Using vacuum gauge to tune engine?

Mopar Performance Issues

  1. LovetheA's

    LovetheA's FABO Gold Member FABO Gold Member

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    “I moved this update to a this new thread because I wasn’t getting feedback on what to do next in this situation.”

    I recently took my edelbrock performer rpm intake to a machinist I trust explained the situation to him and had him mill my intake .040 on each side. That is the same thickness as the two felpro gaskets under and above the valley pan. The intake lined up perfectly now with the gaskets and I’m glad to report no intake or carb leaks. Checked for leaks with starter fluid previously based on the engine idling much higher than before it was taken apart. It used to idle at @1000rpm and with the leaks I had it idled noticeably higher at 1500. Now it is back idling at 1000 again. I sprayed the starter fluid at various points where intake meets heads and carburetor gasket with no sucking in of fluid or change in rpm as it did before. I hooked up a vacuum gauge to the constant vacuum draw on the side of my speed demon. I hope this is the correct port to use for vacuum testing? At idle it currently has 10-12 in vacuum that is after I moved the distributor around to find the steadiest and highest vacuum reading at idle.

    However, it seems like I have another issue. The engine doesn’t idle as smooth as it used to and the gauge bounces around by about 2 inHg. I should mention that it is a 383 4 speed 67 Dart with the older Mopar performance camshaft 484 lift 284 duration. What would be my next logical step check plugs, wires and distributor cap or is idle mixture screw on speed demon carb off? Should I pull each plug wire off of distributor cap one by one and listen to see if I can find if one is misfiring or dead? Some are a bear to get to with TTI headers. I didn't touch the carb just put it back on after changing old cast iron to edelbrock performer heads and updating Harland sharps with Mikes geometry kit from B3 racing. I’m trying to get it running as smooth as it did before by process of elimination and learning as I go. I have included a video of the engine idling and the gauge. When I rev it up it also feels like it is running rough.
     
  2. LovetheA's

    LovetheA's FABO Gold Member FABO Gold Member

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    Here is the engine running warm at idle and what the gauge says. Is this vacuum too low for the combination I’m running?
     
  3. Dubob

    Dubob Well-Known Member

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    You’ve made a bunch of changes to the engine. That cam if I remember is around 241degrees at .050” it will be some what choppy especially in a 383, the vacuum will fluctuate some. How did you set the preload on the lifters? Hot cold. The aluminum heads have a different growth rate than iron. There shouldn’t be lash with hydraulic lifters, they should be preloaded and I believe I hear lifter clatter in the video.
     
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    • Treblig

      Treblig Well-Known Member

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      "The engine doesn’t idle as smooth as it used to and the gauge bounces around by about 2 inHg"

      If the only thing you did is fix all the air leaks then I'm tempted to say that it needs more air????????? When it was leaking the engine ran more smoothly? Of course this was at a higher RPM which might have been hiding the lack of smoothness at a lower RPM??
      A healthy cam usually equals a rough idle, especially at the lower RPMs.
       
      Last edited: Oct 18, 2020 at 9:39 AM
    • fishmens67

      fishmens67 Well-Known Member

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      10 in vacuum is about right with the 284 cam.
      you didn't say where the initial timing ended up.
      It should be in the 20 -22 deg range in most cases
       
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      • fishmens67

        fishmens67 Well-Known Member

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        I'm betting needle bounce is the result of idle mixture
         
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        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          Start by warming up the engine fully. Get it to idle best you can and closest to target idle speed, by adjusting mixture and idle speed screws. Then carefully adjust timing for maximum smooth idle vacuum. "Temper" this with timing light readings so you don't get down the wrong road. This setting may be temporary, depending on your advance curve, IE you may need to re-work that. Temporarily disconnect and plug vacuum advance if used. On a side note I dislike full manifold vacuum for advance

          Now readjust idle speed and mixture. Set mixture alternately for maximum vacuum, readjusting idle speed if needed. "Finish' by touching up idle mixture towards lean just a touch. Vacuum should not drop or just barely start to drop at that point.

