Q: I have a 355-cubic-inch small block Chevy. The carburetor is a Holley 1850, 600 cfm with manual choke and vacuum secondaries. My problem is, the secondaries won’t open at any speed or rpm. When I hit the accelerator, I actually get a bog and have to let up. I can accelerate the car fairly well by controlling the depression rate of the gas pedal, but I really would like to use all four barrels.

I have also had several big backfires and difficult starts. Could the backfire have damaged the carb?

A: We don’t know what kind of intake you have, but we’ll assume it’s a dual-plane street-type manifold. Here’s what you need to do to get those secondaries to work: Open the secondaries by hand with the engine off. If they open easily, the problem is vacuum-related. Then remove the four screws from the vacuum diaphragm housing on the passenger side of the carburetor. Take the top off and see if the diaphragm is okay and the check ball is in place. If the diaphragm is damaged, replace it.

As for your bog, the problem is likely a lack of gas in the accelerator pump nozzle. The discharge nozzle may be too small, or the accelerator is not giving you a long enough fuel squirt. Try a “green” pump cam and a .031-inch discharge nozzle, available from Holley. They should make a big difference.

And yeah, those backfires have probably hurt something in the carb — most likely the power valve. If the valve is blown, replace it with a new one. Also, make sure you have the right size power valve. You can determine this in one of two ways:

  1. Make sure the valve opens when your engine vacuum drops 3 inches below normal cruise rpm vacuum.
  2. Check the vacuum reading when the car is in gear at idle. Divide that number by two and add .5 to get the proper size valve (this works on automatic transmission-equipped cars only).

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

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