318 backfired and now runs like it has a misfire

Discussion in 'Small Block Mopar Engine' started by Rave73, May 16, 2018 at 11:11 AM.

  1. Rave73

    Rave73 New Member

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    Hey guys new to the group. I have a 68 Dart 270 that I swapped a 318 into from a 74 satellite.
    What happened was I had my timing too far advanced and before I realized my mistake I had done a full rev up with it like that and it back fired hard through the carb spitting solid black pieces out of the carb. Since then I dialed in the timing properly but it has always ran since like it is missing a cylinder. The carb has been completely taken off and looked over...all is good. I have also pulled all wires off while running and it's not showing any specific signs as to one cylinder worse than the other. I just ran some seafoam really helped clean out some carbon build up and just pulled off the valve covers to start checking top end compents. At this point I am at a loss of where to go to be honest other than scoping out the the cylinders through the plug holes and pull the push rods out and seeing if one got bent. Any suggestions??

    The only mods done to the 318 is Performer Intake, 600cfm holley, 360 heads from a 76 dart. The plugs are ngk r's(new) and i have a brand new set of wires, new ignition control module and ballast. Only old part left being the coil and distributor....now I do have a brand new distributor coming that is similar to the GM HEI coil on cap distributors to eliminate all electrical concerns.
     
  2. KosmicKuda

    KosmicKuda Well-Known Member

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    Welcome to the camp.
    I'm not a Holley guy but I seem to recall that a backfire will blow out the power valve. Someone will probably chime in with more ideas.
     
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    • oldskool979

      oldskool979 FABO Gold Member FABO Gold Member

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      I would pull all the plugs and run a compression ck as a first step. Any cylinder that is way off (15%) would be first place to look for bad push rod, collapsed lifter, broken spring, etc. If nothing shows up, then you can chase ignition issues. I would not install the new dist until you have done this.
       
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      • RustyRatRod

        RustyRatRod Just another dumbass. FABO Gold Member Technical Editor

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        Perfect answer^^^^^^ This is where to start.
         
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        • 604B1duster

          604B1duster Well-Known Member

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          Have someone slightly brake rev the car and hold the rpm where the engine is missing, then pull one plug wire off at a time until you find the one that has no effect on the engine. That’s your miss. Check the plug, wire, compression on that cylinder
           
        • Rave73

          Rave73 New Member

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          Awesome thanks guys! I'll keep you posted after I run the check!
           
        • tbritton

          tbritton Member

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          All great answers. The black pieces lead me to believe you may have an EGR valve check to make sure the valve is close and didn't get a piece of carbon under it holding it open
           
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          • toolmanmike

            toolmanmike FABO Staff Staff Member FABO Gold Member

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            It belched black through the carb? I would guess carbon. A old trick I used to do. Warm the engine up to temp, open the throttle 1/4 to 1/3 of the way open, (2500 rpm or so) and trickle a pop can or bottle of water down the carb. TRICKLE
            is the key. You can use fuel injection cleaner as well. You may be surprised all the crap that blows out of the tail pipe.
             
          • Dartswinger70

            Dartswinger70 Well-Known Member

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            Pop through the carb is an intake valve, and or timing. Spark happening while intake valve is open. How is the timing chain?
             
          • teringer

            teringer ase master hall of fame

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            idle the engine at 1000 rpms then pull each wire off one at a time if the engine is shaking the bad cylinder will smooth out when the wire is pulled.(intake valve not closing right ,carbon on seat ect) your looking for a drop of 50 rpms each cylinder with wire off this was referred to as a cylinder balance test back in the day, but if you have a compression tester that's where you start like all the other have said. DID YOU BLOW OF A VACUUM LINE WHEN IT BACKFIRED? DID YOU PUMP UP A LIFTER THAT WONT BLEED DOWN?
             
          • Tooljunkie

            Tooljunkie King of cobble/master of the broken bolt FABO Gold Member

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            Many answers.
            Most of them require no parts. Double check your firing order.
            Egr, pull it off and look for a chunk of carbon holding it open. They can leak and make engine run like a real pos...
             
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            • Demonracer

              Demonracer 71 Demon 00 Ram 16 Chrysler 300S 05 Caravan FABO Gold Member

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              Just about anytime you get a back fire on a Holley, it's time to change the power valve. I've only NOT had to change it on two occasions., & that's over about a 50 year period.