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small backfire w/ secondaries

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Sparky, Apr 16, 2002.

  1. Sparky

    Sparky 1/2 ton status

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    Sometimes when i hit the gas quickly i get a small backfire when the secondaries open on my q-jet. Anyone know what typicaly causes this? I was thinking the air flap on the secondaries is either opening too fast or too slow. Any thoughts?

    Sparky
     
  2. 89Blazin

    89Blazin 1/2 ton status

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    Normally, backfires indicate a timing problem.
     
  3. k30gambi

    k30gambi 1/2 ton status

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    There is a little allen head screw on the back passenger side of teh carburetor. If the carb is still on the truck, teh screw will be upside down pointing up. Loosen it and turn teh shaft to tighten the little spring that it holds. You can see if your spring is tighter by pushing down the secondary "flap" My truck did the same thing and I made it tighter and it went away.
     
  4. Sparky

    Sparky 1/2 ton status

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    Yeah, I know how to adjust that spring already, i was just looking to confirm my thoughts. Thanks.

    Oh, I know its not a timeing problem. It has good power never pings and it only does it when i open the secondaries on the carb.

    Thanks again

    Sparky
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    How do you know the secondaries are opening anbd causing this problem, and that its not accelerator pump or timing that is the issue?
     
  6. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Make sure you don't have an ignition problem before you mess with the carb.
     
  7. m j

    m j 1/2 ton status

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    arent quadrabogs vacuum secondary with the big air door?
     
  8. joker89blazer

    joker89blazer 1/2 ton status

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    Could be to lean on fuel mixture. Acelerator pump on carb might be problem.On all the carbs I have seen you can turn the throttle manualy and see the fuel squirt down the carb barrel. If pump acelarator pump is not working engine will not get proper amount of fuel when throttle is increased.
     
  9. Sparky

    Sparky 1/2 ton status

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    Accelerator pump works fine. The only way it could be a timeing issue is if the mechainical advance springs or something in the distributor are jacked up. Its timed perfect at idle, and runs at all rpms just fine. It only does it when i can hear the secondaires start to open. My thoughts are that the vacume is pulling the air door open too quickly and cauing it to go lean for a second and then it backfires. Or, it could be that its going to rich and causing it to flood for a second, and then it backfires a little. The only way I can see it being the ignition is if the springs are sticking inside the dist. a little.


    Sparky
     
  10. dyeager535

    dyeager535 1 ton status Premium Member

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    It would be quite difficult for the secondaries to cause a backfire. Engine vacuum keeps them shut, and airflow keeps them open. Engine vacuum drops, air valve (and metering rods) will move, but only if air speed is enough to open them and keep them open. Soo...if the engine demand isn't high enough, they shut. If it is, they open.

    I've never experimented with the spring set extremely loose, but I'd expect the engine to bog if they were too loose...sure it would dump some extra fuel (but secondary fuel is drawn out via airflow as well IIRC) but engine vacuum would drop to almost nothing, and the fuel wouldn't even reach the cylinders. I *can* see that perhaps this could cause a backfire, but I don't think its real likely.

    If my understanding of backfiring through the carb is correct, a lean condition wouldn't be a likely cause of a backfire through the carb...you need something to burn if you are going to have a flame, excess fuel/sparking at wrong time would seem a likely culprit, not having too little fuel.

    My truck was timed nicely (didn't use a light) but no matter what I adjusted, I couldn't get rid of an off-idle bog/backfire if I floored it. Once I took the distributor cap/rotor off, I noticed that the weights were bound up (bushings were shot) and once I fixed that problem, idle became more consistent, and the bog/backfire was gone. Not hard to pull the cap/rotor to inspect, and its good preventive maintenance to check the cap/rotor/advance mechanism anyways.
     

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