RPM varies wildly small block

General Mopar Tech Discussions

  1. Scott19_72

    Scott19_72 Member

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    Hi, I’ve got a strange one on my hands... my 360 is climbing and dropping by as much as 400 RPM at idle. I’m curious if anyone has ever seen anything like this before.

    360, 30 over. Street/mild purple cam. 16 degrees initial timing (approximate, it jumps around with the unstable idle). Edelbrock 600cfm (cleaned, floats adjusted, mixture screws defaulted to a turn and a half out). Vacuum advance disconnected. I have hosed down the intake gaskets with starting fluid and do not detect vacuum leaks. I removed my spacer setup and put a fresh carb base gasket on there too.

    Here’s a link to about 30 seconds of it running.



    Any insight would be appreciated!
     
  2. 33 IMP

    33 IMP FBBO Gold Member FBBO Gold Member

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    Two things I would look at. What is your idle vacuum compared to your power valve setting. And, are the weights in your distributor sticking?
     
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    • PRND21

      PRND21 FBBO Gold Member FBBO Gold Member

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      The first thing I’m thinking is timing/distributor, as 33 IMP said. I’d also check to see what your vacuum is doing.
       
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      • beanhead

        beanhead Well-Known Member

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        Could be a lot of things happening, and it might help to have a little more info...what type of ignition? Did it just start out of nowhere, or did you do some work and then it started happening? When does it happen, in idle/gear/either one? PCV valve & line clear and working properly?
        Easy/quick stuff first
        I'd start with--bump the initial timing up to 20 and see what it does. It will likely speed up, but you're looking for it to smooth out. If it does, you may need to re-adjust the idle mixture and speed settings a bit afterwards.
         
      • YY1

        YY1 Well-Known Member

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        First ting that comes to my mind is vacuum leak.
         
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        • Scott19_72

          Scott19_72 Member

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          I've actually got the advance disconnected, I'm experiencing this crazy RPM bounce at idle with the advance port plugged.
           
        • Scott19_72

          Scott19_72 Member

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          I did play with the timing but it was in an effort to resolve a miss and high idle. Apparently I've been driving this thing around at 35 degrees of initial timing for a while! I'll continue to work the timing / idle mixture.
           
        • Scott19_72

          Scott19_72 Member

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          It's a mopar drop in electronic distributor kit. I did recently figure out I've been driving around at 35 degrees of initial timing, and the car's always had a miss to it. This entire event has taken place at idle in park. I can hear the cyclical miss at higher RPM though. The plugs all came out roasted marshmallow colored so I know explosions are occurring in all cylinders consistently (a few of them were lighter than others but not much). I also double checked the wire order and stuck my timing light on all of the wires and got a flash. PCV does not appear to be gummed up, and I also capped that port on the carb earlier without any improvement. I have not taken a vacuum gauge to the engine, but do have one available (I was under the impression vacuum numbers wouldn't be as relevant to a cammed motor as it is to a factory setup since I don't have cam specs available).

          I took it down to about 16 degrees of initial and reset the idle mixture/rpm, and it just won't hold steady at any rpm at idle. Faster is a little smoother, but still wanders between 1100-1400. I'm sure setting the initial to the desired total like a dragster covered up whatever is really going on but I'd like this thing to work right for a change! I guess my big question is since I've gone through a can of starting fluid and can't find a vacuum leak, could I have something exotic like a jumped timing chain?

          And of course, as I was playing with it yesterday I found/created a new thing I've got to address before I can get back to the idle circuit. I've got compression close to 125 on 5 cylinders and around 150 on three cylinders. I pulled the valve covers and found out that several of my stamped rocker shafts are so loose I can rattle them by hand, they barely touch the pushrods unless they're deep into opening the valves. Turns out a few of them have elongated mounting holes (some were 30 thousands out of round). Lifters all have about the same amount of resistance pushing on the rods (a lot of resistance) and their surfaces are flat (I fished a few out from under the affected rockers to check them). I know replacing these rockers will help engine operation in general but I don't think this is what's affecting my idle.
           
        • Scott19_72

          Scott19_72 Member

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          This might be related. Have new rocker assemblies on the way. The really loose one was severely elongated, a few of the others aren’t great either.

           
        • Wietse

          Wietse Well-Known Member

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          There are 3 things that can affect the idle rpm:

          -Ignition timing
          -Fuel feed
          -Air feed

          These are the 3 things to focus on, you already checked all over for vacuum leaks, had the PCV capped.
          Other potentials are: the secondary throttle, check for slack on the throttle plate shaft. And if equipped, the power brake booster. Disconnect it and check again.
          After that i think you can rule out the air feed from causing this.

