Got questions?

We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re talking about catalytic converters killing engine vacuum. 

L.R. Baltimore, MD

Q: I have a 1979 Dodge Magnum with a 360 V8. It has a two-barrel carburetor and the Lean Burn ignition system. The only modifications I made to the engine were an MSD 6A ignition, Jacobs Ultra Coil, and MSD Super 8.5 wires. I am using the stock ignition to trigger the MSD box because all of the sensors on the engine and the need to pass emissions.

The problem I’m having is that I cannot keep engine vacuum high enough to get good gas mileage. At idle in park, my vacuum gauge reads between 16 and 18 inches. I really have to get on the gas pedal to get the car to make any power; when the vacuum drops to between five and two inches, then the car will take off.

I have the Splitfire spark plugs gapped a .045 inches and timing set at 12 degrees BTDC per the sticker on the hood. Still, the engine idles rough and doesn’t run right.

A: We suggest you take a look at your catalytic converter and the evaporative charcoal canister that connects your Magnum’s vapor return line to the fuel tank. Problems with either one can cause the lack of power and the big drop in vacuum your are seeing. The converter could be plugged especially if it is the original. Try rapping on it gently with a mallet to see if you can hear anything rattling inside. If you can, it’s time to replace the converter. We would also check the carburetor to see if any of the charcoal canister’s contents are making their way into the carburetor.

Author: David Fuller

David Fuller is OnAllCylinders' managing editor. During his 20-year career in the auto industry, he has covered a variety of races, shows, and industry events and has authored articles for multiple magazines. He has also partnered with mainstream and trade publications on a wide range of editorial projects. In 2012, he helped establish OnAllCylinders, where he enjoys covering all facets of hot rodding and racing.