aspect of this engine that I always took for granted and am now having to go back and learn.

One thing that I found confusing, and pls bear in my that I'm still reading about the system: V. is a system or method to get advance at low rpm's, when the centrifugal system is not spinning fast enough to counteract the springs. How can your system (v.) begin at 9hg and be all in at 14hg? Increased rpm's means decreasing vacuum, right? If your system (v) is all in at 14hg, you'd have about 20* of advance at idle, wouldn't you? The v. canister operates with a diaphram, so as less v. enters, as rpm increase, the v. timing advance would begin to decrease. I'm just going over it in my mind.

From the reading I've done (gofastforless has a good write up) and from your help, I seem to need to change the mechanical advance to be all in at a higher rpm, around 2500. I also need to adjust the v. canister to operate at a lower rpm, so the two are not operating simultaneously. 46* of v. at 1700 rpm could explain my lethargic performance, or?

The v. canister allen screw changes when the canister is all in. I'm guessing that I need to precisely chart the v. readings at diff. rpm's, to know how to adjust the v. advance.

 

Timing is definitely something that I didn't understand when I started in, so I totally understand.

You have two completely separate systems when it comes to mechanical advance and vacuum advance, and they don't correlate. They work at the same time, but they don't work together.

When gas explodes, the explosion travels at the same speed (for a given AFR) no matter if it's at 1000 RPMs or 5000 RPMs. However, at 5000, the piston is moving a whole lot faster.
So, at 5000 RPMs, you want to spark the fuel when the piston is lower in the cylinder, so that by the time the explosion is at it's peak, the piston is at the top. At 1000 RPMs, the piston is moving slower, so you want to fire it later.
This is what the mechanical advance does. The faster the motor is spinning, the sooner it fires the spark so that the explosion reaches its peak as the piston reaches the top.

Then there's a lean mixture vs a rich mixture. A lean mixture burns slower than a rich mixture. So, with a lean mixture, you want to fire it even sooner because it's going to take longer to burn.
This is what the vacuum advance does. The idea is that under most cases, when your vacuum is high, the engine is at low load and is running lean (15 - 16:1 AFR). When vacuum is low, you're usually under high load, hard acceleration and it's running rich (12 - 13:1 AFR). (This is why people who tune their engine for the track usually remove the vacuum advance. They have their engine running at around 13.5:1 AFR no matter the load on the engine, so there's no need to accomodate for a lean/rich mixture.)

This is why they're independent. You can be at 4000 RPMs but at light throttle and low load, just as you can be at 1000 RPMs and have the pedal to the floor and full load on the engine.

1000 RPM - full throttle (no to low mechanical advance, no vacuum advance)
1000 RPM - light throttle (no to low mechanical advance, lots of vacuum advance)
4000 RPM - full throttle (full mechanical advance, no vacuum advance)
4000 RPM - light throttle (full mechanical advance, lots of vacuum advance).




This is what I did on mine:

Mark the base timing, and then get a timing light.
Increase the RPMs by turning the screw on your carb by 250 RPMs and them check the timing and write it down. (I did it minus the base timing, so I could see how much mechanical was being added).
Increase by 250, write down the timing.
etc etc.
Once the timing stops increasing, you're done.

Next drop back down to idle and then get a hand activated vacuum pump.
Hook it to the vacuum advance canister and add 1/2 hg increments of vacuum and mark the timing.
Keep adding 1/2 hg increments until the vacuum advance stops increasing the timing.

This is mine:

750 - 0
1000 - 2
1250 - 6
1500 - 10
1750 - 13
2000 - 15
2250 - 17
2500 - 18
2750 - 20
3000 - 22
3250 - 22
3500 - 22

Vacuum:
0 - 0
1 - 0
2 - 0
3 - 0
4 - 0
5 - 0
6 - 0
7 - 0
8 - 0
9 - 1
10 - 5
11 - 7
12 - 11
13 - 14
14 - 14

So, my mechanical is all in at 3000. My vacuum advance is at 0 at any vacuum below 9hg and then increases up to 14° by 13hg.

I have mine charted out in an Excel sheet for reference, but basically from there you can get your timing at any moment by looking at your numbers.
Base + mechanical + advanced

So, at cruise, I'm roughly at 2250 RPMs and 10hg engine vacuum, I'd be at:
12 + 17 + 5 = 34° advance

At 1500 RPMs and full throttle (0hg vacuum), I'd be at:
12 + 10 + 0 = 22° advance

1000 RPMs and 16hg vacuum (low speed cruise)
12 + 2 + 14 = 28° advance

3000 RPMs and 13hg vacuum
12 + 22 + 14 = 48° advance


etc. etc.