          The fuel feed, bowl level you checked for correct height. But it needs to remain steady also, a bad pump or blocked filter causes trouble. You mentioned that at higher idle rpm the issue becomes less obvious, are you able to check your fuel pressure?

          Regarding ignition timing, you already said you initially found that you were running 35* advance, was this due to having it set and not checked or did you leave it at a lower setting the last time you set it?
          If this wandered off by itself there is a problem.
          You already stated the timing is fluctuating, how much advance do you see, min/max?. You said average is 16* now but if this is going from 12-20* or so, there is your problem.
          Reason for fluctuation could be wear in the drive, or the mech. advance mechanism.
          If you take the rotor cap off and try to turn the rotor by hand cw-ccw, if there is a lot of slack that gives it space to fluctuate and you need to check the wear on the distr. shaft and cam gearing and if not there, the cam drive chain.
          Sticking a heavy spring on the mech. advance will help and rule it out as the trouble maker, with a light spring your mech. advance already might start doing its job while it is not wanted yet and a strong spring will make sure it stays on initial advance only.

          Those rockers showing a lot of clearance, axially and radially.
          The axial play on your rockers normally should be shimmed to a small clearance and as good as centered above the valve stem.
          If yours are showing .030" out of round they are worn, age or no lubrication? What oil about pressure?
          There is something wrong there for sure, although it might not be the reason for the idle issue.
          It could if it affects your vacuum so you need to check if you still have a steady vacuum.
           
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          • beanhead

            beanhead Well-Known Member

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            If the rockers are so worn that they're causing a 'valve lash' effect, that could definitely cause trouble. So can the pushrods being too short, which is possible if a cam with a smaller base circle was put in...I'm not sure how common that really is though. Let us know if the new rockers cure it. From the sound of the first video my first thoughts were vacuum leaks, distributor problem, ignition box problem. Doesn't mean it's not fuel related though, just my first impressions.You should have a vacuum gauge regardless of the cam type, you want to know how the needle behaves, and how vacuum responds to tuning/timing changes, in addition to the actual numbers.
             
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            • Mick56

              Mick56 Active Member

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              I would get a vacuum gauge and use this chart to see what the problem might be. vacumm-reading-1.png
               
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              • Scott19_72

                Scott19_72 Member

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                Thank you for these suggestions, they're a GREAT set of leads. Truth is I've owned this car for so long these are my own mechanical failures coming back to haunt me. As kid in my teens & early 20s on the weekends I would run this thing hard and put it away wet. And then drive it to work and school every day during the week. Everything on this car has been a side/back burner project for me while life happened these last 10 years. I'm pushing 40 and it's time for me to draw a line in the sand - put the work in, pay for the parts and get it to run right.

                I put the electronic ignition on it about 10 years ago because I didn't want to set points any more... I believe I timed it by ear just to see where it ran the smoothest, hence the high advance. I might have even been trying to solve this exact missfire now that I think about it.

                You're right about the rocker shaft play, I can tell it isn't right so I've got new shaft assemblies coming but it will be a surprise if that fixes the issue.

                The timing skips around from around 14-18 at idle no matter the RPM I give it. I did check the distributor shaft just now. It's tight/doesn't move counter clockwise, moves clockwise with some resistance and snaps back crisply to place when I let go. I will hunt down firm springs for it and take it apart to make sure everything is moving freely and rule out the light spring/early advance concern - I hand't thought of that. Inspecting the timing chain for slack will be my next stop once I hear it run with the new rockers. I'll also be sure to get my vacuum gauge on it too and see if I've got steady pull (I'm guessing it'll be all over the place).

                It's always had great oil pressure. That's the one thing I've changed regularly and done well on this car. Doesn't burn a drop of oil. I lost an engine to oil system failure once and I keep one eye on the oil pressure gauge in everything I drive ever since.

                It occurs to me that if running 100LL Avgas and racing anyone who even looked at me twice all those years ago might have contributed to my rocker shaft elongation it definitely didn't do the timing chain any favors. I'll be sure to let you know what I find in there once I get to that step.

                Thanks again for putting the response and suggestions together!
                 
              • Scott19_72

                Scott19_72 Member

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                Will do on the new rocker shafts. I own a vacuum gauge but haven't relied on it much. I'll be sure to read up and the chart Mick56 posted is going to help get me started. I wish I had a known-good carburetor to throw on there for diagnostics. I may go back into mine to clean it up (again) and make sure everything is moving freely too. If it comes down to it and I can't prove it's anything else I may throw an ignition box at it too.
                 
              • Scott19_72

                Scott19_72 Member

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                Very cool chart! I'll see what I get once the new rocker assemblies come in.
                 
              • Mick56

                Mick56 Active Member

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                In the olden days, a vacuum gauge was usually the main diagnostic tool.
                 
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