Vacuum advance can adjustment

Small Block Mopar Engine

  1. JG1966

    JG1966 Well-Known Member

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    I was always under the impression that on a vacuum advance, the amount of timing is set (double what's marked on arm) and you are adjusting the amount of vacuum needed to make the timing come in. My 340 MP distributor from Jegs (part no. 40500) says it's the other way around, at least according to paperwork.
    Has anybody used one of those?
    On instructions, it says "The vacuum advance canister is factory set to produce 5-7° of crankshaft advance at 15” of vacuum. .... Turn the wrench clockwise to increase the amount of vacuum advance or counter-clockwise to decrease."
    That sounds wrong in a couple of ways. For one, I think it should read 10-14° of crankshaft advance. Also, I only get about 12" of vacuum at idle (I'm using manifold port), so if the 15" isn't adjustable, it doesn't do me much good. I called Jegs and didn't really get clarity on it.
    I guess my question is do they make vacuum advance with adjustable timing degrees rather than vacuum needed OR are these instructions maybe inaccurate? Thanks
     
    Last edited: Jul 20, 2018
  2. 4spdragtop

    4spdragtop CONGRATS NORTH AMERICA! FABO Gold Member

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  3. RustyRatRod

    RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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    I always understood the adjustment worked not to adjust the amount of advance, as that was preset, but to adjust at what point it comes in.
     
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    • moper

      moper Well-Known Member

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      The adjustment changes the preload on the vacuum diaphragm. Not the amount of advance. So you are adjusting the amount of vacuum required to add the advance. Not the number of degrees advance itself. That's true with all the vacuum pods I'm aware of.
      Also - manifold vacuum won't give you much of anything at idle especially with a cam/package with limited vacuum. IMO that's a mistake - use ported. And add initial if you need to.
       
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      • toolmanmike

        toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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        Correct me if I'm wrong here guys but you need to tune the other timing events first. (initial, total without vacuum, mechanical advance rate or curve to a total of about 35° at around 3000 rpm. ) Then you can hook up the vac hose and adjust the can to around 50° at 3000 rpm. Test drive it and if it pings while cruising, back off the vac can. If it pings on acceleration your mechanical is coming in too quick or the total is too much. It can also be caused by too much initial. I thought I had a thread saved about this but I can't find it now. May be someone will chime in.
         
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        • RustyRatRod

          RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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          I always just get my initial and total worked out and then just plug in the vacuum can. Normally, it is set "in the middle" and that's just about perfect. It's not abnormal to see total plus vacuum to exceed 60* and that's just fine.
           
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          • RPMagoo

            RPMagoo Just An Old Motorhead

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            Info -- Vacuum - Adjusting The Canister..JPG
             
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            • moper

              moper Well-Known Member

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              Yup. Set initial and centrifugal to what you need. Then set the vacuum can. Best way I've done it is to tape a vacuum gage to the windshield so you can see where things are while driving. Then tune the can to provide max advance without causing a surge or misfire. Race engines in race cars don't benefit from a functioning vacuum advance. Pretty much any engine that experiences widely varying throttle levels will benefit from a properly tuned one. Gas mileage and throttle response are where you see it. I've had cars I drive daily with 4" of vacuum at idle that ran great with the vacuum advance functioning. But - at some point, you get out of the adjustability range of the cans. At that point you're probably in a race engine/race car situation anyway.

              Edit - Thanks MAgoo - I'm not up on the Pertronix stuff. I'd still argue that their instructions are not 100* right though assuming they adjust like others'. You're still adjusting the vacuum level required to move the arm. Not the number of degrees. Once the vacuum climbs high enough you will get 14* added no matter what you adjust.
               
