Stumble (miss) at Idle. Please help. - NCRS Discussion Boards

Stumble (miss) at Idle. Please help.

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

    #16
    Re: Stumble (miss) at Idle. Please help.

    Frank,

    As I read it, you have observed three problems.

    1. An intermittent miss, like a dead or weak cylinder.

    2. An off-idle stumble

    3. A PowerGlide that shifts into Reverse or Drive with an occasional bang.

    If I've misunderstood, then just ignore the rest of this posting.

    1. The intermittent miss.... the suggestions you've already received about ignition issues make sense. I have nothing to add.

    2. Blame the 12-206 cam coupled with a PowerGlide. I have the same combination in my '63 roadster and there is a slight off-idle stumble. Switching to a vacuum can with a lower pull-in vacuum helped detectably. Switching to larger accelerator pump squirters helped detectably. (The engine has an AFB; I bought squirters for the Edelbrock AFB clone.) I've watched the air/fuel ratio in real time and I now know for certain the stumble is a transient lean condition. With the two changes I've made, I can live with what's left of the stumble (and have done so for 14 years). The engine otherwise is very strong and I'm of the opinion the 12-206 is a good cam.

    3. When I had the PG in my car rebuilt, the fellow who did the work noted that the kit was missing a cone shaped cushion washer. He warned me it would bang when going from Drive to Reverse (and vice versa). Yep, it sometimes does. This might be the problem with your PG also. The missing washer, as I understand it, would be in the rear of the transmission and would be visible/serviceable by removing the tail shaft housing. So if yours could be missing, you can verify this and remedy it without having to pull the transmission... just the tail shaft housing.

    Good luck,
    Jim

    Comment

    • Thomas H.
      Extremely Frequent Poster
      • August 31, 2005
      • 1058

      #17
      Re: Stumble (miss) at Idle. Please help.

      Originally posted by Duke Williams (22045)
      Increasing overlap with Powerglide often leads to tuning issues, and your installed cam has considerably more overlap than the OE cam. The excess overlap also kills low end torque, which is not a good idea on the two-speed PG with an 1800-2000 stall converter. Duke
      Duke,

      Am I correct in assuming that the RPM drop of approx 400 when going from park to drive can be attributed to the increased overlap?

      Tom
      1958, 283/245, White/red - Top Flight, October 2016
      1960, Black/black, 283/230 4sp
      1966, Black/Red, 327/350 4sp w/AC
      1967, 427/390, 4sp, Goodwood Green, Coupe
      1971 LS5, 4sp, coupe, Bridgehampton Blue
      2007 Z06, Lemans Blue

      Newsletter Editor, Delaware Valley Chapter

      Comment

      • Carl N.
        Expired
        • April 30, 1984
        • 592

        #18
        Re: Stumble (miss) at Idle. Please help.

        The thought is: a possible crack in the intake manifold allowing exhaust gas from the heat riser to bleed into the intake thus leaning out a cylinder. A bench test with no mufflers (minimal exhaust back pressure) might not indicate a problem, whereas with once mufflers were installed, the increased back pressure might be enough to force exhaust gas through a crack into the intake (leaning out a cylinder).

        Agree with David - after you have checked out everthing else mentioned, you may have a very small crack on a intake runner or a flaw in the intake gasket. I chased this very same issue for about two years and was about to give up. I put an aftermarket intake on and after market carb and everthing was good - problem solved. One day I decided to clean the orignal intake and store - that's when I found the hairline crack. Took it to machine shop - had it welded - back on engine with no issues. These things will drive you crazy - just remember your are dealing with 45+ year old parts !


        Carl

        Comment

        • David H.
          Extremely Frequent Poster
          • June 30, 2001
          • 1526

          #19
          Re: Stumble (miss) at Idle. Please help.

          Originally posted by Frank O'Donnell (44894)
          ... there is one other problem I have always had with this car, even after having the automatic transmission and rear-end rebuilt: the car shifts very hard when moving the shift lever from park to reverse, and from park to drive. ... Frank
          You might check the (front) Differential Cushion for deterioration of the rubber mount or the bolt being loose. Another potential problem would be elongation of the side bolt holes on the Differential Carrier Bracket itself. I think there was a problem with the 1963-64 carrier bracket side bolt holes elongating. There was a fix on the 1965-68 bracket with washers welded in place to strengthen the side bolt holes. I think the elongation problem continued, as a heavy plate was welded on the differential carrier beginning in 1969. I don't think these later (1969 and on) carriers had any elongation problems. If the differential carrier side bolt holes are elongated, then shifting the transmission into reverse and drive might cause the movement you feel.
          Judging Chairman Mid-Way USA (Kansas) Chapter

          Comment

          • Duke W.
            Beyond Control Poster
            • January 1, 1993
            • 15669

            #20
            Re: Stumble (miss) at Idle. Please help.

            Originally posted by Thomas Hoyer (44463)
            Duke,

            Am I correct in assuming that the RPM drop of approx 400 when going from park to drive can be attributed to the increased overlap?

            Tom
            Yes. It's also probably a factor in the jerk when shifting between neutral and D and R.

            As overlap increases, converter stall speed should be increased, but that will also signficantly reduce fuel economy.

            My recommendation for all PG cars - DON'T install a cam with more effective overlap than the OE cam.

            Duke

            Comment

            • Joe C.
              Expired
              • August 31, 1999
              • 4598

              #21
              Re: Stumble (miss) at Idle. Please help.

              Originally posted by Thomas Hoyer (44463)
              Duke,

              Am I correct in assuming that the RPM drop of approx 400 when going from park to drive can be attributed to the increased overlap?

