Got questions? 

We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re diagnosing carbon deposits on an exhaust system and resolving a rich air/fuel mixture.

My Buick Riviera has a 350 Buick engine, a 350 turbo/hydro-automatic transmission, and a Chevelle 12-bolt posi rear-end. I redid the upper end with some Clevite guts and new gaskets, and it runs like a champ.

My exhaust is showing heavy carbon on both tips. It’s not smoking or consuming any oil. The carb is a Rochester two-barrel with two mixture screws on the front. Do you think the carbon could be related to the fuel mixture, or maybe the grade of fuel?

1965 Buick Riviera, Dark Blue passenger side rear

A: The carbon deposits have nothing to do with the grade of fuel. The carbon is most likely the sign of a rich mixture at the carburetor. This could occur on a cold start when the choke is on, at idle with a rich idle mixture, or at part-throttle with jets that are too large. The two screws on your carb control the idle circuit and nothing else, so just set them to get your highest vacuum reading at your desired idle.

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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.