New engine issues

Small Block Mopar Engine

  1. dazedand confused

    dazedand confused Well-Known Member

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    Ok so i recently had a 1985 318 block put together by my buddy at his machine shop. It contains stock rods and crank has after market flat top pistons with no valve relief grooves bored 30 over . All new performance valve train parts ! Erson cam 112 +4 lob separation 449 lift exh/intk 292 duration. Heads are stock 318 heads with light porting and valves redone and performance valves ECT. Performer intake and a Carter afb competition series 750 cfm 4 barrel carb. I recently had a ticking noise coming from the valve covers . Found out that the new comp cam push rods 3/8 wide was rubbing against the push rod holes in the heads! I swapped them out with new solid stock factory rods 5/16 wide solved the problem. Now I have an issue either carb or timing or both? Starts up and idles great but once in awhile has a slight miss ? Maybe low octane gas? You go to rev it up high and it acts like it is struggling or hesitates? But if you do short revs it's fine? I still need to do a vaccum test and cylinder pressure test. I haven't checked the Carter in a while to see if floats are still set right? It also has a brand new mopar performance distributor with performance curv would that be an issue or timing ? It doesn't seem to fluctuate a lot when turning distributor will running unless you turn it real wide then it bogs down or cuts out and dies? That thermoquad is not on the engine I just put it there to take a photo before finishing up engine

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    Last edited: Dec 31, 2018
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    • Steven190

      Steven190 FABO Gold Member FABO Gold Member

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      Sounds to me a carb problem.
       
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      • Plymouth 65

        Plymouth 65 Floorable Deplorable

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        750 cfm is a lot of carb for a 318. 65'
         
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        • BigBlockMopar

          BigBlockMopar BigBlockMember

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          EDIT Just saw the last line in your post, being the Thermoquad is removed already.
          ------
          That's a Thermoquad carb on that engine.
          The strugling most likely comes from the secondaries opening up and things haven't been tuned properly when that happens.
          the secondary airdoor has to be tuned so it opens gradually. If it's just flapping around or loose the engine will suddenly 'see' a big vacuum drop while no additional fuel-flow is initiated yet in the carb.

          The slight miss should be taken care of first. Probably a lean mixture.
          Someone needs a proper working (and willing) brain to tune a Thermoquad. This is why Holley's are so popular...
          ------------

          The above still could apply if your current carb has a secondary 'air-valve'.
           
        • dano

          dano Evil Handy Man

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          What brand of wires? What is your static and total timing? Are you running a vacuum advance and if so have you tried it unplugged? Low Octane won't hesitate, in fact it likes to ignite sooner than higher octane, poor gas, as in water contamination will, but usually you can smell that. The Carter/Edebrock carburetors like a certain fuel PSI and no more, are you running a regulator?
           
        • krazykuda

          krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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          That's a lot of cam for a low compression engine...
           
        • dazedand confused

          dazedand confused Well-Known Member

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          I have a Carter afb competition series 750 on it now and having the issues as described on my post
           
        • dazedand confused

          dazedand confused Well-Known Member

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          I've got a 600 cfm edelbrock?
           
        • dazedand confused

          dazedand confused Well-Known Member

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          I did a compression test today after engine fully warmed up: 120 psi in every cylinder . intake vaccum 20
           
        • dazedand confused

          dazedand confused Well-Known Member

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          9.1 compression flat top pistons bored 30 over
           
        • dazedand confused

          dazedand confused Well-Known Member

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          Running MSD 8mm racing wires plugs gaped a 45. HEI ignition with GM 4 prong performance ignition control module 45,000 super coil . Mopar performance distributor from Mancini racing with performance curv. Stock mechanical fuel pump and Holley fuel pressure regular set 5 -1/2 psi max with fuel pressure gauge. 3/8 racing fuel line. I haven't tried running with out vaccum advance .
           
        • moper

          moper FABO Gold Member FABO Gold Member

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          MP stuff can't be trusted to be right out of the box. Get a timing light with advance function, or a timing tape, and plot the timing curves. I also like to know where the vacuum advance operates too. That can also create a surging just off idle or at light throttle cruise. That sounds like what you have.
           
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          • nm9stheham

            nm9stheham Well-Known Member

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            As for your miss, use a voltmeter to make sure you have solid voltage to the ignition system; the ignition feed voltage ought to be no more than 1 volt less than what is coming out of the alternator. If ignition power is still running through the old wiring and ignition switch, then that is always suspect.

            Is the fuel fresh? Has the fuel tank been cleaned out or replaced? Struggling at high revs could be fuel or ignition. Is this while driving the car or just revving in the driveway? If while in the driveway, then I'd focus on ignition or carb first as the fuel supply system is not being taxed too hard if you are not driving.

            Just one dumb thing to check is that the exhaust is clear; old glasspacks can disintegrate internally and plug themselves up.

            Just as FYI: The cam will not have 292 duration with 20" of vacuum LOL. Some sort of typo there on the cam data; nor does 292 advertised duration make sense with a 112 LSA. And just for info, while the pistons may say they are for 9:1, the reality is going to be around 8:1; the compression readings confirm that.
             
          • PRH

            PRH Well-Known Member

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            And...... whatever the cam actually is..... at 120psi, it’s probably a bit too big.
            I’d be verifying that with a different gauge though before I got too carried away....... since it does seem have good vacuum.

