Backfire through Carb

Small Block Mopar Engine

  1. huljas666

    huljas666 Member

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    Hi All,

    I have a 69 Valiant with a 273. Engine is backfiring through the carb when I accelerate and looses power. I've replaced the accelerator pump, checked the spark plugs, wires, timing and also tried a new distributor. Just can't get rid of it. Any other ideas on what might cause this. Car was running great not too long ago.

    Thanks all for any help!
     
  2. RustyRatRod

    RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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    Is it a rhythmic steady pop? As in a cylinder firing up through the carburetor? If so, look for something keeping an exhaust valve closed. Wiped cam lobe, bad lifter, bent pushrod, bad rocker arm, valve stuck closed. Nothing good.
     
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    • huljas666

      huljas666 Member

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      Yeah, it is a kind of rhythmic steady pop. I'll check those out. Thanks!
       
    • moe7404

      moe7404 moe7404

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      my pappy all way said "all ways check the easiest and cheapest things first." chrysler V8 can have #5 - #7 wires crossed easy. Mopar dist cap wiring... vent
      My dad made this saying about chrysler V8 small and big block dist cap wireing. To reduce wiring confuseion. “the small block has the dist in the wrong place, in the back of the engine. BUT the shaft rotates the right way clockwise. The big block has the dist in the right place, in the front of the engine BUT the shaft rotates the wrong direction counterclockwise. Both wire #5 and #7 on both ALL ways have #5 before #7 on the cap AND block. And most / some stock V8s have the fireing order cast in to the intake manifold. AND: DO plug the vent with JB-weld. Keeps the moursture out.
       
    • RustyRatRod

      RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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      Your dad sounds wise, but I gotta disagree about stopping up the vent.

      Moisture is a by product of the spark inside the distributor. That vent allows it to remain dry.
       
    • AJ/FormS

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

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      I dont see compression test on that list.
      And possible MJ issue.
      Is that still points?
       
    • RustyRatRod

      RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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      With the exhaust valve not opening, she'll still pass a compression test. It will also not have a dead skip as that cylinder is still firing. It is simply exhausting up through the intake and through the carb......IF the exhaust valve remaining closed is the problem.

      Certainly though it will disrupt how the engine runs resulting in the poor power he describes.
       
    • ktcnn94

      ktcnn94 Well-Known Member

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      I agree with RRR, if the timing is correct and plug wires are correct it just about has to be a valve issue.
       
    • OldmanRick

      OldmanRick Well-Known Member

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      Sure sounds like a timing issue, But if this is a high mileage engine, it could actually be some carbon dislodged that's floating around the chamber and holding a intake valve open?

      RRR hit on a lot of possibilities. I'll just add that the "chain" or the "balancer" being off could cause some issues also trying to dial it in.
       
    • 2 Darts

      2 Darts A-body Addicted

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      X 2 on timing chain or cracked distributor cap.

      The OEM timing chain runs a nylon toothed cam gear. Known for shedding teeth and jumping. Replacement is straight forward but a PITA with the engine in the car.

      A cracked distributor cap can be hard to track down. Suggest starting engine in complete darkness. Look for visible blue-ish arc. If you see one, replace cap or wire, which ever is the source.
       
    • Abodybomber

      Abodybomber Breaking street machines , since 1983.....:) Legendary Member

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      273 , damn good engine. To help diagnosis: How about telling us , what you actually have? And how , you actually, how you did, your parts replacement ? (This is absolute vital, for us to help you).
       
    • krazykuda

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

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      Carbon in the valves...
       
    • Woodys cudas

      Woodys cudas Well-Known Member

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      I think you tried all the obvious... take both valve covers off and look for a bent push rod and define which cylinder is the culprit. You could be doing valve or internal damage.
      For example... where is your wing nut for your air cleaner... it's happened. It or something could be stuck in the intake valve.
      Good luck...
       
    • krazykuda

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

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      Compression and leak down test....
       
    • mderoy340

      mderoy340 Well-Known Member

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      Do an ignition drop test at idle and figure out what cylinder or cylinders is the problem.
       
    • TrailBeast

      TrailBeast AKA Mopars4us on Youtube

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      This is probably the very first thing I would do, and then comp test that cylinder, and continue by pulling the valve cover for inspection.

      If it's not electrical, then it pretty much has to be a valve issue.
      It's hard for the average person to tell if it's an intake or exhaust valve but it could be either. (Could be a non opening exhaust valve or a burned/cracked intake valve)
      It's even harder to tell over the internet. (but it doesn't really matter which valve it is, does it?)
       
    • huljas666

      huljas666 Member

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      Hi All, Thanks for all the comments. So the things I have done to the engine: New timing chain (Sprocket is original), New timing cover, new fuel pump, new alternator, new carb, new distributor( tried a new one, still did it), spark plug wires had not been messed with before the problem started but are new, spark plugs are fairly new. I have checked all the easy stuff. I'll try and take a look at the valves this weekend. I just had hernia repair surgery so I'm kinda limited on what I can do right now. Thanks again for all the suggestions on what the problems might be.
       
    • RustyRatRod

      RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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      What have you actually done for diagnosis? This is four days old now with basically zero evidence that you've done any of the suggestions. How about actually doing some diagnosis and reporting back? If we are going to take the time to help you, you can at least keep us in the loop.
       
    • huljas666

      huljas666 Member

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      Well I don't know if you've read my last post, I've had surgery and can't really do too much. I had posted to get some ideas on what I can check.
       
    • RustyRatRod

      RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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      I sure wish you were local. I'd be glad to help. We gotta have a willing member in your area. I never can understand why more people won't speak up when it comes to help.
       
    • dano

      dano Evil Handy Man

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      What brand of wires. Most department store brands and cheap name brands suck.
       
    • RustyRatRod

      RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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      Tell you how you can instantly determine if it is an exhaust valve that's not opening. Remove the air cleaner. Start it up, drop it in drive with the parking brake set. Put your foot on the brake pedal good and tight. Ease into the gas and bring the RPM up to about 1500 loading the transmission in drive. If you get a steady rhythmic pop out of the carburetor that changes with engine RPM, you have an exhaust valve not opening and it is still firing on that cylinder, but blowing the exhaust back up through the intake and carburetor. At that point, you need to remove the valve covers and report back.
       
    • Captainkirk

      Captainkirk Old School Mopar Warrior

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      Check for bent push rods as was mentioned
      Check for major vacuum leak (extreme lean condition-carb or manifold)
      I'm leaning towards a stuck/bent valve or push rod.
       
    • huljas666

      huljas666 Member

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      Hi, all thanks for the great input. From all the things I'm hearing I'm leaning towards a stuck exhaust valve. Once I'm back to 100% and can get out there and check it out I'll report back and let you know what I find. Thanks!
       
    • Woodys cudas

      Woodys cudas Well-Known Member

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      Where are you located.. If your near me I would be glad to help a fellow member. I am in Johnstown,PA
       
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