Prestolite Dual Point Vacuum Advance Adjustment?

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694spd

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Is it possible to make adjustments to the vacuum advance unit on a Prestolite dual point distributor?
 
I don't believe the original vacuum diaphragms are adjustable. May be @halifaxhops can chime in and set the record straight.
 
Yes, but only if the vacuum can has the hex where the vacuum nipple is. Also, the adjustment is not how much advance, but when it comes in. The amount is not adjustable.
 
What size is that Rusty? 5/32"?
 
Very interesting. I have never looked in the manual or spun the nut off the vacuum nipple end.
 
Yup, the ones with the removable nut adjust just like that. I have a Prestolite dual point with one piece hex ended can. Whether it is original or not, I don't know. The original Prestolite that came out of my 56 Hemi has the removable nut.
 
It has the hex on it.
I hooked up my Mityvac on the vacuum advance unit with the engine running at 800 rpm and pumped it up and I ended up getting an additional 25 degs advance from the unit at about 12" hg vac. I was a little surprised that it advanced that much. I was expecting more like 18-20 degs.
At idle, I assuming that there should be very little "port" vacuum at the carb, correct?
I'm going to put a vac gauge on the "port" nipple tomorrow to see what the reading is at 800 rpms

100_4383.JPG
 
It also looks like it has a nut on the end where the vacuum nipple is, so if it does, it adjusts with the washers as described above.
 
It also looks like it has a nut on the end where the vacuum nipple is, so if it does, it adjusts with the washers as described above.
I think I have a photo of mine.
engine 163.jpg
 
Yeah that's the early one! Same as what my Hemi has.
 
The stop for full advance is built on the rod that moves the plate, it bottoms out on the back of the advance can.
I thought I had a picture of one but I cant find it.
 
.. I ended up getting an additional 25 degs advance from the unit at about 12" hg vac. I was a little surprised that it advanced that much. I was expecting more like 18-20 degs.
That's not out of line. It will vary depending on application. What year, engine, transmission? Your testing is good, but did you take into account of the rpm when it advanced with the vac pump? If not, go back and subtract the mechanical timing at that rpm. Hope that makes sense.
At idle, I assuming that there should be very little "port" vacuum at the carb, correct?
Correct. It should be zero. Its gets vacuum from a hole just above the throttle blade when the blade is at idle.
 
Mattax, The car is a bone stock 69 dart 340 4 speed. You are right, I forgot about the added mechanical advance when when the engine speed pick up after I had applied vacuum on the unit.
Earlier, when I tested the mechanical advance alone with the vacuum advance disconnected and plugged. I was getting about 25 degs mechanical advance that was fully in at about 2000 rpm.
The factory spec list the initial timing at TDC at 750 rpm. That setting would only give you a total advance of 25 degs. If I go with a 10 degs initial setting, I will have a more desirable total advance setting of 35 degs.
 
What are the specs on the car? Need application, engine, carb and trans I can scan the sun sheet I have on it if needed. I have seen both types of vac advances on them and if it is stamped and has the larger vac hose hole it s limit can be set with a 3/32 allen through the tube as RRR said.
 
Mattax, The car is a bone stock 69 dart 340 4 speed. You are right, I forgot about the added mechanical advance when when the engine speed pick up after I had applied vacuum on the unit.
Earlier, when I tested the mechanical advance alone with the vacuum advance disconnected and plugged. I was getting about 25 degs mechanical advance that was fully in at about 2000 rpm.
That's probably not full advance at 2000 rpm. Very few factory advance curves look like the hot-rodded curves. Most are two stage. I may have '68 340 curve plotted as an example. If not I'll do it later. Or, if Halifaxhops has the '69, even better. (also, remember, the advance specs in the back of the electrical section are distributor degrees and distributor rpm)

The factory spec list the initial timing at TDC at 750 rpm. That setting would only give you a total advance of 25 degs. If I go with a 10 degs initial setting, I will have a more desirable total advance setting of 35 degs.
For 1/4 mile, yes, you may want more or a little quicker than factory at the top end (3200 - 6000 rpm). Come up with that number by on-track testing for your car, engine, and fuel.
 
I have the averages for 68 and 69 plotted in my spreadsheet. The '69 info is for a CAP because that's what I was comparing. I can't recall if the '69 Dodge FSM only had 340s with CAP or not. The '68 is from my Plymouth FSM.

For testing, the average isn't really the best to use. Better to use the acceptable range and then plot your own. I like to plot at least every 200 or 250 rpm, or every degree or two - depends on how I'm setup.

Anyway, these show that just as it is set now, your distributor may provide around 30 degrees at 4000 rpm. Maybe more, maybe less - that's just the average of the allowable specs.

1968-69-340-timing.png
 
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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?
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?

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.

100_4379.JPG


100_4380.JPG
 
I cannot see the numbers on the advance plate that should be your total mechanical advance at the distributor then the number on the arm of the vac advance is total vacuum. add them together and double them that is the maximum at the crank.
 
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