Mailbag: Finding Possible Causes (and Cures) for Engine Bog

Q: I have a 1970s-era 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, transmission, 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 Avenger EFI system
• B&M Shift Improver Kit

After I installed 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: This is most likely a fuel delivery or tuning problem.

It sounds to us like you’re simply running out of fuel at high rpm. Check the fuel pressure at both idle and under wide open throttle. It should be between 7-9 psi all the way through the powerband for your application. On a carbureted engine, you’d also want to look at things like floats, secondary springs, and throttle cable settings to ensure the carburetor’s secondaries are opening up fully. However, since you’re running an EFI setup, we’ll turn our attention to the ignition tuning instead. 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 port is connected to 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, make sure the pump is located as close to the fuel tank as possible.

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This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.