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Technical Backfiring issue

Discussion in 'The Hokey Ass Message Board' started by Tex Flick, Nov 21, 2017.

  1. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    Good evening everyone!

    I am having a backfiring issue with my '59 GMC pickup with a '72 Chevy 350 in it. It's always started and ran good, but after I put a brand new Holley 4412 2 barrel carb on it it wants to start backfiring after it warms up. It starts by just making some faint popping noises from the exhaust; then if I rev the engine or go to a higher rpm than idle it makes a really loud popping noise from the exhaust. Could this be the engine running to rich and its not done burning the fuel on the exhaust stroke, or could the timing be off since I put this new carb on it. It had a 4412 on it before, so I would think it would be the same. I could be wrong. Any suggestions or tips would be appreciated!

    Thanks,
    Tex
     
  2. 18436572. Check your 5 7
     
  3. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    Yes, firing order is correct. That was my first initial thought.
     
  4. All the same ignition as before tare down? Headers? If yes, bad plug and fuel collecting then firing off?
     
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  5. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,465

    greybeard360
    Member

    Look at the jets in the old one and look up the jet size the new one is built with. See if those are the same. Also, make sure you don't have a vacuum leak.

    Sent from my Moto G Play using The H.A.M.B. mobile app
     
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  6. tractorguy
    Joined: Jan 5, 2008
    Posts: 640

    tractorguy
    Member

    Vacuum leak especially at carb base to manifold. Also is the carb base gasket correct ? Any other vacuum lines or hoses that would have been disturbed in the replacement. Also is carb base tight where it bolts/screws to main body\bowl of the carb.

    Good luck
     
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  7. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    I'm not the one who tore it down. My grandpa did 2 years ago he died and never got the new carb put on it. But yes ignition is the same as the old carburetor.
     
  8. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    Shouldn't be any vacuum leaks. I double checked all that. All ports are plugged that are supposed to be plugged and the base gasket it correct (I compared it to the old one). It does seat tight on the intake.
     
  9. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    Can having jets bigger or smaller jets than the old carb make it backfire like it does? (I'm not questioning your knowledge, I'm genuinely curious.)
     
  10. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,465

    greybeard360
    Member

    Too rich or too lean can cause back firing... Usually lean more than rich.

    Sent from my Moto G Play using The H.A.M.B. mobile app
     
  11. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    So if I stick my vacuum gauge on the port where the vacuum advance goes and re-adjust the mixture screws it might fix it?
     
  12. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,465

    greybeard360
    Member

    Mixture screws will only effect idle. Check the jets and start there.

    Sent from my Moto G Play using The H.A.M.B. mobile app
     
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  13. southcross2631
    Joined: Jan 20, 2013
    Posts: 3,753

    southcross2631
    Member

    check your ignition for a weak condenser .
     
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  14. Is the choke adjusted properly?
     
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  15. What else was torn down and put back by your grandpa BEFORE you got it.
     
  16. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    He said he overhauled it and converted it to electronic ignition. I replaced the ignition module when I inherited it and first got it running because it wasn't getting spark.
     
  17. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    Please excuse my ignorance, I'm no mechanic, but could someone explain how bigger/smaller jets make a difference on an engine? I know what they do, how do different sizes make a difference? (Not being a smartass, just wanting to know)
     
  18. The jets control the maximum amount of fuel. The engine moves the air that it moves and you match the jet to stay in the optimal air fuel ratio.
     
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  19. onetrickpony
    Joined: Sep 21, 2010
    Posts: 427

    onetrickpony
    Member
    from Texas

    Broken valve spring or flat lobe.
     
  20. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    I don't think it's that since he used this as a daily driver for a while and I heard it the last time it ran. it ran really good, no clicks, knocks, or popping to be heard
     
  21. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    It all started after I put this carburetor on.

    Could it be that the engine was not yet all the way warmed up yet? It was like 50 degrees when I started it. I only let it run about 5 or 6 minutes when I did this because it was getting dark out and I'm in a shed with no electricity. But usually when it's cold engines backfire up through the carb and not through the exhaust, right?
     
  22. There you go. Start there because that's the only change aside from tuning. Which is always done at full running temperature.
     
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  23. Tex Flick
    Joined: Sep 30, 2017
    Posts: 29

    Tex Flick

    Thank you all for the replies, I really appreciate it! I'm learning step by step haha.
     
  24. Was there an intake manifold change?

    Quicky guessing help and hints.
    If it's a lean miss, you can close the choke and it should go away. If it's a rich misfire giving it a vacuum leak will make it stop. the plugs would be black as well. Exhaust leaks close to the engine will cause popping in the exhaust.
    Also remember that most carb issues are electronic in nature meaning ignition problems.

    Double triple check the firing order.
     
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  25. R Pope
    Joined: Jan 23, 2006
    Posts: 3,309

    R Pope
    Member

    Put the old carb back on it. If it runs good then, you know where to look.
     
    jeffd1988 likes this.
  26. R Pope
    Joined: Jan 23, 2006
    Posts: 3,309

    R Pope
    Member

    Did it sit for 2 years after Grampa tore it down? And why did he want to change carbs? Was it backfiring then too? If so. I go with the cam lobe wiped idea. It is a small block Chevy, after all.......
     
  27. justpassinthru
    Joined: Jul 23, 2010
    Posts: 260

    justpassinthru
    Member

    Generally, backfiring through the carb is a lean condition or retarded timing. Backfiring through the exhaust is usually ignition issues. I would pull the plugs first and see if there is a problem there, check cap and wires also.
    Bill
     
  28. justpassinthru
    Joined: Jul 23, 2010
    Posts: 260

    justpassinthru
    Member

    Does it have 2 or more year old gas in it? That can cause misfiring also.
    Bill
     

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