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              • JG1966

                JG1966 Well-Known Member

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                I'm using manifold vacuum for two reasons. One, my AED Holley 750DP doesn't have a timed port. Also, my 340 wants a ton of initial advance. I'm at 24 initial, 36 total right now. Manifold vacuum will allow me to lower initial a bit, raise mechanical a bit and still get the initial timing she craves off the vacuum advance (provided it kicks in before 15"). That's the plan, at least. I've never used vacuum advance before with this motor and she runs fine. But I hooked up vacuum advance just as an experiment -- and because so many say it's a must for a 95-percent street-driven car -- and took her for a spin. No pinging, no detonation, and a noticable improvement with idle/drivability.
                 
                Last edited: Jul 20, 2018
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                • mderoy340

                  mderoy340 Well-Known Member

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                  Your increasing or decreasing tension on the diaphragm and spring, doing so changes when advance starts to move and how much at the same time. A mighty vac and timing light is needed to dial in. Max total possible is 2x the lever # at the crank. Either vac port works depending on how you setup the distributor.
                   
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                  • mderoy340

                    mderoy340 Well-Known Member

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                    • lomchivok

                      lomchivok Well-Known Member

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                      I forget who made it but there is a segmented adjustable cam that can be installed in the distributor at the canister diaphragm arm to limit the travel of the canister arm. I used one in a 318 with a big cam to give me 24* of idle timing. At idle I had 10" of vacuum, the can actuated at 5" vacuum and gave a total of 12* timing at 10". All in the 318 liked 42* and total at cruise with the 6* limit on the can arm total timing was 48*. Worked well and softened the in gear idle shake quite a bit...on manifold vacuum. I forget the number of the adjustable canister too...took a bit to find one with such a low actuation point that fit my pertronix distributor.
                       
                      Last edited: Jul 21, 2018
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                      • Mattax

                        Mattax Just the facts, ma'am FABO Gold Member

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                        Crane makes a little plastic cam for limiting the amount of advance for GM/Delco style cannisters.
                        It comes in this kit Crane Cams 99601-1: Adjustable Vacuum Advance Delco Point Type | JEGS
                        I didn't think it would work with Chrysler type internals.

                        Correct.

                        I have not. Do the internals use Chrysler style mechanism?

                        If could be plain wrong as you suspect, or they may have set it so vacuum begins to add advance around 10 to 12*. In that case, it might only have added 7* degrees at 15"; and to hit the maximum advance it can add might take 20" Hg. That's what was found on this older Chrysler built distributor with the adjuster 6 turns ccw.
                        file-php-id-599-jpg-jpg.jpg

                        Only way to make the maximum advance adjustable I've seen is to drill the casting and tap it for a screw. This is the method Mderoy340 used and is shown in How To Limit and Adjust Chrysler Vacuum Advance Cans
                         
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                        • Mattax

                          Mattax Just the facts, ma'am FABO Gold Member

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                          Yes. Manifold vacuum can be used to add timing at idle if needed. 24* at idle is a lot for a stock engine, but could be what a radically cammed engine likes. Variables there include RPM it is idling at, the fuel mixture and throttle opening at idle, and how much load is added when put into gear. If you find it noticibly drops rpm when put into gear, experiment with slightly richer fuel mix and less timing at idle.
                           
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                          • AJ/FormS

                            AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                            I think I know what you might find confusing:
                            Inside that can is a spring and a preload adjuster. When the spring has no additional preload from the tensioner, then vacuum has an easy time to pull the timing in. When the spring receives maximum preload, vacuum has to go to a higher level to pull it all in.
                            For the most part, that little screw you turn, only adjusts the preload, and
                            When you turn the screw CCW you are reducing the preload, and that always confused me,lol. The screw is peened on the end, so it can't come out unless you really force it

                            The amount of advance can only max at what is on the arm, but only if your engine can pull the vacuum to do it.
                            At one time there were a myriad of cans available to suit nearly any engine, but they are getting harder and harder to find.

                            The step on the arm limits the advance. You can cut some step off if you need more advance. If you have too much, you can limit the plate travel inside the D. But you cannot mess with the spring calibration, outside of the provided range.

                            Generally, the vacuum required to max the advance, is somewhere between 9 and 20 inches, and almost any street engine can achieve this by cruising rpm, and that is when this device really shines.

