Prestolite Dual Point Vacuum Advance Adjustment?

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You can use one of these FBO plates to dial in your maximum mechanical advance. FBO Systems

Mr. Gasket has distributor springs if you want the advance to come in earlier. You want to remove the stock heavy spring and replace with one of these:
Mr. Gasket Distributor Advance Kits 925B
Those are nice but man they kick in quick. MP has a few different ones available in different spring tensions also. Forget the colors but white and green are pretty common.
 
Here is a sheet with the different combos and what curve you get, forgot where I got it.
DSC_3219_zps818e19d4.jpg
 
As I understand it, the specs in the FSM are degrees of distributor advance which are 1/2 the degrees of crank advance, but as you are saying the RPM listed is also the distributor RPM and not the crank RPM?
Yes.
On a distributor machine that makes it easy. On the engine, its a bit of a pain.
I guess that makes sense. The FSM shows my distributor mechanical advance curve as:
1'-6.5' @ 550 RPM
9.5'- 11.5' @ 900 RPM
12'-14' @ 1800 RPM
So it would take around crank speed of around 3600 RPM to be fully advanced, correct?
Yes, Basically Correct.
That assumes the advance stops at the last specification. Its reasonable to assume that, but it may not be. I haven't run points distributors on a machine to have confidence is saying its one way or the other.
Here is a picture of the advance springs in my distributor. The light spring I'm assuming would allow the advance curve up to the 9.5'- 11.5' @ 900 RPM range , but to get that last part of the curve you would have the stretch the heavy spring and that would take 1800 RPMs of distributor
rotation or 3600 RPM of crank rotation.
Exactly!
Furthermore, adjusting the initial tension on the light primary spring effects the rpm the curve starts. Adjusting the spring perch on the long looped secondary spring controls the rpm begins to add resistance.
 
Those are nice but man they kick in quick. MP has a few different ones available in different spring tensions also. Forget the colors but white and green are pretty common.
The colored MP ones are for the electronic pickup distributor with the Mallory advance. Mopar Performance does (or did) sell a pair of very light DC springs for drag racing. They are even lighter than the primary spring in the tach distributor. My guess is they were originally for 1/4 mile racing with points distributors.

Being that this is an all original car, unless the goal is to only use it for short drives (1/4 mile down, about the same back), I'd work with it as is. I think its pretty sweet you're tuning it with the original dual point.
One difference today, over back then, is the fuel. So first I'd run it as close to factory spec as possible and see how it does. That's a pretty solid baseline.
Then see how it does with a bit more initial timing (shifting the whole curve). Mostly because its the easiest to do.
There's two things to test here:
One is everyday driving. Try on it on the highway, and when its fully heat soaked, listen for any pinging on part throttle (uphills or mild acceleration or both). If so, note what rpm and load (vacuum). Then you can tune around that.
The other is wide open throttle in top gears. Do this at the strip. See if it likes more advance or not. Probably will, but if you hear any pinging, or see any signs (specs of metal on the spark plug porcelain), then you've got to back off the timing. Also if you keep advancing and there's no gain in the mph at the end of track, then there's nothing being gained by more advance.
 
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I have also used them in points to get the curve to kick in sooner. I was really surprised the early MP dizzies are not adjustubale like the mallories. I have been looking for the specs on them with the curves for each number but best I can find is year and application on them. The prestolight springs are the same compressed length as the stock mopar just the spring rate is different for the lighter weights they use. I Have a Mallory points in my 440 and had a nos Mallory two wire e conversion I put in it, and am just waiting for a later model MP to show up to use. The duel point prestolights I really like, hardly a fail rate on them unless you use points with weak spring pressure (point float), under say 18 in lb. Think the book says 20 or more. Found oem mopar and delco points usually come in above 22 most of the time. Definitely a great distributor to use and keeping it stock is awesome.
 
