You may, or not. It's all part of the experimentation, What does your engine like.
You will need a vac gauge, and I did not see your vac at idle posted. (A vac pump is handy too)

Let's say you have 15" at idle.
Apply that to your adv can and you should get the can's full advance, say 12 degrees.
Thumb rule says it should start to drop off 2-4" below that. So say 13-11"

Let's say you test your can and it starts to drop advance when you drop to 11" (that means you still have full advance at 13") I think 4 below idle is a good place to start.
Go drive it with vac gauge and see if it pings. At 15", 13", 11", 9" (use 3rd & 4th gear) Try it at different rpms (1500-3000). It may only do it at one specific rpm. (due to the mechanical curve) And it may only do it on "tip-in" (before the advance changes quickly enough)
If it still pings at 13" change your vac can so it pulls of at 13" vs 11" (or if you get trailer hitching instead)
Try to set it at least 2" below the idle vac. (b/c It needs to stay steady at idle) ( I believe when this is too close is when you get the trailer hitching)

OTOH If it still does it at 13" you should try limiting the degrees in the vac can instead. I used 10 degrees IIRC.

You already documented your mechanical / full throttle curve so you should not have to change that.
A little light "pinging" at low engine loads is not going to hurt it, for testing anyway. But I would still tune it out.

 

leigh1322,
Thank you for the excellent explanation. I was remiss in not stating my curb vacuum. At 850 rpms curb idle vacuum is 19 inches.
I am of the notion this current 12 degree fixed can and my base timing 12 degrees combined are causing my surging and hot start with the starter issues.
If I could adjust out like you stated maybe 2 degrees of advance with the adjustable version this may clear up my issues.
I am going to check and see that I am not getting any centrifugal advance on top of these numbers at idle.
My dial back was steady at 24 degrees with the current vacuum can hooked up to manifold vac.
I do not believe any additional centrifugal advance was coming in at idle but I will rubber band the weights and check. If it is I will go with a stronger spring combo.
I believe and I just verified via Lars latest timing papers that the two gold springs in the Mr. Gasket 928G kit, which I used and installed should get most engines in the 2800 rpm & 36 degree full advance range. But it maybe that I am a small exception and need to go with slightly stiffer spring(s)? Lars paper does mention this is a possibility.
I will experiment this week end and update this thread. An excellent learning opportunity for me.
Marshal

 

Have you verified your TDC mark on the balancer? If it was off a bit....you would have the exact problems you are stating but the numbers look right.
My own 69' 427/400 loved 14/38 with a 12 degree can and the Mr. Gasket curve with two gold springs......I suspect the whole curve is advanced too much by 6-8 degrees.....
Worth a look.....

Jebby

 

Marshal I know you just put a new balancer on. (Maybe Jebby does not). But he has a good point. Did you verify that the timing mark on the new balancer is at TDC? On SBC there are 2 version about 10 degrees off, on BBC I am not sure, but it is worth a check.

Jebby also has almost the exact same engine as you and had good results with basically the curve you have, so that's good info.

I think it could still be your vacuum can. Your idle vac is pretty high. I suggest you test your vac can with a handheld vac pump. It could be a different model than Jebbys. If it pulls off advance at a very low vac #, it would take a lot of throttle to remove the advance and you could have issues.

With 19# at idle you probably want it to pull off around 15# or so.

Double check. Test and verify. I trust nothing to be "original" or working "as new" after 50 years! Not even my lower back! LOL

 

I think I knew that as I have helped him/you......but whether or not it is new.....you need to eliminate it as a variable....once you know, you know, then you can move on.....

Jebby

 

I just saw jebby's posts. I did not verify the outer balancer ring as it was in the correct position directly in line with the key way as with a big block. I read on the small blocks there is a couple variations. It is a PB1019N.
So this afternoon, I disconnected and plugged the vac advance. I took her for a ride. With the can disconnected and plugged I lugged the engine. I got allot of detonation. I went right home and put a light on her. Base timing was 26 degrees? The surging was gone though.
I had set it for 36 degrees all in at 2800 rooms and let it fall where it may. I reset the base back to 10 degrees.
I then plugged the can back in and it added another 20 degrees. So total base is 30 degrees. I think they vac can is pulling in all the mechanical advance?
I'll have remove the cap and rotor to see what's going on.
Seems odd new balancer should be two steps forward not two steps back.
Marshal

 

You just figured it out....the can is pulling 20....you need to limit it to 10-12 degrees.....contact Lars, he has a stop that he sells for this.
Vacuum cannot pull the mechanical.
Your mechanical advance according to your text is only pulling 10 degrees if you set it at 36 and it falls to 26.....pull the rotor and inspect the weight and mechanism....
You were at 46 degrees at idle with the can hooked up! No wonder it was surging! But I was half right, the whole curve was too advanced...

