Carburetor & Fuel System Tuning

Basic Carburetor and Fuel System Maintenance and Tuning Tips

Carburetor tuningThere is no substitute for a well-designed fuel and air delivery system. By ignoring these two critical areas, all the work of building a strong powerplant is wasted.

Air Delivery

For maximum horsepower, the coolest, most dense air possible should be available at the carburetor inlet. Keeping restriction in the inlet path to a minimum - or better yet, pressurizing the air - is also desirable.

The denser the air, the more you can get into the cylinders. This allows the engine to burn more fuel and make more power. We recommend that a hood scoop or outside air intake should be installed wherever rules allow. Under hood air is heated by the engine and headers and reduces the amount of power that can be produced. A reduction in temperature of 10 degrees F. is approximately equal to a one percent power gain.

There should be a minimum of three inches of clearance between the top of the venturis and a hood scoop. If an air cleaner is installed, the tallest possible element is preferred with four-inch element preferred for racing engines over 500 HP.

When a hood scoop or external air intake is used, it is highly suggested that the carburetor be sealed to it. Otherwise, air will flow across the top of the carb and out of the inlet tract rather than into the air horn. If air is forced past the carburetor it can siphon fuel, causing the engine to run lean. Windshield snorkels are especially notorious for siphoning unless the rear is sealed. Air pan kits for sealing the carburetor to the scoop are available or they can be fabricated. An air bell or radiused intake should be used whenever possible to increase air flow into the carburetor.

It is not unusual for a drag race car to improve ETs by 0.3 second and increase top speed by as much as seven miles an hour after installation of a sealed scoop. A car will not pick up ET after the scoop is sealed off if the scoop is too short or the fuel delivery system is inadequate.

On oval track cars, the same is true. Paying attention to the inle tract design will pay off. Depending upon track length, oval track cars will typically improve lap times by 0.1 to 0.5 second once an optimized air intake system is installed.

Fuel Delivery

Many racers experience fuel delivery problems without ever being aware that something is wrong in their race car's fuel systems. Today's state-of-the-art engines produce a lot more power than a race engine of ten years ago. The process of producing horsepower revolves around the conversion of fuel into energy. The more pounds of fuel an engine can burn efficiently per hour, the more horsepower it produces. Even though your car may not miss, pop, bang, skip or do anything else peculiar, it may not be getting all the fuel it needs to make maximum power.

In oval track applications, a Belt Drive or Hex Drive Fuel Pump is preferred where use of a mechanical fuel pump is specified. These pumps offer the highest fuel delivery volume of any mechanical pump yet maintains low fuel pressure at low engine speeds. This feature alleviates "loading up" of the spark plugs. The BG Six-valve and Super Speedway mechanical fuel pumps will also deliver ample fuel volume when used according to recommendations.

For drag race cars, an Electric Fuel Pump is the best way to guard against fuel starvation. If a car is lazy or lays down at mid-track then pulls well in a higher gear, the engine may be experiencing intermittent fuel starvation.

Why? Typically, the carburetor bowls are full at the starting line so the car leaves hard but in the process, drains the bowls dry. In the lower gears, the car accelerates rapidly with the engine picking up rpm very quickly. This rapid acceleration increases the demand for fuel. When the float bowl fuel level drops, the car lays down because of fuel starvation. In high gear, engine speed increases more slowly allowing the bowls to fill again.

AN fuel fittings and braided hose Fuel pressure regulators  

The Fuel Can Test

There's been no shortage of well-researched and well-written articles and books explaining the workings of the fuel system. However, many racers, both novice and experienced alike do not fully understand the physics of fuel flow and horsepower. To produce torque and horsepower requires a mixture of air and fuel. To produce 1-horsepower for 1-hour requires approximately .5-lbs of gasoline. If you ran a single-cylinder engine, like the one in your lawnmower, under a load of 1-horsepower for 1-hour and weighed the fuel tank before and after, the tank would weigh approximately .5-lb (five-tenths of a pound) lighter. Therefore the equation for fuel flow is 1-H.P. = .5-lb of fuel, per hour.

This is expressed on a dyno sheet as B.S.F.C. (Brake Specific Fuel Consumption). Highly-tuned racing engines can sometimes be more efficient, yielding B.S.F.C. figures of around #.40 which means 4-tenths of a lb. of fuel, per h.p., per hour. Incidentally, the formula for Alcohol is approximately 1-lb of fuel, per h.p. per hour which, as a consequence, necessitates the running of a belt-drive pump, but that's another story.

Typically, a 600-HP engine will require 300-lbs of gasoline per hour and, by the same formula, an 800-HP engine needs 400-lbs per hour. Remember, these quantities of fuel have to be delivered past the needles and seats and the fuel pressure regulator. Consider also, the fuel delivery system has to combat 'G' Forces: loadings that are so formidable they can threaten to stall the fuel in the line (this may also give a clue as to why a fuel line that is too large in diameter can be as harmful as one that is too small). This leads us to the area that is least understood.

When you have only one carburetor it should be easier to feed than two, right? Wrong, in an engine with a tunnel-ram layout, both the needle and seat area and the float bowl capacity have doubled! Whereas the single four-barrel car that is most prevalent today, has a much harder task in keeping the fuel bowls full! A 700-HP tunnel-ram engine needs 350-lbs of fuel per hour which equates to a little over 85-lbs per float bowl. A 700-HP engine running a single four-barrel (not so uncommon these days) needs 175-lbs per float bowl, compared to a 1200-HP Pro Stock engine with demands of 600-lbs max, 150-lbs per bowl.