Prestolite. Do not all Prestolites use a stack of washers on the vacuum spring?
If so, inside it will look like the one in the picture here:
1951 Imperial and Chrysler Service Repair Book - Distributor Repair

The washers are how those are adjusted.

Back to my original question, it looks like "special washers" under the spring is how it adjusted to when the vacuum comes in. Thanks...
I started the car again today and after it was warmed up I removed the plug that I had installed on the "ported" tap of the carb and installed a vacuum gauge. With a idle of 800 RPM. I was seeing 8-9 in-hg vacuum. I wondering why? I would expect little to no vacuum at idle at that port. I'm pretty sure my throttle plates are closed at idle.
Also, If I adjusted the idle mixture screw (which has left hand threads) all the way in until seated the engine keeps running with no slow down. If I back the screw out to about two turns from seated the engine starts to stumble. Kind of strange. The opposite of what I thought it would do .
By the way, I'm running the original AVS carb.
 
The vacuum port for the distributor is the lowest port. The one below the throttle plates. It doesn't have vacuum at idle.

That I know.. but I'm getting vacuum on that port at idle. That's what's strange??
 
B
That I know.. but I'm getting vacuum on that port at idle. That's what's strange??
Big cam? do you have to idle it up to 800 to keep it running?
 
I'm going to have to call this the backwards post. :LOL:

First backwards. Any port that is connected underneath the throttle sees manifold vacuum.
Normally 'ported' or 'timed' vacuum port connects to a hole that is just above the throttle plate when the throttle is at normal (hot) idle. So typically those ports have zero vacuum at idle as you expected.
So that's basically what Mike was saying (I think). On your carb, externally, there is one really low vacuum port that does have manifold vacuum at all times. That pulls the choke open just a bit once the car is started.

Second backwords. That mix screw doesn't control fuel. That idle mix valve controls the idle air bleed. I know that was used on some Chrysler Holley's. Have you looked at the factory lit on this one? Besides 69 FSM, Barry has most of the service bulletins, corrections and updates in Hamtramck Historical Library.

Got me curious. gonna have to look myself... and think about what you wrote...
edit 2: The FSM says that counter-clockwise (in) is richer, and you should initially set it at one turn out (clockwise) from gently seated. So the idea was that just like a typical idle mix screw, turning counterclockwise makes it richer. I think what we'll find is that this controlling air bleed to the idle ports, rather than the fuel.
 
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I'm pretty sure my throttle plates are closed at idle.
Worth checking this. There's probably (hopefully) some contact with the idle speed screw. If so, its worth trying to back off a little bit. If the ported vacuum drops, then we know the blade was close to the port (without taking the carb off and physically looking).
If the engine slows down too much, give it more initial timing ( 2-3 degrees).

If I back the screw out to about two turns from seated the engine starts to stumble. Kind of strange. The opposite of what I thought it would do .
The stumble is too lean.
Opening that mix screw lets in more air bleed at idle. The fuel mixture screws for each idle port are hidden under the lead seals.
There is also an off idle mix screw at the top.
Two guesses on why your engine continues to run when the idle air screw is all the way in. One, it doesn't restrict the fuel coming out of the port. That air bleed is just a fine tune on the amount of air mixed into the fuel before it mixes with air in the manifold. Two, a portion of the fuel is is also coming out of the transistion slot. Possibly a little more than normal because the throttle is open a little more - that's just a hypthesis.

All that comes back to trying less throttle opening, and if needed a bit more initial timing.
Also see if tweaking that idle mix screw brings up the manifold vacuum and idle speed.

Here's the explaination to Chrysler techs about the new idle mix (starts with introduction of the AVS in 68) 1968 Chrysler Imperial Carburetor Service And Repair Highlights from the Master Technicians Service Conference Session 243
It looks like Figure 15 shows the air bleed has two passages, one that always is open, the other controled by the mix screw. If so, that's another reason why it will run even with the valve closed.

and a more schematic version Carburetion Fundamentals & Facts (Session 273) from the Master Technician's Service Conference
 
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B

Big cam? do you have to idle it up to 800 to keep it running?

