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Discussion Starter #1
For some time now, I have been using the accell adjustable VA can #31035. Based upon my prior understanding, it was necessary in many instances to limit the diaphragm travel (amount of vacuum advance) by making up a stop device attached to the can arm. I had done this many months ago.

Recently, I decided to change my carb from a Holley HP 830 race, to a QFT SS 850 in order to gain the electric choke and further adjustablity over the Holley. In doing so, I started from scratch with timing, etc.

What I found is that the Accell unit not only adjusts the point at which it activates via the adjusting screw in the hose nipple, but also limits the travel of the plunger which limits the total VA applied. I thought the unit may be defective, so I removed it from the dizzy and hooked up my hand vacuum pump to find that as I adjusted the set screw, I limited the overall travel from 0 VA to 15 or so.

I ordered a Crane model #99600 and did the same test and found that regardless of the setting of the screw within the hose nipple, there was full travel of the plunger. This kit comes with a small eccentric plate that you install separately in the dizzy that you use to limit the overall plunger travel which in turn limits the total VA applied regardless of the amount of manifold vacuum applied. This assumes that there is sufficient manifold vacuum to actuate the unit which I believe is about 5hg.

Has anyone else seen the same results or had the same experiences?
 

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Rather than going that route, I wanted to keep the
shape of the VA looking stock in my 70 Z28. So I
bought a B28 VA can and made a limiter plate to give
me an additional 4.5* of timing at idle (20.5*) to give
me a smoother idle with my low vacuum 9-10" at idle.
My initial is 16* and total 36* with vacuum plugged.

Using my orig dist housing which needs to be bushed for the mockup.
 

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Discussion Starter #4
Rather than going that route, I wanted to keep the
shape of the VA looking stock in my 70 Z28. So I
bought a B28 VA can and made a limiter plate to give
me an additional 4.5* of timing at idle (20.5*) to give
me a smoother idle with my low vacuum 9-10" at idle.
My initial is 16* and total 36* with vacuum plugged.

Using my orig dist housing which needs to be bushed for the mockup.
Hard to tell rom the photo, but it looks that same as the adjustable unit other than the stamp. Looks good..
 

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Yep what u found is correct.
Have covered this many times in old posts...
Also different VAs can have different specs , but all as the degs are reduced so does the vacuum range reduce and visa versa
There is also out there a VA where u can push hard on the adjusting allen screw and turn the whole inside adjustment mechanism into 6 different postions ...each with different vaccuum starting points and degree range.

One can also mod std VA simply by extending the slot the lever works in and as indicated above using stops ...which can be mounted either end or both ends.

I strongly suggest thu, when using stops, have the lever sit in a V otherwise one can get inconsistency.
This is one of my very early attempts from around 35+ yrs ago when messing around on an old points dizzy, and still today use the same principle using several adjustment choices on the same stops


With further investigation u will fond changing the shape of the counter weight tails... rounding reshape between .25 and 1mm u change the amount of degs in the cent (or as some do change the shape of the anvil instead)
And on std points and HEIs there are holes in the top plate, tap 1 out , take a grub head screw make the head in a cam shape... flats or oval.. u have an adjustable cent stop.Caution thu, take too much of a counter weight tail and the counter weight can fall off the edge of the top plate locking the advance open... u will notice this next time u start the engine.
replacing the plastic/ nylon slide plates by epoxy on larger ones sorts that... espec when dialling in a hei designed for EGR on non eger engine requiring large number cent .
 

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Discussion Starter #8
Yep what u found is correct.
Have covered this many times in old posts...
Also different VAs can have different specs , but all as the degs are reduced so does the vacuum range reduce and visa versa
There is also out there a VA where u can push hard on the adjusting allen screw and turn the whole inside adjustment mechanism into 6 different postions ...each with different vaccuum starting points and degree range.

One can also mod std VA simply by extending the slot the lever works in and as indicated above using stops ...which can be mounted either end or both ends.

