Distributor rotor tang pattern - NCRS Discussion Boards

Distributor rotor tang pattern

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  • Mike E.
    Extremely Frequent Poster
    • February 28, 1975
    • 5136

    Distributor rotor tang pattern

    I pulled this rotor out of a 71 big block that wants to cut out above 2800 rpm. I have never noticed a pattern like this on a rotor before--maybe I have just missed it. Do the small dots (8 in the circle and 1 in the center) indicate anything to those of you who are for more experienced with engine diagnosis than I am? (Other than it is an 8 cylinder). Car has original coil, replacement TI amp, new GM TI pickup coil, new GM braided plug wires, new AC plugs. Ready to replace rotor with extended tip one. Any help much appreciated.
    distributor rotor pattern (1).jpgdistributor rotor pattern (2).jpg
  • Donald O.
    Extremely Frequent Poster
    • May 31, 1990
    • 1585

    #2
    Re: Distributor rotor tang pattern

    What does the distributor cap look like on the inside for the center post?
    The light at the end of the tunnel has been turned off.

    Comment

    • Dick W.
      Former NCRS Director Region IV
      • June 30, 1985
      • 10483

      #3
      Re: Distributor rotor tang pattern

      I think your tang might be a wee bit short from the looks of the pattern. Go to your local NAPA and get a ECH RR1670 rotor and I believe it will solve at least that problem
      Dick Whittington

      Comment

      • Larry M.
        Extremely Frequent Poster
        • January 1, 1992
        • 2688

        #4
        Re: Distributor rotor tang pattern

        Never liked the later replacement gray or white colored rotors. Buy a NOS Delco-Remy or an extended tip rotor made in the USA. Then give it another try.

        Agree to check cap center electrode also for issues. Who made the cap??

        Larry

        Comment

        • Mike E.
          Extremely Frequent Poster
          • February 28, 1975
          • 5136

          #5
          Re: Distributor rotor tang pattern

          Originally posted by Donald Olson (17357)
          What does the distributor cap look like on the inside for the center post?
          Looks just fine--like it should.

          Comment

          • Mike E.
            Extremely Frequent Poster
            • February 28, 1975
            • 5136

            #6
            Re: Distributor rotor tang pattern

            As I indicated, I'm installing an extended tip one. I'm asking about what is the read on this one besides cheap and poor quality.

            Comment

            • Owen L.
              Very Frequent User
              • September 30, 1991
              • 864

              #7
              Re: Distributor rotor tang pattern

              Originally posted by Mike Ernst (211)
              As I indicated, I'm installing an extended tip one. I'm asking about what is the read on this one besides cheap and poor quality.

              I don't recall ever seeing a pattern on a rotor -- just a dot where the coil's center tower is. It kinda looks like the rotor is walking around in a regular pattern. When the cap is screwed on, does it seem to wobble or is it pretty stationary with respect to side:side movement? Does the fit for distributor in the intake feel secure side:side without the clamp? Perhaps pull the distributor and check the drive gear as well as the shaft's play in the bearings.

              Comment

              • Joe L.
                Beyond Control Poster
                • February 1, 1988
                • 43211

                #8
                Re: Distributor rotor tang pattern

                Originally posted by Mike Ernst (211)
                As I indicated, I'm installing an extended tip one. I'm asking about what is the read on this one besides cheap and poor quality.

                Mike------


                I don't know what could have caused this pattern on the contact. One might opine that it was caused by some "arcing" between the rotor contact and the carbon contact in the distributor cap. However, I don't see how that could create the pattern seen since the contact point should be the same throughout the rotation of the rotor.

                This does not appear to me to be a Delco rotor. I have never seen a Delco rotor of this light gray material and I've not seen a Delco "light colored" rotor which used a rivet to retain the brass contact. You could check for the possibility it is a Delco piece, though. Usually, these will have the Delco logo on the underside. No logo=not Delco.
                In Appreciation of John Hinckley

                Comment

                • Ray K.
                  Very Frequent User
                  • July 31, 1985
                  • 369

                  #9
                  Re: Distributor rotor tang pattern

                  Does not look like Delco rotor to me. I would try to locate a Delco black rotor if possible. Also, as pointed out, make sure that the distributor cap is also a Delco Remy unit.

                  Ray

                  Comment

                  • Jim L.
                    Extremely Frequent Poster
                    • September 30, 1979
                    • 1808

                    #10
                    Re: Distributor rotor tang pattern

                    Originally posted by Joe Lucia (12484)
                    Mike------


                    One might opine that it was caused by some "arcing" between the rotor contact and the carbon contact in the distributor cap. However, I don't see how that could create the pattern seen since the contact point should be the same throughout the rotation of the rotor..
                    I think you've explained it, Joe.

                    If the axis of rotation of the rotor were to be aligned with the tip on the inside of the cap, there would, indeed be a single point of contact. I believe that rotor axis is NOT aligned with the contact tip causing the rotor movement to look more cam-like than rotor-like. So each time a plug fires, the tip inside the cap is making contact at a different point on that tang, producing the 8 little arc dots.

                    That's my theory and I'm stickin' to it.

                    Comment

                    • Gary B.
                      Very Frequent User
                      • July 31, 1979
                      • 926

                      #11
                      Re: Distributor rotor tang pattern

                      One dot for each spark plug wire location on the cap. When the rotor turns and catches the terminal in the cap, other's cross-fire to the center.
                      Replace the cap. If you have an extended tip rotor, use that also.
                      Gary B

                      Comment

                      • Joe L.
                        Beyond Control Poster
                        • February 1, 1988
                        • 43211

                        #12
                        Re: Distributor rotor tang pattern

                        Originally posted by Jim Lockwood (2750)
                        I think you've explained it, Joe.

                        If the axis of rotation of the rotor were to be aligned with the tip on the inside of the cap, there would, indeed be a single point of contact. I believe that rotor axis is NOT aligned with the contact tip causing the rotor movement to look more cam-like than rotor-like. So each time a plug fires, the tip inside the cap is making contact at a different point on that tang, producing the 8 little arc dots.

                        That's my theory and I'm stickin' to it.

                        Jim------


                        That was my original thought, too. However, I then figured if the rotor was turning with this much eccentric, the engine would be running horribly.
                        In Appreciation of John Hinckley

                        Comment

                        • Jim L.
                          Extremely Frequent Poster
                          • September 30, 1979
                          • 1808

                          #13
                          Re: Distributor rotor tang pattern

                          Originally posted by Joe Lucia (12484)
                          Jim------


                          That was my original thought, too. However, I then figured if the rotor was turning with this much eccentric, the engine would be running horribly.
                          Weeeellllll, I'd say that cutting out above 2800 RPM qualifies as "horrible".

                          Comment

                          • Joe L.
                            Beyond Control Poster
                            • February 1, 1988
                            • 43211

                            #14
                            Re: Distributor rotor tang pattern

                            Originally posted by Jim Lockwood (2750)
                            Weeeellllll, I'd say that cutting out above 2800 RPM qualifies as "horrible".

                            Jim-----


                            I would think that it would be far worse than even that.
                            In Appreciation of John Hinckley

                            Comment

                            • Dan D.
                              Extremely Frequent Poster
                              • November 5, 2008
                              • 1323

                              #15
                              Re: Distributor rotor tang pattern

                              Looks to me like the contact piece is placed too far in on the rotor. This is putting the axis of rotation off center, as previously said, and causing an excessive gap at the plug wire terminal. I would say the rivet hole in the contact is not in the correct location, putting the contact too far in and increasing the spark gap to the plug terminals. This will cause a misfire.

                              -Dan-

                              Comment

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