            As has been said..... verify the timing, fuel pressure, fresh gas, etc....... put a kit in the carb, leave the vacuum advance off for now, set timing to 36* “total”.

            Looks like this is the cam:
            Who knows where they’re measuring that 292* from.

            I’d have used something more like a Comp 260H or the 700 Voodoo.
             
            Last edited: Jan 1, 2019
          • dazedand confused

            dazedand confused Well-Known Member

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            I followed the step by step instructions for converting over to HEI and keeping factory mopar distributor. True dual exhaust with super 40's Flo masters no cats. The pistion comes right to the top of the cylinder no gap between piston and top of the cylinder by the head gasket . Not a smog engine so it has high compression maybe the rings havent seated in yet?
             
          • PRH

            PRH Well-Known Member

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            If it’s really about 9:1, then I’d be more interested in seeing if that compression tester is accurate....... cuz if it’s 9:1 and you’ve only got 120psi cranking pressure...... “something” isn’t right.

            If the bores are honed with the proper finish, and you’ve got any kind of normal rings in it...... they should have seated within 10 minutes after start up.
             
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            • PRH

              PRH Well-Known Member

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              Btw..... the math doesn’t add up here.....

              318+.030, zero deck, pistons with no dish or valve pockets, std composition gasket...... the heads would have to be about 73cc’s for that to only be “9:1”.

              Did anyone cc a head?
              What casting number are they?

              Did you verify that the pushrods you swapped in were the same length as what you took out?
               
            • nm9stheham

              nm9stheham Well-Known Member

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              Ahah.... that is one of the Erson RV grinds.
               
            • thesiren74

              thesiren74 Well-Known Member

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              Had that same cam in my stock 318 with 2.45 gears and tall tires very soft up to 3,000 then it came on strong, they are advertised at 292 duration but very good idle.
               
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              • nm9stheham

                nm9stheham Well-Known Member

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                The initial post says 'aftermarket flat-top pistons', but did not say that they are zero decked. So I suspect they are something like stock replacement 285's or 814's, which are down in the hole by .070-075". Now the 120 psi compression readings make more sense....
                 
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                • AJ/FormS

                  AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                  At 700ft and with an Ica of 76*
                  Static compression ratio of 9.0:1.

                  Effective stroke is 2.27 inches.
                  Your dynamic compression ratio is 6.48:1 .
                  Your dynamic cranking pressure is 120.14 PSI.
                  V/P (Volume to Pressure Index) is 79
                  It's a 292 in a 318, at very low dynamic compression ratio. With closed throttle your EFFECTIVE IDLE compression ratio might be 3/1
                  I see a whole new combo in your future.
                   
                • PRH

                  PRH Well-Known Member

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                  If that info is correct, it’s only 9:1 if the heads are 73 cc’s.

                  661.4- swept
                  73-head
                  9-gasket
                  1-area above rings
                  ————
                  744.4
                  83-total minus swept
                  ————-
                  8.968cr- 744.4/83
                   
                  Last edited: Jan 1, 2019
                • PRH

                  PRH Well-Known Member

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                  Here’s a question regarding the on line cranking compression calculators......

                  Or something you can try when you’ve got the compression tester out.....

                  Assuming you have an adjustable valvetrain........ how far can you crack the intake valve off the seat by removing any lash, and going a bit further...... before the compression tester shows “zero” compression?

                  For example, if you tag a valve by over reving the engine and tweak it a little...... the compression reading is lower, but unless it’s severely bent, it’s not showing “zero”.
                  Yet, a bent valve never completely “closes”.

                  What’s the point of this?
                  My feeling is........ with cams that have extremely slow closing ramps, some compression starts to build before the valve is actually “seated”.
                  The valve is closing, the piston is ascending....... and if the piston is displacing more area than can squeeze through the small opening the almost seated valve has provided, some pressure will start to build.
                  So, if you tested two cams with the same exact actual closing points, but had radically different closing ramps(one really slow vs one that just slams the valve down), they would show different readings on the compression tester.
                  Although the calculation would say the cranking pressure would be the same.

                  It’s the same reason that a 292* cam could have 20* of vacuum.
                  The number of overlap degrees might be large, but the area under the overlap curve is still rather small because the valves aren’t open very far.

                  This 292/292-112 cam has the same overlap as the MP 484 cam, which is 284/284-108. Both have 68*.

                  Think that 484 cam would idle at 20” in this 318?
                   
                  Last edited: Jan 1, 2019
                • dazedand confused

                  dazedand confused Well-Known Member

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                  All I know by inspecting my engine after it was done that when the pistons come to the top of the cylinder the pistion position is flush with the gasket surface of the block! I'm using a set of felpro permatex head gaskets. The pistion according to the machine shop guy he had to bore the block 30 over. I did not see any valve grooves on top of the pistons. He used the rods that where originally in the block as well as the crank ! I dont know if they where swapped by a former engine build? The 318 1985 model came out of a plymouth fury cop car. The cam is an Erson brand and all I did is type the specs off of the paper that came with the cam. The heads are stock heads from mid to late 70's or early 80's?
                   
                • dazedand confused

                  dazedand confused Well-Known Member

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                  This engine has stock non adjustable rockers. I bought a chrome gauge from a speed shop that works off of vaccum " fuel saver" with color coded face plate to adjust: distributor, idle, fuel/air screws on carb to get the maximum efficiency in performance and fuel economy. It was reading 20 while engine was running? I didn't know if this is the same vaccum test you guys where talking about?
                   
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