              Tom
              Measure the engine's manifold vacuum @ 550 RPM and report back.

              Your Comp High Energy cam may not necessarily have more effective overlap than the 929 cam because of the 929's very lazy ramps and your cam's faster ramps. The speed of the valves' opening and closing plays a very large part in what is known as effective overlap. Simply comparing the 929's 195/202 degree durations at 0.050" valve lift with the 212/212 of your 12-206-2 camshaft is meaningless unless you also compare them both at J604d durations (advertised, or "seat-to-seat"), and then averaging the two. A reliable indicator of a cam's ramp speed is the delta between its 0.050" valve lift and J604d durations. Faster ramp speed is indicated by smaller duration differences between these 2, and notwithstanding its larger 0.293" lobe lift, your cam shows a 48 degree difference. OEM flat tappet camshafts typically were designed with as much as a 100 degree difference, and considering the very conservative lobe lifts involved, you can see that the ramps were exceedingly sluggish. This sluggishness causes considerable bleed-off of cylinder pressure before the valve(s) close fully, or to the point where they will effectively no longer pass any more gas. When calculating overlap, the lobe separation is employed as well. The 929 uses 112 degrees, and your Comp cam uses 110 degrees.....................essentially the same for purposes of this discussion.

              Comp's reproduction of the 929 cam is 12-105-3:
              http://www.compcams.com/Company/CC/c...px?csid=1&sb=2

              Comp's High Energy 12-206-2:
              http://www.compcams.com/Company/CC/c...x?csid=72&sb=2

              Comparing overlaps of yours versus the reproduction 929 yields the following:

              12-105-3 @ 0.050" valve lift = -36 degrees (no overlap)
              12-206-2 @ 0.050" valve lift = -8 degrees (no overlap)

              12-105-3 @ advertised duration* = 91 degrees
              12-206-2 @ advertised duration* = 40 degrees

              The 929 cam has more than twice the overlap at the seat than does your High Energy cam. Both cams have zero overlap @ 0.050" valve lift.



              * Advertised duration is the valve lift where, effectively, gas flow becomes negligible.
              Last edited by Joe C.; March 14, 2012, 05:41 PM.

              Comment

              • Timothy B.
                Extremely Frequent Poster
                • April 30, 1983
                • 5186

                #22
                Re: Stumble (miss) at Idle. Please help.

                I don't own a powerglide car but could the clunk could be from a 1000rpm idle then dropping into drive. Can you tell us how much idle vacuum the engine has and where the inital timing is set.

                IMO, I don't think any stumble is acceptable, these are tuning issues and when you start using hot rod parts the original specs are no longer good. Larger accelerator squirters, reducing timing because of less vacuum to gas the mixture, proper throttle plate position exposing transfer slot, enlarging the idle feed restriction are all things that are experimented with. There is no reason why installing a performance camshaft should lead to issues that you have to accept.

                Comment

                • Frank D.
                  Expired
                  • December 1, 2005
                  • 80

                  #23
                  Re: Stumble (miss) at Idle. Please help.

                  Originally posted by Thomas Hoyer (44463)
                  Frank,

                  Since you have ruled out a fault in the ignition system by doing this, I'd be looking into the mixture and then vacuum leak possibilities. Maybe make a run at the valve adjustment also.

                  Is the side that seems to have weaker exhaust the side that has the heat riser?

                  Tom
                  TOM:
                  The side that shows the stumble is the left side (the side w/o the heat riser). The no. 7 plug that shows oil fouling? but no gas smell is left side.
                  FRANK

                  Comment

                  • Frank D.
                    Expired
                    • December 1, 2005
                    • 80

                    #24
                    Re: Stumble (miss) at Idle. Please help.

                    David:
                    When we bench-tested the engine, we did have the pipes and the mufflers installed. When you mentioned the possibility of a crack in the intake manifold, I remember my mechanic mentioning saying something about a seeing a repair somewhere on the engine but that it looked "okay". However, I went back to my notes while researching the history of the car and one of the owners in the 70's DID repair the, "porous runner on the intake manifold" with epoxy.

                    If after I: (1)change out the breaker-less points, (2) re-set the dwell and timing, and (3) check everywhere for a vacuum leak, and still don't find the problem would you suggest I try replacing the intake manifold with an after-market one?

                    FRANK

                    Comment

                    • David H.
                      Extremely Frequent Poster
                      • June 30, 2001
                      • 1526

                      #25
                      Re: Stumble (miss) at Idle. Please help.

                      Originally posted by Frank O'Donnell (44894)
                      ... and still don't find the problem would you suggest I try replacing the intake manifold with an after-market one? ... FRANK


                      When in doubt, I start with "cheap and easy" and progress towards the more expensive and difficult solutions. I wouldn't purchase an aftermarket manifold until I was sure the manifold was the problem and repair costs were prohibitive. If the epoxy repair has failed, or there is another crack, I would start a new thread asking for repair suggestions.

                      Perhaps you can borrow an intake to use for a test if a visual inspection does not reveal a fault in your manifold or gasket. Was the oil splash shield on the bottom of the intake manifold removed and replaced for inspection during the rebuild? Any cracks around the oil shield rivets?
                      Judging Chairman Mid-Way USA (Kansas) Chapter

                      Comment

                      • Frank D.
                        Expired
                        • December 1, 2005
                        • 80

                        #26
                        Re: Stumble (miss) at Idle. Please help.

                        David:
                        When we removed the engine, transmission and rear end during the restoration, we replaced the rubber mount as it was trashed and wet with transmission fluid. There were no problems with elongation of the holes, but thanks for following and replying to my post.
                        Frank

                        Comment

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