I strongly suggest thu, when using stops, have the lever sit in a V otherwise one can get inconsistency.
This is one of my very early attempts from around 35+ yrs ago when messing around on an old points dizzy, and still today use the same principle using several adjustment choices on the same stops


With further investigation u will fond changing the shape of the counter weight tails... rounding reshape between .25 and 1mm u change the amount of degs in the cent (or as some do change the shape of the anvil instead)
And on std points and HEIs there are holes in the top plate, tap 1 out , take a grub head screw make the head in a cam shape... flats or oval.. u have an adjustable cent stop.Caution thu, take too much of a counter weight tail and the counter weight can fall off the edge of the top plate locking the advance open... u will notice this next time u start the engine.
replacing the plastic/ nylon slide plates by epoxy on larger ones sorts that... espec when dialling in a hei designed for EGR on non eger engine requiring large number cent .
Glad to see that my findings are somewhat accurate.

Spent some today tuning the new carb and playing around with the crane VA can and here is where I am at. I have the static timing at 18 degrees and have limited the mechanical to 18 for a total of 36. The VA connected to manifold vacuum is set to engage just off idle and adds a total 15 degrees. At idle(1000 rpm) I have about 6hg. If I raise the idle, I get the anticipated increase in vacuum, but can't stop the VA from jumping in and once it does and it won't go back to any form of idle, but rather stays at 1400- 1500 rpm where my mechanical is already in play. If I leave it there, it pre ignites if I shut it off. Also, once the VA engages in that scenario, I have to disconnect the hose to get it back to idle and then reconnect it.

If I connect the VA to ported, it eliminates the issue, but it is much less responsive off idle as expected until the blades are open enough to create the required vacuum.

I also disassembled the dizzy today to inspect and clean it because I suspected that it was sticking a bit. It is nice and snappy now on the return so I believe that I took that out of the equation.

It seems that the mechanical and VA are in a big dispute and need a divorce lawyer. :D

Any suggestions are much appreciated.
 

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Glad to see that my findings are somewhat accurate.

Spent some today tuning the new carb and playing around with the crane VA can and here is where I am at. I have the static timing at 18 degrees and have limited the mechanical to 18 for a total of 36. The VA connected to manifold vacuum is set to engage just off idle and adds a total 15 degrees. At idle(1000 rpm) I have about 6hg. If I raise the idle, I get the anticipated increase in vacuum, but can't stop the VA from jumping in and once it does and it won't go back to any form of idle, but rather stays at 1400- 1500 rpm where my mechanical is already in play. If I leave it there, it pre ignites if I shut it off. Also, once the VA engages in that scenario, I have to disconnect the hose to get it back to idle and then reconnect it.

If I connect the VA to ported, it eliminates the issue, but it is much less responsive off idle as expected until the blades are open enough to create the required vacuum.

I also disassembled the dizzy today to inspect and clean it because I suspected that it was sticking a bit. It is nice and snappy now on the return so I believe that I took that out of the equation.

It seems that the mechanical and VA are in a big dispute and need a divorce lawyer. :D

Any suggestions are much appreciated.
Your bringing in too much advance with your cam when the VA is connected
and you won't be able to adjust the idle lower.
Try and limit the VA to bring in 5 additional degrees so at idle you have 23*.
Remember that the VA is zero and not in the picture in your total timing at WOT.
 

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I have the static timing at 18 degrees and have limited the mechanical to 18 for a total of 36.
Are u runing EGR? if not ideally u should have manifold vaccuum and the idle made up of intial + VA
So if your ideal (best stable idle at lowest prefered rpms and mixture slightly on lean side) idle is 18 .. And your best performance/ power is 36 deg with a rich mixture....it is un wise to have more than 40/ 42 deg initial+cent+ VA without dialling in with a knock sensor
so 41 deg less 36 gives a VA max of 5 deg
And if ideal idle is 18 18 minus 5 gives an intial of 13 degs
The VA needs to be all in at idle vacuum.. or no more than 1/2 to 1" above.. MAX
It also need to be all in at approx 60 mph flat road and '1/4 acceration... and start to come in when hit a gentle slope

Of not thu.. with modern pump fuels, cam designed stc I doubt very much that 36 deg is your best total (intial+cent) advance... ball park would more likely be around 32 @ around 3000/ 3200 rpms
This would then give 41-32= 9/10 degs in the VA
idle still 18 intial 18-9= 9 deg and
cent 32-9= 21 deg .