          Now see what you have.

          If you suspect port to manifold leaks don't forget to check around UNDER the manifold as well!!! And ports can leak from within the valley if mismatched enough. At least on a B/RB you can examine the valley gasket to port match on the heads.
           
        • fishmens67

          fishmens67 Well-Known Member

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          Initial timing needs to be sorted out 1st, 383's like a fair amount of initial timing .
           
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          • LovetheA's

            LovetheA's FABO Gold Member FABO Gold Member

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            Thank you everyone for all the advise. I’m testing the vacuum off of the vacuum port on the carb not the manifold and the distributor is an electronic one without vacuum advance on it. Should the timing be about 10-15 BTDC on initial and about 30-34deg BTDC in when fully advanced?
            I set the pre-load on the Harland sharps when I was working on that particular cylinder intake and exhaust by feel. I set the rocker so that it just starts to compress down on the pushrod and hydraulic lifter then went another half turn about .030-.050 thousands to lock it in. Then tightened down on the lock nut above to lock it in. It does sound like there is a lot of clatter with the rockers. Do you think I should recheck the settings?
             
            Last edited: Oct 18, 2020 at 12:49 PM
          • 67Dart273

            67Dart273 FABO Gold Member FABO Gold Member

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            "Depends". Different engine builds "want" different timing, a hot cam, etc, but you can not just "pick" numbers because of the curve in your dist. The engine may "want" more at idle (initial) but if you are driving it you don't want it to over--advance, so you may have to adjust it for max performance at high RPM and "put up with" less than optimum at idle.

            The answer..........recurve the dist.

            But since you seem to be trying to get it to idle, I didn't bother you with all that.

            So what you need to do is find out where it needs initial / low RPM, and find out what it wants under full power/ mid-high RPM, then take those figures and recurve the dist. to suit.
             
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            • gilberttitan08

              gilberttitan08 Mexican Hillbilly!!!

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              Go Oakland Athletics, I LovetheA’s too!!!
               
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              • LovetheA's

                LovetheA's FABO Gold Member FABO Gold Member

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                That’s funny gilberttitan08. I didn’t think about other uses for lovetheA’s.
                 
              • LovetheA's

                LovetheA's FABO Gold Member FABO Gold Member

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                I finally adjusted the carburetor air fuel mixture screws and set the idle screws after dialing in the timing the best I could. It idles at 1000 rpm, has 12-13 inHG and runs much smoother but what’s great is the choppy cam at idle. I’m over the moon for now.

                 
                Last edited: Oct 18, 2020 at 10:44 PM
              • pishta

                pishta I know I'm right....

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                I had 14" vacuum on a '68 spec 340 with that cam and a Strip Dominator with a 625 Demon. I got my idle down to 800/750 in gear with a MP 175K convertor. It idled better with a Holley spread bore though. Hated that Demon, it was posessed with an ill machined needle/seat boss that would shear the O-ring every time you installed one and would always end up fouling the needle seat and flooding it. I would think a 383 would be more mild than a 340 with the same cam profile as that cam (031?) was called the "Road Runner cam" thinking it was probably mild enough for a stock 383 RR?
                 
              • mopardude318

                mopardude318 Well-Known Member

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                This video helped me out on tuning.
                 
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                • LovetheA's

                  LovetheA's FABO Gold Member FABO Gold Member

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                  Than you everyone for all the advice and mopardude318 for the video. I do have another question for everyone. Everyone talks about ported and manifold vacuum. If my 383 has an electronic distributor with no vacuum advance how do you set the advanced timing? Also if my manifold doesn’t have a manifold port how can I take vacuum off of the manifold? I adjusted the timing and fuel/air mixture by using the rear port off of the demon carburetor on the passengers side. I read that is what you use. See photo.

                  4F00923E-04A9-4883-92CF-2789EE265D9C.jpeg
                   
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