                            Really the preload adjuster is just to delay the timing change to prevent detonation at low-rpm/part throttle, on engines that are susceptible to that condition. I bring mine in as fast and hard as it can.
                             
                            Last edited: Jul 22, 2018
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                            • JG1966

                              JG1966 Well-Known Member

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                              Thanks Moper. Please help me with this. I taped vacuum gauge to windshield and went for a cruise. I was at 18" cruising at 50 at 2,600. I had to use a manifold port. Hope that's OK. It would jump to 20-22 when I let off gas completely. But right around 17-18" while cruising. So what's next? I know take a mityvac, pull vacuum to 18* and check timing. I'm assumimg I do this with engine at 2,600 rpm. Correct? Then what? Let's say, for example, timing then is 52. Is that OK? What am I aiming for when I adjust vacuum can? If it matters, right now I've set timing at 16 initial, 36 total and it's at 53 all in at about 2,400 with advance hooked up. Thanks.
                               
                            • moper

                              moper Well-Known Member

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                              I'm curious - what cam is in this engine? What pistons and head? What intake? Have you taken cylinder pressure readings?
                               
                            • JG1966

                              JG1966 Well-Known Member

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                              It has a Hughes 2832 cam (228@50 .530 lift), flat top pistons, .202 X heads, Edelbrock Performer Air Gap intake. Motor was refreshed this winter (rings, bearings, new cam, etc) and I haven't taken compression readings since then. But they were about 165-170 (except for two) when I did compression test before getting work done.
                               
                              Last edited: Jul 24, 2018
                            • AJ/FormS

                              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                              I had the next smaller cam, the 2330; and now have the 3037. I WAS gonna get that 2832, but got gready .
                              I think I woulda liked that 2832
                               
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                              • moper

                                moper Well-Known Member

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                                I won't run Hughes product so I don't have a point of reference. Have you contacted Hughes and asked if that vacuum level is typical for that camshaft in that engine? IMO, if you need that much advance, something is wrong. What happens with the usual 15* initial, and a curve that brings another 20* before 2500? Did you tune the carb?
                                 
                              • mopar head

                                mopar head Well-Known Member

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                                Just curious why, attitude? poor quality?
                                 
                              • JG1966

                                JG1966 Well-Known Member

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                                I've changed the jets on the carb but waiting til I get timing set before completely tuning it. If I set 16 initial/36 total, it runs fine. But it idles better -- and has more vacuum obviously -- at 24 initial. Like I said before, the motor likes a lot of initial. When I had my old cam (.509 Purple cam), I once timed motor by ear using a vacuum gauge. I got smoothest idle and highest vacuum and I looked and was at 40 initial. That wouldn't work. I'd like to run vacuum advance, but if I cant it's no big deal. As for my recent post, how do I adjust can based on my timing reading when I use mityvac on advance? Do I check timing at 2,600? Thanks
                                 
                                Last edited: Jul 25, 2018
                              • halifaxhops

                                halifaxhops It's going to get stupid around here! FABO Gold Member

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                                The number marked on the arm is not vacuum it is distributor degrees usually around 14" of vacuum. The adjustment is usually set in the middle of a new can say 7 (14 at the crank) you can only get 14 (28 crank) out of it at the most do it almost every day here. There are a lot of myths regarding them. Most of the mopar are adjustable with a 3/32 allen wrench. Now the engine each one has different timing demands depending on what it is so usually get the mechanical right then go from there with the vacuum. I have had no complaints so far.
                                 
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                                • halifaxhops

                                  halifaxhops It's going to get stupid around here! FABO Gold Member

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                                  Sorry yes you can use the mighty vac at idle if you want just have to know where the mechanical is at 2600 and add the vac to it.
                                   
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                                  • halifaxhops

                                    halifaxhops It's going to get stupid around here! FABO Gold Member

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                                    Last edited by a moderator: Jul 25, 2018
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