The engine is bone stock...

Thanks Mattax for the attachments. Those and you help has cleared up a lot of confusion. Now it make sense why the idle mixture screw adjustment is acting the way it is and why the factory made the screw a left handed thread. "Clockwise=Lean"
I reconnected the vacuum advance and drove the car some yesterday. It overall ran pretty good. I still have get the light out again and see where the timing is running.
Yesterday, the one thing that I noticed is that the idle speed is a little inconsistent when I come to a light. Sometimes it is a little lower that I would like. Other times it is a little too high. I would a imagine that the timing might be causing that to happen.

The carb still has the factory dashpot on it. I have it backed off right now so it does not come in to play. I read the write up in the FSM on how to adjust it, but what they say does not make sense.
 
Glad I could be some help.
Yesterday, the one thing that I noticed is that the idle speed is a little inconsistent when I come to a light. Sometimes it is a little lower that I would like. Other times it is a little too high. I would a imagine that the timing might be causing that to happen.
Yes, its possible.
It is probably worth looking to whether this carb & distributor were intended to have the spark vacuum port above or below the throttle plate. Manifold vacuum was sometimes used - I remember that - but what cars, what years - no idea.

Another possiblity is some air is a small air leak (PCV ? or dirt or ?)

The carb still has the factory dashpot on it. I have it backed off right now so it does not come in to play. I read the write up in the FSM on how to adjust it, but what they say does not make sense.
Sorry I don't have experience with, or know enough about, them. It is supposed control the emmissions on deceleration, so maybe it plays into the timing - keeps the vacuum signal to the spark port from advancing too quick when the throttle is shut.
 
That I know.. but I'm getting vacuum on that port at idle. That's what's strange??

I'm going to have to call this the backwards post. :LOL:

First backwards. Any port that is connected underneath the throttle sees manifold vacuum.
Normally 'ported' or 'timed' vacuum port connects to a hole that is just above the throttle plate when the throttle is at normal (hot) idle. So typically those ports have zero vacuum at idle as you expected.
So that's basically what Mike was saying (I think). On your carb, externally, there is one really low vacuum port that does have manifold vacuum at all times. That pulls the choke open just a bit once the car is started.

Second backwords. That mix screw doesn't control fuel. That idle mix valve controls the idle air bleed. I know that was used on some Chrysler Holley's. Have you looked at the factory lit on this one? Besides 69 FSM, Barry has most of the service bulletins, corrections and updates in Hamtramck Historical Library.

Got me curious. gonna have to look myself... and think about what you wrote...
edit 2: The FSM says that counter-clockwise (in) is richer, and you should initially set it at one turn out (clockwise) from gently seated. So the idea was that just like a typical idle mix screw, turning counterclockwise makes it richer. I think what we'll find is that this controlling air bleed to the idle ports, rather than the fuel.

Thanks for the clarification Mattax. I always preach the distributor vacuum line goes to the "ported" vacuum nipple above the throttle plates.
 
Thanks for the clarification Mattax. I always preach the distributor vacuum line goes to the "ported" vacuum nipple above the throttle plates.
Its interesting figuring out what the factory did.
I think your approach (using ported) is best for hot-rodded and is also what the factory most often did. I'm just not 100% sure with this carb because the illustration (Fig 6 in 1969 FSM) appears to show the port passage going below the throttle - but it might be wrong or I may be interpreting it wrong. Some cars got an external vacuum signal mixing valve, but not this one.
 
Its interesting figuring out what the factory did.
I think your approach (using ported) is best for hot-rodded and is also what the factory most often did. I'm just not 100% sure with this carb because the illustration (Fig 6 in 1969 FSM) appears to show the port passage going below the throttle - but it might be wrong or I may be interpreting it wrong. Some cars got an external vacuum signal mixing valve, but not this one.
If I can get to my original carb off my 66 HP273 today I will snap a photo. I am sure the distributor has constant vacuum from a port below the throttle plates and that carb has only 1 vacuum port.
 
