Thumpr cam crap

Discussion in 'Mopar Performance Issues' started by N1ce2k1llu, Apr 10, 2018.

  1. N1ce2k1llu

    N1ce2k1llu Member

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    Oh I'm sure this has been brought up. I rebuilt a 1971 LA 318 and Installed a thumpr cam, headers, aluminum heads and edelbrock 1406 carb. Problem is that engine shakes alot at idle below 1000rpm. People say 1000rpm is about right but Im looking for that low choppy sound instead of a high idling motor. Already drilled 1/8" hole on my primary butterflies for afm adjustment but still can't get it to stop shaking anything below 1000rpm. Locked timing at 36 and still shakes. I know edelbrock 1406 is lean so I already changed the jets but that has nothing to do with idle. Changed to the weakest step up spring to. Haven't been able to do any tweeks lately do to an almost electrical fire.
     
  2. pishta

    pishta I know I'm right....

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    Thumper is a long duration cam and you wont get a clean idle with that. Id say 650-700 is about right for an idle...have you checked for a vacuum leak? 30°?
     
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    • SGBARRACUDA

      SGBARRACUDA ROY FABO Gold Member

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      Which Thumpr cam is it?
       
    • AJ/FormS

      AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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      Back up the bus; we are not the racer's forum so I assume this is a street engine.
      What did you set the installed centerline to?
      What compression ratio?
      What cylinder pressure?
      What idle vacuum?
      How much lifter preload?

      Do a leakdown test

      1/8 per plate is too much, maybe waaay too much; make sure the secondaries are completely closed with those sewer drains, and you may have to bypass the PCV with holes that big.
      Make sure the metering rods are staying down; tune the idle with the springs out if you have to
      Too much timing IMO.And why locked?
      Is this it?
      20-600-4 - Thumpr™ Hydraulic Flat Tappet Camshafts
      279/296/107+5 is a pretty ugly idling cam for a 318; posting 73.5* overlap. This cam needs a lotta compression, with aluminum heads. I have heard these in 318s at 700 or less and they do sound awesome.
      EDIT
      Those holes and all that timing, has thrown your T-Port sync off; that's why it's lean. I would set the sync first,with the mixture-screws out 2.75 turns, at 18*idle timing, and no butterfly holes at all, and plug all other vacuum sources on the intake, including the PCV. Then I would check the valley for vacuum, and if none found,I would reconnect the PCV.And finally prove no external vacuum leaks.
      Now, you can begin the tune, but do not disturb the T-port sync much, if at all. Idle speed setting will be with idle-air bypass, with perhaps a couple of degrees of timing one way or another. Do not attempt any other tuning until the idle is close to perfect. Then tune the off-idle to about 2200rpm.
      And if you just can't get it, make sure that all the lobes are still on the cam.
       
      Last edited: Apr 10, 2018
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      • dartgarth

        dartgarth Well-Known Member

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        what he said... ^^^^^
         
      • crackedback

        crackedback FABO Gold Member FABO Gold Member

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        If the cam is installed properly including preload, it's not the cam. Unless it has a flat lobe. They idle fine and just as you want.

        There is a different fly in the ointment. Drilling the primaries is a bad deal and a crutch to cover other issues.

        Vaccum leak, needles not held down at idle (lighter springs) are a couple of the issues that may be causing problems.
         
      • RustyRatRod

        RustyRatRod Just another dumbass. FABO Gold Member Technical Editor

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        If this is a new build, are you certain you installed the correct balancer and flex plate or flywheel?
         
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        • 273

          273 Well-Known Member

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          Isin't that what is supposed to do ??

          I got a 380hp create engine, Im pretty sure the cam they use in these is a mopar's version of a thumper cam. It lumpy has hell seem to want to shake my car apart at idle :)
           
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          • harrisonm

            harrisonm Well-Known Member

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            I wonder why they call them THUMPERS!
             
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            • j par

              j par Well-hung Member

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              Because the idol real smooth....
               
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              • Cope

                Cope Well-Known Member

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                That cam will eat your motor mounts. If they were old the're done now...
                Solid mounts=WAY better. Shake the fenders, not the engine....
                 
                Last edited: Apr 11, 2018
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                • bbrroowwnn

                  bbrroowwnn never re member FABO Gold Member

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                  still better than the "mutha thumpa"
                   
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                  • AJ/FormS

                    AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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                    I drove the 292/292/108 for about a year. It has 76* overlap. If you put it in at 107similar to what I did at first, it will have the overlap split very similar to the 279 Thumpr. If you then give her what she wants as to timing and fuel.....and compression...... and a stock heavy flywheel, mine was not all that rough, and I could pull it down to 650 or a little less with the clutch; but she sure wouldn;t do that with timing locked at 36*. More like 5* down there and 14* idle and with aluminum heads and 11.3Scr, she liked 32/34 power-timing...... and burned 87E10.
                    If you have a TC you can run a lot more timing, and pretty early with the bigger TCs, cuz it doesn't get as jumpy down low;the TC smoothes it out. But.... if you fire it a little later as the peak-pressure occurs further down the bore, it smoothes out the idle. You just have to bring the timing in a little faster to be similar at stall, and then slow it down on the way to 3200ish,to prevent detonation. But with aluminum heads on a 318, and running premium gas;getting detonation might be a non-issue; cuz 9.5Scr is about all you can get at zero deck. And with that and an ICA of 61/62 the pressure should be fairly low.
                    This is a tricky cam, it idles like a 292, but as soon as it gets to about 2000/2200 it becomes a pussy-cat, like the 227@.050 that it is.
                     
