You Can Be a Carb-Tuning Hero

This is definitely cut-and-try type work. If you go too lean on theidle-feed restrictors, the engine will surge at cruise and hesitateunder light acceleration, since most engines don't like to accelerate atlean air/fuel ratios. If you tune the idle-feed restrictors too lean,the engine will most likely suffer from an off-idle lean stumble as anearly indicator that you've gone too far. It's the transition circuitsthat are more seriously challenged and the ones that will falter firstwhen you begin to lean out the idle and primary main circuits.

An ideal accelerator-pump shot is just enough to maximize acceleration.Additional fuel only kills power.

Tuning Challenge 1

Our first real-world example started out with a 9.0:1-compression 455ciOlds with a mild hydraulic cam, dual-plane Edelbrock, the stock 455Q-jet, headers, and an HEI ignition. The Innovate meter reported an idlemixture of around 12.0:1 that went lean at around 13.8:1 as the throttleopened up in mild acceleration. At steady-state highway cruise speeds,the air/ fuel ratio went back rich at around 12.5:1. At WOT, the enginewas actually a bit lean.

The first thing we did was to adjust WOT tuning by swapping the stock CHsecondary rods to thinner rods (eventually a pair of aluminum rods)until we had a WOT air/fuel that came in around 12.8:1. Because of thecam, intake, and header swap, part-throttle transition fuel was alsoslightly lean, so we changed to a slightly weaker power-valve spring topull the metering rods out of the primary jets sooner as load increased.The engine responded with a slightly richer mixture in lightacceleration of around 13.2:1, but we still had a slightly rich cruiseair/fuel ratio. When we tried larger (leaner 52B) primary metering rods,it hurt light-throttle acceleration. Some late-model Q-jets are set upto allow you to adjust the position or depth of the primary meteringrods in the jets. If our carb had been equipped with this feature, wecould have adjusted the metering rods deeper in the primary jets to leanout the part-throttle metering.

Tuning Challenge 2

We also tried a 350 Chevy equipped with a long-duration cam and a Holley750-cfm 0-3310 vacuum-secondary carb that suffered from a bad off-idlestumble and a pig-rich 10.5:1 ratio at part-throttle cruise. The WOTair/fuel ratio was only slightly rich at 12.2:1. The challenge was toimprove the driveability and mileage without sacrificing WOT power.After properly adjusting the primary accelerator-pump linkage, thestumble disappeared. We also replaced the large 0.035-inchaccelerator-pump nozzle to a 0.028 to further improve the throttleresponse.

Setting idle mixture will change when you bolt on the air cleaner. Ifpossible, set your idle-mixture ratios with the air cleaner in place.

The LM-1 meter told us the engine was way rich at just off idle, so wefirst tried leaning out the idle-mixture screws, but it didn't help.Next we disassembled the carb and found that a previous hacker haddrilled out the idle-feed restrictors to 0.052 inch. Stock 750 Holleycarbs come with idle-feed restriction of around 0.032 to 0.035 inch.Some quick math revealed that the 0.052-inch orifice increased the areaby more than 100 percent. As a temporary fix, we tried a0.020-inch-diameter wire stuffed into the idle-fuel jet to reduce theflow area. The larger wire reduced the area by roughly 15 percent, whichleaned out the part-throttle air/fuel ratio to roughly 11.8:1, which wasstill very rich. Further experimenting with reducing the main jets from72 to 69 and using a slightly larger 0.025-inch wire in the idle-feedjet finally got the part-throttle air/fuel ratio close to 12.8:1. Wecould not tune any leaner without creating an off-idle stumble andpart-throttle lean-surge condition because of the cam. The WOT air/fuelalso improved with the leaner primary jetting to 12.5:1. These simplechanges improved the fuel mileage by over 30 percent.

CC Tuning Tip

Poor ignition performance can be confusing when using the LM-1air/fuel-ratio meter. When a cylinder misfires, the air and fuel do notcombust, meaning the oxygen in the air moves into the exhaust where theO2 sensor will immediately pick up the additional air and indicate alean condition. Simply put, this means a dead cylinder or occasionalmisfire will show up on the Innovate meter as a lean condition, not arich mixture as you might think. Fix the misfiring cylinder(s) and theair/fuel ratio will read as a richer mixture.