Jebby

 

Woah! 20 degrees vac advance. And too much initial!

Glad you found it.

It did take a village.

 

Get Lars papers...current edition. It has the part number for a can that pulls only 12 degrees.....at the correct vacuum drop. This is an easy fix.
The mechanical on the other hand needs to be inspected. It either isn't pulling enough, or is not going back to stop.......spend some time and get it right, and you will be very happy with the outcome.
Once it is set, you should never have to mess with it again.......the numbers won't lie once it is correct. If it doesn't run right after setting timing to the letter......then we/you can move on.

Jebby

 

Tonight I inspected the weights in the distributor. They were moving freely but there was dirt gumed up on the underside. I cleaned them real good and they glide back n forth on the advance plate. See pic.
The springs are both gold from the 928G kit installed about 6 years ago.
They did not appeared stretched.
I have play clockwise and up n down on the main shaft.
I hooked a miti vac to the can. It starts moving the plate at 7" and stops at 15". It meets the 2" rule but? as jebby says it's too much.
Thats where I am at for now.
Do I need to pull the distributor to swap out the vac can?
At 15" of vacuum any way to translate that into crank degrees?
Lars sent me his latest papers too.
Thanks,
Marshal

 

As part of my rebuild, I shimmed a brand-new distributor to take out endplay. It could have been a few cam degrees by itself. Check @lars' papers to get the proper final dimension. Before replacing the distributor, the old one was visibly "knotchy" in mechanical advance response as seen on the timing light.

I used a Moroso 26140 shim set, if that helps (SBC, dunno if yours is different).

You can see in Post 19 what 12 degrees of advance looks like using the Accel adjustable vacuum can. It starts at 6 or 7", and is all in by 10". Your best bet might be to use whatever @lars suggests is proper for your engine and your distributor. EDIT: and to add the vac advance limiter. But the Accel unit works in a pinch.

 

You could just add a vacuum limiter to limit the movement to 10 or 12 degrees. (20 is way too much). Lars and Crane have them. Screws on, Stops the pin from moving. Or change the can.

You probably don't have to pull the distrib, but I wouldn't want to lean over that big fiberglass fender that much, and/or drop small parts into the "dark abyss".
It's easier to pull & reset.
Plus then you can shim it to get rid of the "up & down" movement.

Here is a pic of the Crane one installed so you can see how it limits movement.
Personally I like Lars unit better, this one is adjustable, but it looks like it could "slip"
His is one piece.
He calls it a VAC Vacuum Advance Corrector. It goes in the same place.

 

leigh1322,
I ordered the Accel adjustable vacuum can for points distributors. I'm hopeful, I can limit the initial vacuum down to 10 degrees with 10 degrees of initial mechanical.
As of now with the 10 initial mechanical and 20 degrees vacuum advance added she starts right up no heat drag on starter. The temperature remains constant at 190-200 degrees.
I will get the vacuum advance unit installed and attempt to retime it with a little more initial than 10 degrees with the reduced vacuum advance.
Marshal

 

What is initial mechanical? You have initial advance. Then you have mechanical advance. Then you have vacuum advance. Total advance is initial + mechanical. A nice performance ignition curve is 14 initial + 22 mechanical for a total of 36 all in around 2500-3000 rpm. Vacuum advance only comes into play at part throttle where it will add advance to the total advance for fuel economy. Too much vacuum advance will cause ping. Reduce vacuum advance only to remove ping at cruise speed. Vacuum advance does not come into play when starting the engine nor does mechanical until the rpm's are great enough to advance the timing. Once the engine is started, you will probably have full vacuum advance and initial advance but no mechanical unless the springs are really weak. At wide open throttle, vacuum advance is 0. If there is pinging at wot, reduce initia and or mechanical advance. Better to just reduce mechanical.

