Engine & Exhaust / Q&A / Tech

Mailbag: More Solutions for a Rich Air/Fuel Mixture

Got questions?

We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. Last week, we tackled a rich air/fuel condition. This week, we’re revisiting the popular subject and offering up a few more possible solutions as well as some general tuning tips.

J.R. • Summerland, CA

Q: I have a 1964 Nova that I built from scratch. I put in a 355-cubic-inch small block, with 9.75:1 TRW flat top pistons, 64cc heads, 280-degree duration/.486-lift COMP Cams camshaft, a Holley vacuum secondary carburetor on an Edelbrock Torker Intake, and an HEI distributor. The car also has a Turbo 350 transmission, and a 10-bolt rear-end with 3.73 gears and a posi.

The problem is, the engine is running too rich! I have rejetted the carb several times with no improvement. I have checked the primary power valve weekly, and it is good. I am now running a #65 jet in the secondaries, and the thing is still too rich. I gap the spark plugs at .050-inches. What gives?

A: The possibilities are almost endless when talking about a rich condition. Running on the rich side can be caused by a spark plug with too hot a heat range, carburetor float level set too high, and dozens of other things. I think you have a carburetor that is too large cfm-wise. Here is a rule of thumb on Holley carburetor sizing: “If you have to go more than four jet sizes up or down from the jets Holley installed at the factory to make it go rich or lean, then you need to go to a different size carburetor.”

In your case, you made all the adjustments you could make, and you’re still running too rich. That means you should go to a smaller cfm carburetor. If you shift gears manually, try a 650 cfm carb with mechanical secondaries. This should also yield a crisper, cleaner throttle response thanks to the higher air/fuel velocity.

Tags: , , ,

One Comment

Leave a Reply

Your email address will not be published.


This site uses Akismet to reduce spam. Learn how your comment data is processed.