Now a couple things to watch
I mentioned above where a counterweight can fall off the sliders and over the edge above
If a VA is not all in at idle (non egr on manifold vaccuum) the u are most likey get a idle that speeds up and slows down... as the VA pulls advance increases ,that increases the rpms.. then sice u are running on the fixe metering carb idle circuits.. the engine runs out of fuel to maintin those rpms slows..the Va start pulling again....repeating
And a similar effect if the cent springs are too light and dont hold the counterweights in at idle... I run a very fast cent curve because of the propane fuel and highe octane and over 11:1 compression.. so push the limits here... my idle in gear is a very stable 550 450 in N(because of propane) and they come off around 700 rpms and the curve all in around 2600.
For a petrol engine which needs slower curve but more advance up top... the all in around 3000/ 3200 will have the weights coming off around 1200 to 1500 rpms
300 to 500 rpms above idle speed
Another thing to watch is that the carb secondary butterflies MUST BE set corectly, and the carb IS idling on the idle circuits only.


The stop limiter plate at this end is not recommended as it puts stress
and a constant load on the VA spring.
Strip out a VA and see how big that spring is....also u will find that the spring is already loaded inside the canister anyway... and in real world , the diaphamn will perish and get a hole yrs before the spring gives up the ghost..and if it doesnt the spring will rust and break 1st.
If anything if the VA mounting screw is not screwed in , torqued correctly the spring will damage the mounting hole threads.. and this can occur at the other end.. that diaphamn pulls far more pressure on that lever than the spring can ever apply.
 

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Discussion Starter #12
Your bringing in too much advance with your cam when the VA is connected
and you won't be able to adjust the idle lower.
Try and limit the VA to bring in 5 additional degrees so at idle you have 23*.
Remember that the VA is zero and not in the picture in your total timing at WOT.
At 23 degrees, my idle is about 1100 and I have no way to lower it although it is very responsive from there.

One of the things that I think I am fighting is the point at which the mechanical advance begins to add advance. If the curve for most of these dizzy spring sets (at least the ones I have seen) begin to retard the spark at 800 rpm and my idle is 1000 or 1100, I am already advancing at idle and not getting a true initial setting. If I lower the idle via retarding the initial timing, to let's say 10 degrees, the engine won't easily start and barely runs until the VA picks up the slack.

I understand that Steps is suggesting that I lower the initial and add the VA to gain back the total of about 18, but it still needs to be able to start at 10 degrees in that scenario

I will give all of this a try today.
 

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Discussion Starter #13
Are u runing EGR? if not ideally u should have manifold vaccuum and the idle made up of intial + VA
So if your ideal (best stable idle at lowest prefered rpms and mixture slightly on lean side) idle is 18 .. And your best performance/ power is 36 deg with a rich mixture....it is un wise to have more than 40/ 42 deg initial+cent+ VA without dialling in with a knock sensor
so 41 deg less 36 gives a VA max of 5 deg
And if ideal idle is 18 18 minus 5 gives an intial of 13 degs
The VA needs to be all in at idle vacuum.. or no more than 1/2 to 1" above.. MAX
It also need to be all in at approx 60 mph flat road and '1/4 acceration... and start to come in when hit a gentle slope

Of not thu.. with modern pump fuels, cam designed stc I doubt very much that 36 deg is your best total (intial+cent) advance... ball park would more likely be around 32 @ around 3000/ 3200 rpms
This would then give 41-32= 9/10 degs in the VA
idle still 18 intial 18-9= 9 deg and
cent 32-9= 21 deg .