 

Resdoggie,
I have 10 degrees initial mechanical. When I attach the vacuum advance can to full manifold vacuum which is 19 inches
I have 20 additional degrees of timing added to the 10 initial. I have not rechecked the 2800-3000 rpm timing. The car starts and runs well and runs cool at this point.
I have been instructed by other forum members to reduce the vacuum advance to 10 degrees.
The excessive advance was causing the car to experience heat soak and surging at low rpms with 14 initial and 20 degrees vacuum advance.
Marshal

 

The reason I asked is because I'm not sure your on the same page as the others. There is no such thing as initial mechanical. Again, there is initial advance, period. Then mechanical advance. Total timing is a combination of the two. Vacuum advance is not even needed sometimes but its main purpose is to smooth the idle and provide fuel economy. If you want to truly determine your initial, either put on the factory springs or better again, lock out the mechanical advance mechanism. Then disconnect the vacuum advance and plug the carb port. Hook up timing light and reduce idle speed to about 600 rpm. The timing you see now on the timing tab is your initial timing. Try it.

 

Been following this thread. I’m also in the midst of trying to dial in my timing advance. The issue I’m struggling with is that my mechanical advance seems to be starting to come on below 850 rpm. I’ve got a heavy and medium spring in there. My max mechanical advance is all in by 2600. So ok.

I’ve heard that mechanical advance should start at about 1000 rpm. Is this correct? The earlier post about locking out the mechanical advance has relevance because if it’s coming on really early like mine, you are not getting a true initial timing number. This is how I interpret this scenario. Maybe I need to change weights to counter the too early advance. Not sure. Trying to figure it out.

 

You shouldn't need any mech'l advance below 1000 rpm which is basically idle speed. So the curve can start at say 1000-1200 rpm. No hard and fast rules. But put the next heavier set of springs in to prevent mech'l advance coming in too soon. Can't hurt to try. If I recall years ago, when I had too light put in, the idle was not steady i.e. it was hunting. Heavier springs fixed it.

Like Jebby said, set timing to 36* at 2500-3000 rpm with vacuum advance disconnected. If you know for sure that the mech'l advance for the dist'r is say 22*, then the initial is 14*. Done. Take car for spin and check for ping without the vacuum advance hooked up. No ping is good of course. Now hook up vacuum advance. Test drive. If there is ping, the vacuum advance is the cause and needs to be addressed if the ping is excessive. Momentary ping at light load going up a hill may be expected and tolerable. To eliminate ping, easy way, reduce total advance. Better way is to limit vacuum advance. I got tired of playing with springs, vacuum, etc so I went with a MSD programmable ignition which uses a MAP sensor. My initial is locked out at 42* but programming reduces that to 17* initial at start up only. It's a great piece of kit and I won't be going back to springs anytime soon. I can't help Marshal because I don't understand what his timing is to start with. His initial could be 10* ATDC. You need to do what Jebby says first and then deal with vacuum advance afterwards.

 

Yes you do want the advance to be stable at idle speeds. It should not even start to kick in till 200rpm above idle speed to be sure.
You could add a one step stronger spring that should help.
It would be OK if it raised the "all-in" rpm a little bit, as long as it is by 3000-3300 rpm you're good.

The reason behind this is their are many different football and weight shapes out there, both OEM and aftermarket. There is no telling what combination you have in there now after 50 years. In fact most people do not like the results of the advance weights included with the aftermarket kits and throw them away. In the old days the racers would grind the football cam and weights to get the curve shape they want. You may not have GM advance weights any longer and that could be causing your problem. The springs should completely close the weights at idle / no speed. Sometimes they do not because of the shapes involved.
I also noticed there are two different football shapes on the replacement distributor shafts. You really need the high performance one for optimum results with a high performance advance curve.

That's all probably getting deeper into it than you need to. Try some heavier springs and see if that works. If not there are other options.

 

resdoggie,
I am not using the right terminology and I have caused some confusion on this thread. I apologize as this is a learning endeavor for me.
With no mechanical nor vacuum advance, I am set at a constant 10 degrees BTDC initial timing using a dial back light set to 10. The new balancers witness mark and the timing tab zero are lined up.
Now going on the premise that the mechanical is not coming in off idle. (I need to rubber band the weights and confirm this.) If the springs are holding everything in place at idle I can then set the timing of the mechanical phase to show 36 degrees on my dial back at 2500-3000 rpms.
I'm awaiting an Accel adjustable can to show up at my door and I will see if it allows me to shave off or dial back the vacuum advance to 10 degrees at idle only.
This is where I am at and I know this has been more complicated than it needed to be but the vacuum advance threw me for a loop during the timing process.
Do I need to pull the distributor to swap out the vacuum advance can or can I do it in place?
Thank you and hope fully this is helping others as well.
Marshal