This article is not intended as a do it yourself article but rather a guide to the modifications required to make 2 bbl carburetors competitive in oval track racing. Make no mistake; there are very few truly stock 2 barrel carburetors in oval track racing at any level.
Since the Holley 4412 and 7448 are the 2 most popular carburetors used today, this will be the primary focus here. They are rated at 500 and 350 cfm respectively at 3 inches of vacuum. The 350 carb is most prevalent in the South and the 500 more popular in the North. The same problems exist in the Rochester and Motorcraft carbs but they are addressed a bit differently.
Now is a good time to understand carb flow ratings. 2 barrel carbs are rated at 3 (40.80) inches of vacuum and 4 bbl carbs are rated at 1.5 (20.40) inches. A 650 carb on a 358 cubic inch engine will seldom see 1.5 inches but a 350 carb on the same engine will frequently see 5 to 8 inches. At 7 inches of vacuum the 350 carb is really flowing about 430-440 cfm.
The main characteristic of these carbs is the tendency to run excessively rich at high rpm. At high rpm fuel flows from every orifice in the carburetor whether it is supposed to or not. Thus when you jet for high rpm the idle and midrange are very lean. Most carb builders compensate for this by closing the power valve to lean out the mixture as the vacuum increases at higher rpm. Doing this with a stock carb can be dangerous as the power valve circuit flows a significant amount of fuel and losing all of that fuel when the valve closes will create a lean condition. This is where internal modifications come into play to reduce the amount of fuel supplied through the power valve so that when it closes there is still sufficient flow to maintain proper air to fuel ratio. The flow is increased through the main circuit.
Don’t think the modifications stop there. Since fuel flows through every orifice, changes are made to all circuits to maintain proper fuel control at all rpm.
Carburetor modifications are very engine specific thus carbs from 1 engine will not always work on another. The main difference is that the signal to the booster is greatly affected by the port velocity and camshaft selection. Cam selection is determined by rod length and the size of the track as well as driver preference. Sometimes the cam selection is based on what carb the builder was using when he tried that particular cam.
Air-fuel ratio should be greatest at maximum torque. This is where the engine is most efficient. Car should be geared to come out of the turn at 2-300 rpm above Max torque, NEVER below max torque. The fuel mixture should be a bit leaner at maximum horsepower. A carb that is excessively rich will have a narrower power band.
Overview of alterations done to Holley carburetors
Based on each application's requirements
(In no particular order)