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Prestolite Dual Point Advance Curve BACKS OFF! (retards 4 degrees)

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  • Electrical: Prestolite Dual Point Advance Curve BACKS OFF! (retards 4 degrees)

    Timing advances as rpm increases to max advance.

    Then as rpm is increased more, timing backs off (retards) 4 degrees.

    Further rpm increase, timing is constant.

    QUESTION: Has anyone observed this phenomenon?


  • #2
    First things I'd check:

    1. Is the vacuum advance disconnected when you're checking the advance?

    2. Are the pivot pins for the centrifugal weights worn?

    3. Do the shaft bushings have any play?

    jack vines


    • #3
      Thanks Jack,

      To answer your questions:

      1. Yes, vacuum disconnected
      2. Pins should be OK. Weight bores (un-bushed) will have wear. Looks decent from top but since cam plate was not removed, bottom wear is unknown.
      3. Main shaft is OK and is shimmed. Cam plate is another matter - I've never seen one with clearances that I would find acceptable.

      The history of this distributor is that it came with the R2 Lark. Car has about 70k on it. I street raced it for maybe 20k miles. Set total advance many, many times and never observed this behavior. So I'm a little perplexed why it's showing up now. (Of course I quit racing in the mid-70's and since then the car has only a few thousand miles on it.)

      I'm undecided whether to spend more time with the Presto.

      Due to 1) the difficulty of the cam-plate removal (see next paragraph), and 2) the total lack of parts availability (hours spent on the internet with NO leads on where to get parts,) I want to get as much information before I decide if I'm moving forward to Disassembly and Inspection.

      Taking the cam-plate out is problematic since this distributor does not have the "wire snap ring" retainer - instead it has the accursed self locking external retaining ring which while non-reusable can be removed with a lot of effort. And this would be the second time it's been removed. And if I break it, well, that would be very bad.


      • #4
        I have Prestolite dual point distributors in both my 62 Hawk and 63 Avanti and neither one of them has any retard as the rpm is advanced. The R2 distributors have less centrifugal advance and a faster advance rate than the normally aspirated distributors. Also any of the Prestolite dual point and single point distributors that I have serviced over the years have shown any retard at higher rpm. I own a distributor machine so any distributor I recondition is checked on the machine. Like Jack Vines says all of the moving parts and the retainer clip on the cam need to be checked for wear or looseness. McMaster Carr has the retaining clips listed in their catalogue if you need them. Bud


        • #5
          APOLOGIES! (this is a way long post - but it does have pictures - haha)

          It appears there could be something else at play.

          Presto issues are well known, but since no one is saying they've seen this phenomenon, there is a good chance if I totally go over this distributor, the problem could still remain. This is especially true if they've been checked on distributor machines which would reveal this phenomenon in a heartbeat.

          I'm beginning to suspect it is not worn distributor kinematic components which leaves: points, condenser, and coil. I'm ruling out plugs which are new and wires.

          However, there is more information that might help explain (actually, it will probably obfuscate the problem):

          In the Beginning: THE STRANGE MISS:
          This is "starting at the beginning."

          A Few Years Ago:
          A few years ago the engine would enter a very chaotic miss - so severe it was difficult to manually keep the engine running in neutral. There was some backfire out the tailpipe I believe due to unburnt fuel making its way into the exhaust. I don't think there was much coughing through the carb.

          I tried just about everything I could think of. Condensor, coil, plugs with no effect. As I remember it, I believe I ultrasonically cleaned the plugs and that may have solved it - but the onset of all this was when cold weather hit, and it just so happened it went away when warm weather was showing up.

          I'm not sure I fixed it and that it may have simply (mysteriously) subsided.

          A Few Months Ago:

          While I hadn't driven the car many miles, mostly because if this miss occurs, it is nearly impossible to move the car under its own power. But I started driving it around the neighborhood and then expanded my range. Soon I felt pretty confident that everything was fine.

          Then I had to put the car outdoors to free up my heated garage for another car repair. It sat for 3 days in the 30's without rain. Then 2 days in scattered rain showers.

