Adj vac advance on factory dist.
- Thread starter Bewy
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Simple answer is barely. That adjustment is relative to environment same as the adjust screws on a carburetor. If I recall, the advance adjustment was mentioned on factory service manuals were high altitude tuning was the topic.
Allen screw in the vacuum nipple.
Just adjusts the rate of advance not the total advance, Should be around section 8-27 in the fsm
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Once in a great while a car with a fresh tune-up would come in complaining about pinging. With adjustable advance I could add a little more advance and cure it. So depending on the car, weather, gas, driving habits, etc, this feature could be helpful, but not normally.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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the Pic in post 4 shows the arm pointing down, with the stopper tabs visible.
On that arm is a number which translates to how many degrees the unit was designed to advance.
You can file those tabs off, little by little, to get a max of about 22/24 .
Most of our performance small blocks, with overdrives, can really really use in the neighborhood of 50>56 degrees of cruise timing, depending on the rpm. At 2240, mine likes the 50. sometimes more.
But I also use it to augment my Part-Throttle-timing, adding all what it can do, all the time, and as fast as it will roll in, Thus, in Second gear, I could be rolling along at 1500rpm=38*, or at 2000=44 , or at 2800= 52* ..But I can dial out some to all of that 22/24* with just a lil pedal.
That makes her pretty peppy in traffic.
On that arm is a number which translates to how many degrees the unit was designed to advance.
You can file those tabs off, little by little, to get a max of about 22/24 .
Most of our performance small blocks, with overdrives, can really really use in the neighborhood of 50>56 degrees of cruise timing, depending on the rpm. At 2240, mine likes the 50. sometimes more.
But I also use it to augment my Part-Throttle-timing, adding all what it can do, all the time, and as fast as it will roll in, Thus, in Second gear, I could be rolling along at 1500rpm=38*, or at 2000=44 , or at 2800= 52* ..But I can dial out some to all of that 22/24* with just a lil pedal.
That makes her pretty peppy in traffic.
TT5.9mag
Two atmospheres are better than one
You added advance and the pinging went away?
jos51700
Green Bearing thread connoisseur
The slot is the only adjustment to the total amount (and requires cutting or welding), but I've never seen that in the FSM. Mopar Action had a complete article on that stuff a few years before it went out of print, if you can find a copy.
There is another kind of "adjustable" advance, which came on the dual point distributors (the ones with the hex on the nipple). You unscrewed the hex and added or subtracted washers to adjust the advance. I have never seen any documentation on how to do this systematically, or which washer pack came with which engine combo. But at one time I had 6 dual point small block distributors, and different ones had different washer combinations — for example, Commando 273 with or without California Clean Air Package.
Sorry, tighten the Allen screw one turn- remove some timing.
I knew I had this photo somewhere... just took me awhile to find it. This shows the hex end removed, and the washer pack (in this case, one thick washer) for the distributor for a 1967 Commando 273 non-CAP, auto trans.
Mattax
Just the facts, ma'am
Good photo Max.
That's covered in the Master Tech Conference.
Adjusting was all covered in the "how to" article.
I can't answer the OP's question as to the percentages of adjustable vs. non-adjustable vacuum advances were on the later style factory Chrysler built distributors.
That's covered in the Master Tech Conference.
Adjusting was all covered in the "how to" article.
I can't answer the OP's question as to the percentages of adjustable vs. non-adjustable vacuum advances were on the later style factory Chrysler built distributors.
Only does the rate not the amount of advance. I tried.......
TT5.9mag
Two atmospheres are better than one
It was cheaper and easier to build one adjustable VA unit than a specific VA can for each individual application.
Yep, the total advance in an OEM can is not changed contrary to what the OP said in another post.
Adjustment changes the potential start point and rate of advance, not total in the can.
Want to change the amount of advance, shim the can/arm movement. Same type of approach with welding the slots for mechanical adv in an OEM distributor.
TT5.9,
That makes sense & I thought that might be the case [ pardon the pun ]. But if installing a replacement due to failure, what is the adjustment procedure??
Example: when I convert an engine to MVA, I tighten the spring [ CCW ] 2 turns at time, check the timing with each adjustment. When the timing drops or is no longer steady, I go 3 turns CW. Since Chrys used PVA, how was it adjusted?
That makes sense & I thought that might be the case [ pardon the pun ]. But if installing a replacement due to failure, what is the adjustment procedure??
Example: when I convert an engine to MVA, I tighten the spring [ CCW ] 2 turns at time, check the timing with each adjustment. When the timing drops or is no longer steady, I go 3 turns CW. Since Chrys used PVA, how was it adjusted?
I always start with the screw tightened clockwise all the way, lightly seated. This removes as much spring tension off the diaphragm as possible and allows timing to be pulled in as early as possible. If you get spark knock, then go counter clockwise one turn at a time until it goes away. If you don't get any spark knock, then leave it adjusted clockwise all the way, as this makes the vacuum advance come in as early as possible. No reason not to have it all as early as you can as long as there's no spark knock.
Yeah, seems very few people paid attention to your cutaway photo. Makes it clear as a bell how it works. But people still argue.
Mattax
Just the facts, ma'am
Strictly speaking the rate does not change unless the spring is changed, just the preload. Changing the preload changes the start and stop.
It's in the Chrysler Master Tech Conference. It's measurable with a vacuum pump. I posted this up 2018 and its easy enough to prove to ones self with distributor and a mity-vac or similar.
It's in the Chrysler Master Tech Conference. It's measurable with a vacuum pump. I posted this up 2018 and its easy enough to prove to ones self with distributor and a mity-vac or similar.
I do believe we're on the same page with it.
TT5.9mag
Two atmospheres are better than one
I’ve never seen anything published by mopar on how they adjusted them. I do it exactly how Rusty said above.
Adjusting the spring either compresses it or reduces the tension. If the Allen screw is fully CW, less vacuum is needed to move the actuator compared to when the screw is would a few turns CCW. So the Allen screw affects spring tension, which in turn affects the amount of vac reqd to move the actuator.
Back in the mid seventies or so, my younger brother had a 318 Road Runner, do not remember what year the car was. He had to run premium gas or it would ping. Then I found out about this adjustable vac can. Played with it until he could use regular gas again.
Exactly. And no matter how you try to explain it, someone always thinks the opposite. That tightening the adjuster screw makes it take more vacuum to work. It's just like you said.
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