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Thread: Exhaust popping under hard acceleration

  1. #1
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    Exhaust popping under hard acceleration

    Hey guys,
    I’m hearing popping/backfire only under hard acceleration, usually starts after 3000 rpm, and only hearing it from the passenger side pipe. At each pop I can feel an extremely brief loss of power, similar to a misfire. Could this loss of power be due to expansion upon detonation in the exhaust which is causing exhaust valves to open during power stroke? I don’t believe it’s missing at low rpm, as it actually runs very smoothly at idle and lower speeds. Have driven the car since new, now has 8,000 miles and first time I’ve experienced this. Please chime in.

    Some more info on the car:
    427W
    Holley 750 carb 4 barrel
    Champion spark plugs
    MSD distributor
    3/8” fuel line, mechanical pump
    Running 91 pump gas with octane booster

    Thanks guys

  2. #2
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    Quote Originally Posted by Riflebolt View Post

    Could this loss of power be due to expansion upon detonation in the exhaust which is causing exhaust valves to open during power stroke?
    No.


    "Popping" on acceleration *usually* indicates "lean", and if it's only one side, and "new"....


    I'd take the primary metering block out of it, hose it down with carb cleaner, blow it out "thoroughly" with compressed air, and see if that changes anything or cures it.


    If not, proceed to primary + secondary metering blocks, and carb body with the same process described above.


    Good luck - proceed with caution.

  3. #3
    Senior Member brewha's Avatar
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    I agree it could be the carb but I would pull the plugs on that passenger side to see if its running lean or if its one plug misfiring. I seen cracked plugs run fine at idle but misfire under load. Spark Wires I assume are not frayed and in good shape.
    If plugs and the carb are good, Id check the coil next. If that was good....distributor issues, valve or push rod issues...I think some more info the engine would be helpful.
    Good luck
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    Senior Member brewha's Avatar
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    Orrrrrr check you header on the passenger side and make sure it hasnt loosened up and is letting air into the exhaust system.....
    Mark4 - 331 Stroker - Fitech 600 -TKO600 - Moser 3.55

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    Senior Member wareaglescott's Avatar
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    Quote Originally Posted by brewha View Post
    Orrrrrr check you header on the passenger side and make sure it hasn’t loosened up and is letting air into the exhaust system.....
    I was thinking possible exhaust leak also since only coming from one side.
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    Not a waxer Jeff Kleiner's Avatar
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    Quote Originally Posted by brewha View Post
    Orrrrrr check you header on the passenger side and make sure it hasn’t loosened up and is letting air into the exhaust system.....
    Quote Originally Posted by wareaglescott View Post
    I was thinking possible exhaust leak also since only coming from one side.
    Popping due to an exhaust leak will only occur upon deceleration, not under load. Since the OP reports being able to feel it as well as hear it I'm also thinking a lean misfire and agree to pull the plugs and see what they're showing.

    Jeff

  7. #7
    Seasoned Citizen NAZ's Avatar
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    I've seen this before, here's something to consider:

    My guess is you have one or more cylinders on the right bank that are misfiring that creates a loss of power you can feel in your butt dyno. The popping noise is an air/fuel charge (misfire = unburned fuel in exhaust) ignited in the collector. Note that two things that place a heavier load on an ignition system is cylinder pressure and RPM. Cylinder pressure increases as you open the throttle and RPM follows so it is more likely to see a misfire from a compromised ignition system when the engine is under load at WOT and higher RPM.

    A misfire can be caused by many things so you need to take a systematic approach to troubleshooting.
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    8000 miles, new problem, only one side so maybe only 1 cylinder... I'd pull the plugs. Plug breakdown or gaps that are too big will breakdown under higher rpm and higher cylinder pressures. When I used to test aviation plugs, it was amazing how little of an increased gap would create a complete shutdown of the plug at higher rpm / cylinder pressures.
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    Wow thanks very much for the replies guys. Im going to start with the easy stuff as recommended and will report back.

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    Senior Member Avalanche325's Avatar
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    Definitely read your plugs. Pay attention when you pull the wires off, one may not be on tight. Also make sure you don't have one that got burnt. Then the other things that were mentioned.

    Here is one that got me. I started getting misfires over 5000rpm with about 4000 miles on the car. It felt like the rev limiter was kicking in. The distributor cap contacts were badly corroded. I live at the beach, but it looked more like I live in the ocean.

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  14. #11
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    Shoot the headers with a temperature gun. Sounds like a plug wire/spark plug.

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    Hey guys,

    Today I pulled the spark plugs and found each of them pretty badly fouled. Im going to replace all of them and see if this does it. Be back soon...

    Also, this is a Champion RC12YC plug. I tried doing some research and found 20 ft lbs to be the torque. Sound right? I was also going to use anti seize as it was clearly put on by the engine builder.
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    Seasoned Citizen NAZ's Avatar
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    If you're going to use anti-seize be careful not to get it on the ground strap or electrode portion of the plug. Also, be advised that torque specs can change significantly when you lube the threads and if you use a dry torque value on lubed threads you can easily tighten the plug too much and damage the plug or heads or both. I'm a firm believer in apply a torque wrench to critical fasteners or in some cases, measure actual stretch. However, I never torque spark plugs. When installing new plugs tighten them until the crush washer is flattened and just a tad more to snug them down. I've never had one blow out.
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    Quote Originally Posted by NAZ View Post
    If you're going to use anti-seize be careful not to get it on the ground strap or electrode portion of the plug. Also, be advised that torque specs can change significantly when you lube the threads and if you use a dry torque value on lubed threads you can easily tighten the plug too much and damage the plug or heads or both. I'm a firm believer in apply a torque wrench to critical fasteners or in some cases, measure actual stretch. However, I never torque spark plugs. When installing new plugs tighten them until the crush washer is flattened and just a tad more to snug them down. I've never had one blow out.
    I appreciate the tip. Thank you

  18. #15
    Senior Member CraigS's Avatar
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    I agree. As you tighten the plug you can feel when it starts to crush the washer. That crushing action takes a certain amount of increased effort which stays the same for maybe one to 1.5 turns. At the point where the washer is fully crushed, the effort to keep turning suddenly increases. At that point stop turning, you are done.
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    Thank you Craig and thanks to everyone for the replies.

    Spark plug it is. Replaced all 8 and took her out today for a heated lap. Not a single pop. Problem resolved. Thanks guys

  20. #17

    Steve >> aka: GoDadGo
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    Quote Originally Posted by Riflebolt View Post
    Thank you Craig and thanks to everyone for the replies.

    Spark plug it is. Replaced all 8 and took her out today for a heated lap. Not a single pop. Problem resolved. Thanks guys
    I'll be changing my plugs today as well after experiencing some popping yesterday.
    Thanks for posting this thread because it is appreciated.

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