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                    • N1ce2k1llu

                      N1ce2k1llu Member

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                      I locked timing at 36 because I keep reading these cams like as much advancing as possible. This will be a weekend cruser. Here's the cam card and link. I have 10 hg for vacuum at 1000rpm. COMP Cams Thumpr Hydraulic Flat Tappet Cam and Lifter Kits K20-600-4

                      20180411_081347-1.jpg
                       
                    • doogievlg

                      doogievlg Well-Known Member

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                      What is the issue with the Mutha Thumpa cam? I’ve seen those make some really good power in a 10:1 small block.
                       
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                      • Mattax

                        Mattax Just the facts, ma'am FABO Gold Member

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                        Timing is advanced or delayed from top dead center to account for rate of burn.
                        At idle, even with all the exhaust dilution with this cam, 36* is too early.
                        Somewhere between 17* and 20* is going to be right with that cam in a 318.
                        Then it will burn a richer mixture at the correct rate to push the pistons down efficiently.

                        Too bad about the holes in the throttle plates. Probably not needed and may make it harder to get right now. But give it a shot. If you find the blades are too far closed at idle of 700-900 rpm, you'll have to make them smaller.


                        With an automagic, set the idle timing so when placed into D, you get the best vac and the rpms drop very little. Start at 17 - engine fully warm. Keep advancing, until there is no more gain. That's enough. Any more is a waste of energy and will result in less power at idle - off-idle.
                         
                      • RustyRatRod

                        RustyRatRod Just another dumbass. FABO Gold Member Technical Editor

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                        • IQ52

                          IQ52 Well-Known Member

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                          Shoulda gone with the blue muscle suit with a big red "S" on the chest.
                           
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                          • gtsdude

                            gtsdude Well-Known Member

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                            Like RRR said and i think someone else mentioned it also, I would start by getting the initial ignition timing set lower than 36 degrees. Maybe 15-20 degree range, then see if it idles down.
                             
                          • N1ce2k1llu

                            N1ce2k1llu Member

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                            What is t-port sync? Tried looking it up and all I get is motorcycle info.
                             
                          • Mattax

                            Mattax Just the facts, ma'am FABO Gold Member

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                            On a Holley type carburator, the throttle blade relation to the transfer slot is a big factor in the idle - and cruising circuit.
                            On Carter and Autolites etc, it is still important but the way the sub-circuits branch off and position second idle restrictions etc make it less an issue.
                            Drilling holes in the the throttle blades is done so they can be closed sufficiently at idle. Otherwise too much transfer slot is exposed at idle. This results in a dead spot as throttle open. The master tech series has videos (slide shows actually) and pamphlets on how the carburetor circuits work if you want to learn about that.
                            Anyway, fix your electrical problems. Check with CC on whether the cam should have been advanced on installation. Then work on the initial timing and fueling.

                            This one is as good as any.
                            Carburetion Fundamentals & Facts (Session 273) from the Master Technician's Service Conference
                             
                            Last edited: Apr 11, 2018
                          • N1ce2k1llu

                            N1ce2k1llu Member

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                            Thanks everyone , this is my first build (obviously) I will make notes of everyones suggestions. I don't think wearing a superman costume would help so I'll just leave that one out :D
                            After I fix my wiring and get my cam issue solved I'll post an update. It might help someone else later. Thanks again
                             
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                            • crackedback

                              crackedback FABO Gold Member FABO Gold Member

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                              I'll guess that the high idle and drilled holes is activating the main circuit in the carb. Standing behind the car when running at idle, does it smell like a fuel truck dumping gas on the ground.

                              That cam sould pull somewhere around 12inchs at idle. You are below that at a higher RPM.
                               
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                              • AJ/FormS

                                AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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                                10"@1000 with 36* idle-timing means something is seriously mechanically wrong.Or the firing order is out.
                                My guess is the intakes are not closing properly or she's sucking air somewhere other than the carb, or both. Yeah probably both.
                                If you decked the block to get a zero deck, "occasionally" , the machining gets out of whack and the intake sucks air on the valley side. To check for this, put a vacuum gauge on the dipstick tube, flip the PCV out of the VC (you do have a PCV right), and seal all openings into the crankcase; then start her up and watch the vacuum gauge. Got vacuum? Pull the intake and get it machined to fit. If you are lucky, it will just need the bolt-holes slotted a tad.
                                While the intake is off, you can check where split overlap occurs; and all your lifter preloads.
                                After that,or if no vacuum in the valley, then I would do a leakdown test.
                                 
                                Last edited: Apr 11, 2018
                              • harrisonm

                                harrisonm Well-Known Member

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                                I still like my Comp 268XE. It has a nice "thumping" idle and gives me oodles of power. For a street cruiser in a small block, I think you'd be hard pressed to find a better cam. Just IMO, of course.
                                 
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