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  • Engine backfire

    My 305 GM in the 78 Avanti II "backfires" intermittent at low rpm. May have a fouled plugged. What brand of Fuel additive should I try to first clean out the plug/ fuel lines/Carb ? I looked at STP/Prestone etc. Does any of these liquids work ?

  • #2
    If you have fouled spark plugs, you have to find the cause of the problem as no additive can clean a fouled plug. The problem could be as simple as the wrong heat range plug has been installed to bad wires, low compression, carburetor problems, plugged air filter etc. The late 70's Chevy engines also suffered from camshaft lobe failures and that can cause engine back firing. The best additive that I've found is Chevron Techron. it's expensive but it does work. Bud

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    • #3
      Yes Techron is great works fantastic in Motorcycles too. We swear by it.

      Mabel 1949 Champion
      Hawk 1957 Silverhawk
      Gus 1958 Transtar
      The Prez 1955 President State
      Blu 1957 Golden Hawk
      Daisy 1954 Commander Regal Coupe
      Fresno,Ca
      Mabel 1949 Champion
      Hawk 1957 Silverhawk
      Gus 1958 Transtar
      The Prez 1955 President State
      Blu 1957 Golden Hawk
      Daisy 1954 Regal Commander Starlight Coupe
      Fresno,Ca

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      • #4
        First....find out what caused the backfire.
        Then fix that.

        A bad spark plug will cause a "misfire" NOT a "backfire".

        IF...you have a or a bad set of plugs...just replace them with a new set. Much better way of doing things.

        The first place I'd look is into the distributor -
        1. bad (oily) points
        2. bad condensor
        3. loose point plate
        4. bad rotor
        5. bad dist. cap
        6. bad plug wire(s). Burnt, oily, OLD
        7. loose timing chain
        8. worn cam (timing) gear

        As for cleaners, I've had very good luck with Lucas, Fuel Injector Cleaner.
        Use it every oil change or long trip, both in my cars and motorcycles.

        Mike

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        • #5
          Is it a true "backfire"...flame from the carb? Or does it "pop" through the exhaust?



          Dick Steinkamp
          Bellingham, WA

          Dick Steinkamp
          Bellingham, WA

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          • #6
            May I suggest that this may be due to a vacuum leak? An easy way to test for vacuum leaks is to use a propane torch, NOT LIT, with gas valve slightly open, and play the gas jet around all intake surfaces, vacuum hoses etc. with engine running. You don't want to shoot the propane gas into air cleaner snorkel or carb throat or directly at distributor cap, and please wear safety goggles.

            If engine speeds up when gas is directed at a particular area you have found your vacuum leak.

            To test distributor vacuum advance remove cap, with engine not running of course. Apply a little bit of suction on end of hose disconnected at engine vacuum source [near carb] and watch to see if points advance plate moves. Also hold the suction for a moment to see if vacuum cannister loses vacuum, causing advance plate to drift back to static position.

            JimmieD

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            • #7
              It's a misfire thru the exhaust system I'm hearing.
              Been raining every day down here in South Fla. Can't get the car on the road as yet or check under the hood.

              Is ZMAX worth trying ?

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              • #8
                quote:Originally posted by geojerry

                It's a misfire thru the exhaust system I'm hearing.
                Been raining every day down here in South Fla. Can't get the car on the road as yet or check under the hood.

                Is ZMAX worth trying ?
                I haven't ever used that product so no opinion, sorry.

                There's another rarely ocurring possibility for backfiring through exhaust at idle only. There may be an exhaust leak very close to exhaust valve port on one cylinder, or maybe 2 but unlikely. That is, a cracked manifold or a void in gasket sealing area. The engine may be running rich in carburetion. The combination of the 2 can cause the over-rich exhaust vapors coming out of cylinder, laden with unburnt fuel, to re-ignite in the presence of fresh oxygen at leak area!

                It may not be that large of a leak so it's tricky to diagnose. What may work is using a piece of tubing or any type of flexible hose as a stethoscope, listening as you carefully move it around [hot!] exhaust manifold area. You'd hear that characteristic spitting sound of exhaust escaping. But don't hold the tube too close to your ear, as a backfire could be well beyond unpleasant, even deafening.

                It's also possible that a weak condenser, faulty points, a loose or corroded ignition electrical connection, bad ground [most likely] or even a failing coil could be the culprit. Here the electrical system is putting out the least total voltage at idle, so maybe not enough to consistently spark mixture, causing intermittent miss. But when reved up the increased voltage bridges the gap. A bad sparkplug lead wire or connector [second most likely cause] can do the same.

                The coil depends on electrical saturation, then a discharge across points gap to ground through condenser, to break the magnetic field & cause spark discharge through secondary coil windings & through plug wire to plug. Any weak link...

                .

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                • #9
                  quote:Originally posted by geojerry

                  It's a misfire thru the exhaust system I'm hearing.
                  So it stumbles every now and then like a cylinder didn't fire, then smooths out, then stumbles again? No "popping" through the exhaust?

                  Dick Steinkamp
                  Bellingham, WA

                  Dick Steinkamp
                  Bellingham, WA

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                  • #10
                    Remember that GM V8s of about that vintage had a "soft cam" issue. When the cam goes you get sort of a muffled backfire that sounds like popcorn being popped inside the air cleaner.

                    I'd get vacuum gauge on it, and look for low or erratic vacuum readings when the noise occurs.

                    Gord Richmond, within Weasel range of the Alberta Badlands
                    Gord Richmond, within Weasel range of the Alberta Badlands

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                    • #11
                      Drove the Avanti yesterday around the neihborhood to gas up with Shell 93 Octane. No misfires at any speed to and from. Wiring looked ok, idle ok, carb looks ok. Very strange. Same thing happened about 6 months ago, then it went away. Maybe getting water vapor from all the South Fla rain storms this past month.

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                      • #12
                        For what it's worth, my '56 Ford sometimes gets a pop or two through the exhaust on rapid deceleration from highway speeds. It's intermittent and unpredictable. There is a cracked ear on the driver's side exhaust manifold (yes, I'm going to replace it) and I suspect the cause is as JimmieD described above. Don't know if your exhaust manifold might have a crack somewhere, but the symptoms sound similar to my car's.

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                        • #13
                          I can bake potatoes on my exhaust manifolds, but everytime I try to pop popcorn in my air cleaner, my gas mileage goes way down..

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