2 Barrel Carbs

The 2 barrel carburetor in Oval track racing

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 inches of vacuum and 4 bbl carbs are rated at 1.5 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 loosing 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.

Carb 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)

• Plug all unneeded vacuum ports permanently, along with throttle shaft end hole.

• Install/size screw-in air bleeds. (either hidden or in traditional locations)

• Install/size power valve channel restrictors (either press in or screw-in)

• Sometimes relocate Power valve channel restriction holes by plugging old and machining new, relocated ones. Install PV blowout protection where needed.

• Install either custom sized squirter or enlarged pump system and improve geometry of linkage. Select proper pump cam for application.

• Shafts get thinned, de-burred, straightened, and then re-plated for corrosion protection if legal. Special stainless screws used where legal.

• On alcohol carbs, the throttle bores are oversized approx. .010″ to help eliminate sticking butterflies,

• Whenever shafts are re-bushed, Teflon or bronze bushings are used. Many times, base plates are enlarged significantly and custom butterflies used. (But only when rules allow).

• The angle of taper is changed in the venturi and entire bores are polished. Casting line is removed on stock diameter venturi as well as on custom sized ones if rules permit.

• Boosters are installed with professional tooling, tightened, checked, and aligned for proper booster signal. In many cases, custom sized and machined “stepped” boosters are used. (Where rules allow).

• All gasket surfaces are flat milled and then re-plated to correct casting flaws and promote better gasket seal.

• Based on specific needs, idle feed restrictors installed either in main body or in metering block. On standard body carbs (where rules allow) the choke horn is milled off and entry area of carb is rounded, de-burred and new vent tube installed. On HP bodies, all small casting imperfections are removed.

• Choke rod hole permanently blocked (not epoxy’d shut) to protect from unfiltered air/dirt entering.

• Main wells are resized to application’s needs Emulsion channels enlarged and emulsion bleed system totally built from scratch. As many as 5-7 bleeds are installed. Many times removable and tunable screw-in bleeds installed. Proper jets, PV, and restrictors are installed based on each application. Stainless idle mixture screws installed using O-rings instead of cork seals. Vent baffles are installed

• In alcohol applications, completely custom metering blocks are built from scratch using bare unfinished blank castings.

• All machined surfaces re-plated for corrosion protection.

• Oval track floats are installed where applicable, or in alky carbs, new brass floats used. High flow Viton-tipped needle/seats installed, or large stainless steel alky needle/seat assemblies are used.

• All screw washers are made of re-usable Nylon instead of paper, which allow unlimited adjustments or removal and never leak

• Fuel inlet washers are Nylon as well. In alcohol applications, special pump diaphragms are used that withstand corrosion from methanol

• Float settings are done dry using a standard jig eliminating human error from “eyeballing”.

• After assembly with all top quality gaskets, carbs are flowed on a wet flow bench and proper final tuning of the “real life” fuel curve is mapped out and double-checked. While not necessary, occasionally “dry” flow bench work is done. This mainly yields only “advertised” cfm figures. Wet and dry flow numbers do not relate to each other. An increase in dry flow may well yield lower wet flow numbers. Many carbs go right to the dyno for testing as well.

• Finally we get caring for your carburetor. A positive throttle stop should always be used to prevent damage to the throttle shaft and bushings. Also multiple throttle return springs should be used for safety. Be sure they are in line with the linkage and at the proper angle to for maximum effectiveness. Finally, during extended storage, remove carb from the engine and dump all the fuel. Shoot a good amount of WD-40 or similar lube into the bowl through the vent tube and work the throttle a few times to get it in the accelerator pump passages. Spray a little on the outside of the carb and put it in a plastic bag. It is amazing what can get into a carb that is just left on the car.


Dayton Umholtz
Kinetic Performance (607) 277-6462
373 Van Dorn Rd. North
Ithaca, NY 14850

John Dickey
Advanced Engine Design
(804) 271-9107
2530 Willis Road
Richmond, VA 23237

Steve Cole
Barry Grant Inc (706) 864-8544
1450 McDonald Rd
Dahlonega, GA 30533