No EGR. I will try what you area guesting and see where it goes. More to come:yes:
 

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I tried just about everything with VA connected on my set-up. The car just would not idle correctly and the response fell off. I gave up and disconnected it. The car runs so much better without it. I do not seem to have any of the symptoms that are mentioned without the VA connected such as running hot, plugs fouled etc. I may try some of the things suggested to see if I can dial it in. Good info.
 

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At 23 degrees, my idle is about 1100 and I have no way to lower it although it is very responsive from there.

One of the things that I think I am fighting is the point at which the mechanical advance begins to add advance. If the curve for most of these dizzy spring sets (at least the ones I have seen) begin to retard the spark at 800 rpm and my idle is 1000 or 1100, I am already advancing at idle and not getting a true initial setting. If I lower the idle via retarding the initial timing, to let's say 10 degrees, the engine won't easily start and barely runs until the VA picks up the slack.

I understand that Steps is suggesting that I lower the initial and add the VA to gain back the total of about 18, but it still needs to be able to start at 10 degrees in that scenario

I will give all of this a try today.
Work with changing the springs. 1 med and 1 light then test with your timing light.
 

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I tried just about everything with VA connected on my set-up. The car just would not idle correctly and the response fell off. I gave up and disconnected it. The car runs so much better without it. I do not seem to have any of the symptoms that are mentioned without the VA connected such as running hot, plugs fouled etc. I may try some of the things suggested to see if I can dial it in. Good info.
If your cam is not stock and you still have the stock VA can then it most likely won't start to work with the current cam. My current roller cam has a 112 LSA (9-10" hg at idle) compared to the 70 LT1 cam 116 LSA with probably a 15" hg at idle. My stock 201-15 VA can only gave me 1 or 2* of timing due to the stiff spring. That's why I bought a B28 can but without the limiter plate it gave me 34* at idle and the idle would surge up and down with black plugs plus crap out of the tail pipes.

List of Vacuum Advance cans and their specs - http://www.corvette-restoration.com/resources/technical_papers/Vac_Adv_Spec.pdf
 

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Discussion Starter #18
Here is latest. It is simply not happy at 10 - 12 initial which is where I can keep the idle low enough to avoid the mechanical from adding any advance. I set the adjustable VA can so that I was adding about 9 to the initial 10 to get 19 at idle and 42 total. It idled okay, but as soon as I gave it a little gas to accelerate from a stop, the VA drops out and I am back to the initial of 10 and have to tickle the accelerator to keep it running.

Other than completely disconnecting the VA and setting the initial at 18, I can't seem to find a happy spot. With the initial set at 18, I can reach in the car and just bump the key and it fires right up. With a lower static timing, I have to get in and touch the gas pedal in order to get it started. Then, it slowly climbs in rpm and is less constant.

The best scenario is when I have the static set at 18 and the VA set not to come in at idle, but rather slightly off idle. The problem then is that it doesn't go back to the static timing once the VA engages and hangs at 1500 rpm at which point the mechanical is definately in play. The only way to resolve it then is to unplug the VA, plug it back in and then I am back to 18 until I touch the gas again. I am going in circles. :mad:
 

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Don't go in circles - get yourself the right VA can. You want one that's all-in at your idle vacuum.
 

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Discussion Starter #20
Don't go in circles - get yourself the right VA can. You want one that's all-in at your idle vacuum.
I can easily adjust the one I have to be all in at idle vacuum. I currently have the static set at 16 (mechanical back to 20) and it is moderately happy there, starts easily, etc. I adjusted the can to be all in at idle as suggested and it is adding 8 more degrees of advance for a total at idle (static + VA) of 24. I have tried limiting the VA to 5 degrees and the results are the same.

With this setup, I am still dealing with a high idle which brings the mechanical in since the idle is over 1200 rpm. The carb blades are completely closed (including closing the secondaries). I could live with this setup, but due to the high idle, I get preignition again when I shut it off.

If I had a way of lowering the idle, it would probably be good like that. I suspected a vacuum leak but have tried all of the tests and don't find any.
 
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