          When the job was over, I started it, blipped the throttle a couple times - everything was just fine, and then drove it into the garage. The total run time was probably 30 seconds. Everything was just fine.

          Then as if someone turned a switch, the chaotic miss returned.

          What Was Tried:

          1. [*=1]New Cap and Rotor (Accel)
            [*=1]Swapped another in need of rebuilt R2 distributor
            [*=1]Checked carb needle and seats and float level (grasping at a fuel problem)
            [*=1]Checked for broken cam (desperate act)
            [*=1]New Autolite 85's and a new condensor
          Although these two were done sequentially, between them, I had installed the distributor about 3-cylinders out of time. Before I realized this, I concluded the plugs did not work, so I replaced the condenser and then discovered the timing issue.

          The old condenser checked OK on a DVM making the needle blip both directions. The new condenser produced less of a blip. I didn't like that since the "new" condensor could easily have been over 20 years old, but I installed the "new" one.

          However, when cranking the engine to let the distributor drop into the oil pump, I observed extreme point arcing in the passenger side point set. I'm accustomed to seeing just a small blue arc, but this was completely different and much worse.

          I cranked the engine and blew compressed air into the contacts to bow any metal dust that may have found its way there when I was setting dwell - some of my screw drivers have gotten magnetized and since I worked on my lathe, metal dust/debris is present.

          Nevertheless, I buttoned up the distributor, started the engine, and the miss was gone.

          TIMING LIGHT:

          However, what about the timing light?

          I have a "classic" chrome Sears Penske inductive light

          that has always seemed to fire properly. It is the light I used when observing this phenomenon.

          I borrowed a friends Snap-On digital light

          and it exhibited something completely different: The strobe was steady all the way to 34*, but when I was hoping to observe the 4* of retard as rpm was increased, on the balancer timing tape, I could catch glimpse of about a 20* jitter from 40-ish to 60-ish degrees.

          Thinking about this now, it is highly suspicious that the SO would "fail" just where the strangeness is occurring. But it is just as strange that if the SO was correct, just how was the Sears light smoothing this jitter?



          • Delco Window, Pertronix Ignitor III and Flame Thrower III.
          • $50 Chinese Pontiac HEI and fab it.
          • WILL NOT regut the Presto with the Mopar electronic ignition.


          • #6
            Does a dwell change coincide with the timing retard observed?


            • #7
              An oscilloscope would probably be required to reveal the symptom, if not the disease, but certainly a wacky unsteady dwell reading would indicate there is indeed "a problem."


              • #8
                Ah, a dwell change... I would like to answer that question. If dwell changes due to point contact duration, then there is a 50/50 chance the timing would change (depending on which point was varying.)

                I actually have two of those Heathkit Ignition Oscilloscopes - might be time to break one out and use it.

                I'm also warming up to the Window w/Ignitor and FireBall III Pertronics...


                • #9
                  If you have a scope, I recommend using it to look at what is happening with the ignition. A variation in dwell at different engine rpm is definitely a cause for concern as it shows either you have excessive wear on the distributor shaft and bushings or you could have a problem with point bounce or float which will give an unsteady dwell reading. If the dwell is changing then it's a guarantee that the timing is changing. If your scope doesn't show anything, then it's time to find someone with a distributor machine to check out the distributor. Also keep in mind there have been several people reporting including myself about new stock condensers failing shortly after they were installed. Bud


                  • #10
                    I don't think I have point bounce or float around 3000-ish. The springs of my points are much stiffer than the Standard X suffix points.

                    Timing is pretty solid and does not jitter much at all, maybe 1*. NOTE: I have power steering which is a real pain to see timing - it is not possible to look on a radial line, so there is parallax error.

                    Now hearing this about "new" caps failing, given the cost of points/cond, I'm really leaning towards Pertronix (but which one) or the Crane XRi which seems to have lots of complaints on the internet.

                    I could make a hobby of getting the Presto right, but seriously, why should I do that? Even points for the Window are almost $20 which is totally insane in my world. And then there is getting HD ones and how do they vary? Pointless definitely has advantages. And seeing how my "points" have almost stranded me a couple blocks from my home, there is no argument that points are more reliable. I put this all in the "50 year old failure modes" category.