Hopefully that helps clear stuff up!

 

For whatever it is worth, I found some of my old notes and found this from last year, after returning from a road trip to OK.

Centrifugal is 20 degrees @ 2800 RPMs + initial [16*] = 36 degrees. Vacuum is 10 degrees.

This is on a stock engine [1986 4.9L] with Duraspark distributor, MSD ignition box and no EGR.

 

Okay! Well, that was a wonderful lesson. I should have been more specific. All I needed to know was the working of the v.advance system, and how to dial it in more specifically.

HOW TO DIAL IN YOUR VACUUM ADVANCE:

Step 1.

Determine the range of vacuum advance adjustment.

1. Insert a 1/8" Allen wrench into the vacuum advance port and engage the adjustment screw.
   
2. Turn the adjustment screw all the way clockwise, until it stops turning.
   
3. Turn the adjustment screw all the way counter-clockwise, counting each full revolution, until the last screw thread "clicks" when it exits the nut on the diaphragm spring.
   
4. Make a note of the total number of turns from maximum to minimum sensitivity.
   
5. Make sure you re-connect the vacuum hose from the carburetor spark port to the vacuum advance canister.
   

Step 2.

Set vacuum advance to maximum sensitivity.

• Turn the adjusting screw clockwise until it stops.
  
• Make a note of the vacuum advance adjustment.
  

Step 3.

Test drive.

Take a test drive with your current vacuum advance setting. Warm up the engine to normal operating temperature before you start driving.

• Try to drive over a variety of conditions typical for your usage – moderate to brisk acceleration from a variety of steady speeds, acceleration while climbing hills, etc.
  
• Try accelerating both mildly and aggressively from various steady-state conditions.
  

Make a note of any conditions under which the engine pings, and how pronounced the pinging is.

• No pinging at all means you can advance the ignition base timing more. In that case, refer to the procedure for setting ignition base timing on this page .
  
• Significant pinging when the throttle is opened means you should reduce the vacuum advance sensitivity.
  

Step 4.

Re-adjust vacuum advance sensitivity.

Set the vacuum advance to the middle of its sensitivity adjustment range.

• Use the 1/8" Allen wrench and turn the adjustment screw counter-clockwise by half the total number of turns from all the way in to all the way out.
  
  For example, if the total range is 16 turns, turn the adjustment screw counter-clockwise by 8 turns.
  
• Make a note of the new vacuum advance adjustment.
  

Step 5.

Test drive again.

Repeat the previous test drive and try to duplicate the test conditions.

Make a note of any changes in pinging or engine performance.

• If significant part-throttle, throttle-opening pinging continues, try reducing vacuum advance sensitivity further.
  
• If there is no throttle-opening pinging, try increasing the vacuum advance sensitivity.
  

Step 6.

Re-adjust vacuum advance sensitivity.

Adjust vacuum advance sensitivity (increase or decrease it) by half of the remaining adjustment range.

For example, if the total range is 16 turns and the current setting is in the middle (8 turns out), turn the adjustment screw clockwise by 4 turns to increase sensitivity, or counter-clockwise by 4 turns to decrease sensitivity.

Make a note of the new vacuum advance adjustment.

Step 7.

Repeat steps 5 and 6 as necessary.

Keep repeating the test drive and adjustment cycle until you get to the maximum vacuum advance timing without excessive pinging.

Each time you make further adjustments in the vacuum advance sensitivity, reduce the size of the adjustment by half of the previous adjustment.

For example, if you adjusted the vacuum advance sensitivity by 4 turns last time, make the next adjustment by 2 turns.

 

I had no v.advance! I pulled apart the dizzy and also found the one spring that the dizzy guy put on. As I looked at it I remembered everything he told me about the work he did: He said to run 16* initial and the dizzy supplied 16* more. The mechanical is in slot 13, which would normally mean 26* advance. But my dizzy guy welded a small tab in the slot to limit the advance to 16*. I purchased a Moroso kit (GM) and used their lightest spring, and installed it on the other side of my mechanical. I didn't have time to hit it with the light. I also dialed in the v.adv. can. Now I can see it working when I hit the gas. It is a bit peppier. Nice.

 

Awesome description of advance abandoned.
i would rep you if i could