Spark Port: Best I can figure from reading the '68 shop manual, its probably just at the throttle blade. Several times its mentioned that the vacuum must be at or under 6"Hg at idle. When using the external mixing valve, its supposed to take at least 3 seconds to drop from manifold vacuum - given as 16" Hg, and when finished it must be below 6"Hg.
Therefore, its reasonable to try lowing the idle speed with the idle speed screw.

Dashpot: There's a lever hanging on the outside of the primary throttle shaft. Open the throttle until it just touches the dashpot. If its above or below 2000 RPM, turn the dashpot so it just touches at 2000 rpm.
 
I put some miles on the car this weekend. I think I got the timing about as good as I can with a stock distributor and carburetor. With the vacuum not connected, at 750 rpm I'm about 10-12 degrees advanced. At 2000 rpm I'm about 33 degrees advanced. With the vacuum connected, I'm still at 10-12 degrees advanced at 750 rpm and I'm seeing about 50 degrees total advance at 2000 RPM.
I'm getting about 5"hg vacuum at the ported tap at 750 rpm. The vacuum reading is just below the point to where the vacuum advance starts to come in allowing the timing at idle to just run off its initial setting.
It took a little bit of playing to the throttle plates closed enough to lower the ported vacuum down to 5"hg.
The car pulls real strong through the gears with no engine pinging.
I have added the picture so you can see the ported vacuum tap that my distributor is connected to.

100_4552.JPG


100_4553.JPG
 
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Sounds good. If you ever get a chance, measure out the entire advance. On the car that will require timing tape or a dial back light.

My suggestion, if you take it to the drag strip, is back off the timing a couple degrees before the first run. Assuming your distributor advances at the rate I graphed, it will pick up another 4 degrees from 2000 to 4000 rpm. That will put it at 36. Possibly OK, but safer to test out at 34 first.
 
The way I see it is that the heavy distributor spring is holding back the last 3-4 degrees of mechanical advance and it would take a much higher engine speed to realize it. I could remove the heavy spring and just go with a single spring. With the single spring my total mechanical would be in at around 2000 rpm, but I will probably just leave it as is due to the car running really good now.
I do have an Innova digital timing light that makes checking total advance very easy.
Thanks for the dash pot info. Your description make a little more sense than the FSM write up.
 
The way I see it is that the heavy distributor spring is holding back the last 3-4 degrees of mechanical advance and it would take a much higher engine speed to realize it. I could remove the heavy spring and just go with a single spring. With the single spring my total mechanical would be in at around 2000 rpm, but I will probably just leave it as is due to the car running really good now.
I do have an Innova digital timing light that makes checking total advance very easy.
Thanks for the dash pot info. Your description make a little more sense than the FSM write up.
I took the heavy spring out of my 66 273 dual point and installed a Mr. Gasket spring from Kit #925B. I am all in at 2000rpm. 4 speed car but no ping.
 
That's the timing light I've been using most with my current motor. It's convenient to have the rpm right there.

My point about the last 4 degrees isn't that its needed earlier. It would take extensive testing to determine that. My point is that under full throttle+full load too much advance will be destructive. So at the drag strip, be conservative.

I used the assumed additional 4 degrees to estimate the advance when its at 4000 rpm. The only time the engine is going to see this type of load is 3rd and 4th gears with pedal floored through rpm of max power (roughly 5400 rpm). On the drag strip, the best timing is the one that produce max pressure over the most effective crank angles. For most of us, the way to test this is best mph in the lights.

The ideal timing curve for max torque and power at wide open throttle will vary between vehicles with different loads (gear ratios), fuel, combustion chamber conditions, even though they are all '340s'.

Again, I was just trying to make sure anyone reading this (hey its the internet) understood that IF its taken to a track or a similar venue, be conservative on the timing in the high rpm range.
 
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