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  • Pertronix Distributor

    Still tweaking the engine for best performance. As you know, I have a FAST EZ EFI system. I swapped my OE distributor with a Pertronix and that made a noticeable improvement. This has a GM style adavance so is a lot easier to tune. I've been working with FAST on their help forum for the correct settings. However still running rough under load, and mpg is less than 13.

    I opened up the distributor cap and found that the carbon button was worn to nothing. So I replaced the cap and rotor. No difference at all.

    I analyzed using an oscilloscope and identical clamp type probes. One on the coil wire and the second on a plug wire. Here's a picture of representative results. It looks like I'm losing a lot of energy through the distributor cap. I did this with all eight plug wires and all results were similar.

    Yellow is the coil wire and blue is a spark plug wire.
    Attached Files

  • #2
    This afternoon I cut a large window in the old cap and used that to measure clearance between the cap and rotor electrodes. I get about 0.039 to 0.048. This is larger than my plug gap.
    Attached Files

    Comment


    • #3
      Even though that gap seems large in the cap you cut up, you need to compare the diameter distance between the contacts across the cap from the cut up cap to the new cap you are running. Its not quite apples to apples as you have two different caps.

      Also you should try to get a good guestimate of the length of the 2 rotors by measuring from the center mark made by the carbon center post to the end.

      I would assume the differences to be slight, but y'all know how that goes and you are trying to track down something important. Easy to check and perhaps another dead end, but it would be a known dead end! Perhaps a different combination of brands would make the gap tighter??

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      • #4
        The old and new cap and rotor are all pertonix. Should be identical. Also I don't have deep calipers to measure the new cap. I figured this would give the most accurate results.

        Comment


        • #5
          I agree, dimensions should be identical across the same brand. Still would be interesting to even fit a wood strip across and compare how it fits to an old Ford cap.

          That gap seems really big to me too. I've never liked aluminum contacts either in a cap, but that is just me. Keep checking - you'll find the problem.

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          • #6
            I'm going to take the dremel to the old cap and make the contact closer, see how that works.

            Comment


            • #7
              Originally posted by Yadkin View Post
              This afternoon I cut a large window in the old cap and used that to measure clearance between the cap and rotor electrodes. I get about 0.039 to 0.048. This is larger than my plug gap.
              But the spark doesn't have to fight compression, possible fuel wash, heat etc inside the distributor cap so I don't think that gap is unreasonable. (I have run motors with 40 thou plug gap)
              A Thunderbirder from the Land of the Long White Cloud.

              Comment


              • #8
                Good point, but how do you explain the oscilloscope graphs?

                Comment


                • #9
                  I'm on par with Tom's explanations.
                  As far as the graphs, did you try swapping oscilloscope leads? Do they give exact same results?

                  If you're worried about .03" gap, solder a wire extension onto the brass rotor (or peen it closer). My bet is, it won't make a bit of difference.

                  Did you ever have a plug that wouldn't fire? Sometimes pulling the boot off the plug 1/2" will wake it up again. To rationalize, I think the collapsing field in the coil builds to a higher voltage before it can jump across the gap. That's why more modern vehicles incorporate a resistor-plug. - Dave
                  My latest project:
                  CLICK HERE to see my custom hydraulic roller 390 FE build.

                  "We've got to pause and ask ourselves: How much clean air do we need?"
                  --Lee Iacocca

                  Comment


                  • #10
                    Yes swapped the leads from the coil, same results. Also tried every plug.

                    I decided to experiment a bit and apply the dremel to the rotor contact. The first picture is my first grind and the second is my final, after several trial and error fits. I brought the clearance from 0.040 or larger down to 0.018.

                    Too late to fire her up, I'll try to find time tomorrow and get a new scope reading.
                    Attached Files

                    Comment


                    • #11
                      With reduced clearance, no improvement. In fact more erratic.

