Mailbag: What Does Carbon on Exhaust Tips Mean?

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.

J.K. • Mountaintop, PA
Q:  My 1970 Buick Skylark 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?

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.