                    • #11
                      Pertonix has it's fans, but I convert them to the Mopar solid state trigger.

                      FWIW, it costs me $300 to completely rebuild one correctly with all new parts.

                      Jack Vines


                      • #12
                        UPDATE: Just borrowed a like new Sun timing light to break the disagreement from my other two timing light readings.

                        I'm totally undecided what way to go:

                        • OEM look
                        • OEM HEI systems
                        • Delco Window Distributors
                        • Delco HEI Distributor performance
                        • Pertronix & Crane modules that don't use an external box (OEM look)

                        • Points and condensers (they are getting just too expensive compared to electronic)
                        • Prestolite Dual Point with its bushing issues (shaft and weights)
                        • External ignition boxes
                        • Delco HEI Distributor looks (hideously non-OEM look)
                        • Pertronix Ignitor III is over $100 which is too much for inferior to OEM aftermarket stuff.


                        • #13
                          UPDATE: Checked timing with hardly used, babied Sun timing light and it completely agreed with the Sears Penske.

                          Also learned that Mallory's 61001M is a point eliminator. If I had to pick between Mallory, Crane, and Pertronix, I'd be compelled to pick Mallory even though I've never liked their products.

                          I also tore down my spare Presto and it was as I suspected, the cam plate slot limits the weight travel. This is a reaction that is out of plane with the CG so that the weight wants to "curl" up when it is limited. I was hoping to rebush the weights, but the bottom face is worn too. Not worth the effort to repair (of course, I haven't priced new bushed weights - would not be surprised that they are $50 a set - heh heh)

                          DELCO WINDOW:
                          I was looking at my 2 Delco's and they do not have roll pins in them. One I remember re-pinning after shimming and I used welding rod with mushroomed ends to retain, the other looks factory where the pin was quite mushroomed with a heavy grid pattern where pressed. I know they are supposed to be shear pins, not sure what to use. Also, the one I shimmed, has excessive end play when originally set to .005-.010 inches. Don't know where all that wear came from.

                          Maybe I'll just clean up the Delco and put it in to see what the timing does.


                          • #14

                            Simply cleaned up my Delco that was setup maybe 20 years ago.

                            It is a joy to see rock steady timing and a proper advance curve.

                            Nest step it to decide on which Pertronix to use. Can get a good deal on an Ignitor I and will probably go that route although I'd like to try the III.

                            Then there is the bypassing the PINK resistance wire - I detest working under the dash of the 63 Lark. Tempted to install a lug mounted 12v 20 amp relay at the coil so I don't have to deal with the ignition switch.
                            NOTE: If I have to tap into the ignition switch, it might be worth the effort to conveniently locate a "sub-box" for the IGN and ACC terminals that would make any future electrical mods easier.

                            RE: Presto weight/timing issue:

                            The problem is that that top for the weights is the end of the the cam plate slot. When this happens the force on the top and the centrifugal force on the weight CG produces a couple that is reacted by the pins. The result is that you get wear that allows the weight to "curl" upward.

                            Note that before the weight hits the stop, the centrifugal force is reacted in-plane with the springs so there is no couple.

                            A solution to this would be to shim the cam plate so that it is very close to the weight top face so that when it wants to lift, it hits the bottom of the cam plate. Unfortunately, this would required shims to be placed under the cam plate clip (and reducing the thickness of the shaft spacer the cam plate rests on.

                            While it would be interesting to solve all these issues, I don't have the time or money to invest in this "Presto hobby."


                            • #15
                              1962 was a sad year for studebakers enviable reputation as possibly the car least likely to leave you stranded on the side of the road--particularly if the road was in the heart of central africa in the zambezi valley bordering both zambia and zimbabwe and teaming with REAL WILD ANIMALS---1962 WAS THE ADVENT OF PRESTOLITE DISTRIBUTORS--GENERATORS[ALTERNATORS IN 63] AND STARTERS--THE GENS.AND STARTERS WERE O.K--THE DISTRIBUTORS AND ALTERNATORS WERE ABYSMAL--TALK ABOUT THE WEAKEST LINK LETTING THE SIDE DOWN//