Mailbag: Troubleshooting a Backfiring 350 Small Block in a ’73 El Camino

Q: I have a 1973 Chevy El Camino with a 350 engine, a TH-350 transmission, and a 10-bolt rear axle with 2.73 gears. All of the parts inside the engine, trans, and axle are stock. The car is a daily driver. I have made some modifications that I hope will not only increase horsepower, but maintain decent fuel mileage. Mods include:

  • Doug Thorley headers
  • Mallory Unilite distributor with vacuum advance
  • Mallory Promaster coil
  • Accel 8.8mm plug wires
  • Holley dual plane intake manifold
  • Holley 2D Pro-jection kit
  • B&M Shift Improver kit

After installing these parts, I ran into a problem. When I accelerate from a dead stop, the engine bogs to the point that it begins to backfire. Once moving, the engine pulls strong until 4,600 rpm when the power abruptly drops off and the engine begins to backfire sporadically. The plugs are clean and show no signs of running rich or lean. What’s wrong?

A: It sounds to us like you’re running out of fuel at higher rpm.
Check the fuel pressure both at idle and at wide open throttle. It should be between 7-9 psi all the way through the power band. Double check how many degrees of advance are in the ignition and adjust to stock specs if necessary. Also, make sure the distributor’s vacuum advance is connected to a ported/timed vacuum port and not a full-time vacuum port.
Another thing to look at is the placement of the electric fuel pump. Since the pump pushes fuel through instead of pulling it like a mechanical unit, you should make sure the pump is located as close to the fuel tank as possible.

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  1. Rob Snyder says:

    Why do you call for ported vacuum when manifold vacuum is typically called for?

    • Manifold vacuum is not called for. If you use manifold vacuum the advance would be all in at idle and decrease with throttle. Ported vacuum works in conjunction with the throttle plates and increases as the throttle does.

  2. Victor Villa says:

    Needs a time chain its got to lose that’s why running rich any go no power .

  3. The one in the picture is a 74 not a 73

  4. Check valve adjustment

  5. According to the Holley installation manual the fuel pressure regulator is factory set at 21 PSI. A SBC may not need this much pressure, but it certainly needs more 7 to 9 PSI. Holley suggests 13 PSI as a minimum. Also if the fuel filters are a directional type are they properly installed.

  6. I think someone overlooked the fact that Holley pro-jection unit is installed when they recommended 7-9 psi.

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