                      Comment


                      • #12
                        After my issue last week I ordered a new Pertonix "Second Strike" box and Flame Thrower II coil. The Pertronix box is lighter than the FAST box I'm replacing and does not seem to be as robust. But now distributor, Ignitor II module, coil and box are all the same manufacturer who lists them as working together.

                        I took about a 15 mile test run including speeds up to about 70. Misty rain, now the car's dirty. This seems to run somewhat smoother. Stopped twice to adjust the second strike and it seems to be smoothest with the second strike turned off. Still not perfect though.

                        Scope on the system shows occasional spikes on the coil wire (channel one) which coincide with the plug wire (channel 2). Random, about every third or fourth cycle. Channel 2 pattern not as consistent as with the FAST box, "bouncing down" from the coil wire spikes.

                        I put the battery voltage on Channel 3 and it won't read as a DC source, so set it to AC. This shows a series of pulses, like an AC wave form with the negative cut off. Interesting thing is that I see spikes from it that coincide with the coil wire spikes. So much for the battery acting as a filter.

                        The alternator is a Tuff Stuff with the regulator integral in the unit. Some call this a one-wire. It gives me a consistent 14.4 volts on a digital multimeter.

                        I purchased a 150 amp voltage filter, a NewMar 150A, and will install between the alternator and the battery. Hopefully this will smooth out the electrical and solve the problem.

                        Comment


                        • #13
                          An automobile's electrical system is EXTREMELY noisy, as you found out. The battery does act as a filter but not nearly as fast as a capacitor (even though we are happy with a battery's response for starting and charging). If it were possible to disconnected the battery while the engine is running, you would see far more noise.

                          So, capacitors work great. Did you ever see a circuit board with a whole bunch of little caps all connected in parallel? That's because little caps work much faster than one huge cap.

                          I'm not suggesting you run out and buy a couple thousand .22-uf caps and tie them all together, but that is the value we use on your points, on your mechanical voltage regulator and on the original generator. All those caps are mounted close to the spike source for maximum efficiency, otherwise your radio antenna will pick up the frequency and amplify the noise over your speaker. They seemingly do nothing so most of us simply let them be. The only one that ever gets replaced is the 'points condenser'.

                          That filter you bought... mount it closer to the alternator and distributor than the battery. Shorter wires are faster than long wires. - Dave
                          My latest project:
                          CLICK HERE to see my custom hydraulic roller 390 FE build.

                          "We've got to pause and ask ourselves: How much clean air do we need?"
                          --Lee Iacocca

                          Comment


                          • #14
                            According to my genius son this is a "huge LC filter", which is an inductor and capacitor, or low pass filter. I read about it on wikipedia and I get the basic jist.

                            My alternator, battery and 100 amp breaker between them are all on the passenger side. I plan to mount this on the radiator support just below the breaker. I might even use a short bus bar to connect the filter to the breaker.

                            Either that or the shortest #4 cable on record.

                            Comment


                            • #15
                              Steve, what is the function of an inductor? What is the function of a capacitor? Then, when you put them in series? Or, in parallel?

                              We use LC in speaker crossovers but they deal in frequencies of 20kHz. This is a car with frequencies far lower.

                              A capacitor opposes a change in voltage.
                              An inductor opposes a change in current.

                              Together in series, they form a tank circuit that oscillates when matched. That's why your ignition circuit works. If your coil OR your condenser crap out, no spark.
                              In parallel, they cancel each other, producing a resistive circuit.

                              I can understand putting a capacitor somewhere around your engine but not by the battery.

                              Did you ever use HID headlamps? They require a cap, mounted close to each lamp or the light flickers. I'm sure you've seen that at night, where strong, annoying flickering headlights are shining from a car.

                              BTW, short copper wires can carry far more current. I have used a #14 copper wire as a 100-amp fuse, but it was 2" long. - Dave
                              My latest project:
                              CLICK HERE to see my custom hydraulic roller 390 FE build.

                              "We've got to pause and ask ourselves: How much clean air do we need?"
                              --Lee Iacocca

                              Comment

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