So what happens if fuel delivery is weak? Your engine may miss or "burn-up" parts. It may just not perform to expectations. The new camshaft, racing-carburetor, or flowed-heads that didn't pick-you-up may have overstressed an already taxed fuel delivery system. Carburetors cannot disperse the optimum air/fuel mixture unless the fuel system has the ability to maintain correct float bowl levels. Fuel levels that are two low may not cause the motor to miss or "burn" a piston, but they will reduce fuel flow and performance will suffer. It is not uncommon after upgrading a fuel system with a single four-barrel carburetor to pick up 1- to 4-tenths of a second. In extreme cases, E.T.s have been known to decrease by as much as 1 second!

Can a fuel system that is too large hurt performance? No, it assures your combination will reach its full potential: the needles and seats will shut when the float bowls are full. Conversely, if your fuel system is marginal, fluctuations in battery voltage will cause fuel flow changes from run-to-run which affects the float levels in the carburetors and out-the-window goes your consistency! So how do you know if your volume is adequate?

Test your fuel system by obtaining a 1-gallon gas-can (do not use a moulded-plactic gas container, or marked super-jug, or antifreeze-jug as you will not get accurate readings). Open up the top of the tin-can and insert the two or four carburetor fuel lines from your regulator, switch on the system and carefully measure the time it takes to fill it. High 10-second cars will need to pump 1-gallon in 25-seconds or less. A 9-second car should fill the can in 20-seconds or less, 15-seconds is all it takes for an 8-second car and under 12-seconds for 7-second vehicles. Important Note: It's essential to observe two strict rules during the test. One, keep a fire extinguisher handy and two, do not carry-out the test by yourself.

How do you know you're getting all of the performance from your car? Perform the gas-can test even if your car is running well - you have nothing to lose and everything to gain: including the prospects of improving your E.T. and gaining increased consistency. When your car isn't performing, always carry out the gas-can test first - it's one of the least-expensive diagnostic aids you'll encounter. Keep in mind that valve springs, ignitions systems, torque converters, even engines have been changed, when all the time the fuel system was at fault.

Fuel Filters

For the same reason, only fuel filters specifically designed for racing, such as the BG5000 or BG Inline filter, should be installed. Use of a fuel filter is strongly advised as long as it doesn't restrict fuel flow. The fuel filter should be installed in the line before the fuel pump. This filters the fuel, preventing any damaging material from entering the fuel pump or the rest of the system

Controlling Fuel Pressure Settings

Fuel pressure should be set between 6 and 8 psi for a gasoline carburetor. An alcohol carburetor is a different animal with very different requirements. The Alky carburetor will require 4 to 5 psi at idle and 9 to 12 psi at wide open throttle. Remember, fuel pressure is not a substitute for volume! If the fuel bowls are not full, the pressure is meaningless. In fact, fuel pressure is simply an indication of the amount of restriction in the fuel system.

Regulators and Bypasses

Most electrical fuel pump systems require the use of a fuel pressure regulator. One regulator is sufficient in many applications. The use of two regulators is recommended in high horsepower engines to avoid excessive fuel restriction and provide adequate volume. With mechanical fuel pumps, and some electrical pumps, a bypass is preferred rather than a regulator. A diaphragm bypass without an idle bleed is recommended when constant fuel pressure is needed from an electrical or mechanical pump. A belt driven fuel pump, using gasoline or alcohol, requires a diaphragm bypass with an idle bleed. Higher pressure mechanical fuel pumps delivering alcohol, such as the 15-psi BG Six-valve, require a throttle bypass to supply the variable fuel pressure required by the carburetor.

Setting the Idle Mixture & Idle Speed

The engine should be at optimum operating temperatures. Lightly seat the idle mixture screws, then back them out 1-1/2 turns to establish a starting point. With the engine running, slowly turn the mixture screws in or out as needed to establish the best idle quality. Do this twice. The first time is a coarse adjustment; the second one is a final fine adjustment.

If idle quality can't be adjusted properly, or if the idle mixture screws can't be backed out far enough to obtain proper adjustment, it may be necessary to have the idle circuits reworked (the cam may not be pulling enough idle vacuum). A common reason for lack of adjustability in the idle mixture screws is having the primary butterflies adjusted too far open, which prevents the idle mixture screws from being capable of controlling the idle quality. When setting idle speed, the butterfly openings should be adjusted equally both primary and secondary side. However, the transfer slot should not have more 0.040" exposed (viewed with the carburetor removed from the engine and turned upside down). Secondary adjustment is set with a screw accessible from the underside of the carburetor.

Remember, the secondary throttles should be just slightly cracked open at idle. Even on carburetors with non-adjustable secondary idle mixture, secondary throttle position at idle may be adjusted. It may be necessary to open or close the throttle adjustment due to the cam design. It's a trial-and-error proposition to find the setting that gives you the best idle. However, here is a starting guideline. On engines that idle at or above 1000 rpm, start with the primary and secondary butterflies open the same amount. For engines that idle below 1000 rpm, begin with the primary butterflies open about 0.020" and the secondary butterflies positioned at the bottom of the transfer slot. From these points, begin to adjust the idle adjustment screws until the idle is steady and smooth.

Adjusting the Carb Jetting

Whether you have a Demon or a stock HOLLEY carburetor, jetting should be fairly close if the carburetor is used in the intended application. For maximum performance, increase or decrease jet size two numbers (primary and secondary) as required. As long as the performance continues to improve, continue increasing or decreasing jet size. At some point, ET or lap times will start to fall off, which means the mixture has been moved past the optimum air/fuel ratio. At that point, move one jet size at a time in the opposite direction until optimum performance is achieved.

Always jet for performance, not spark plug color. Most high-energy ignitions will leave very little residue on the plugs. With a drag car, plugs can remain bone white so attempting to read spark plugs is a waste of time. With an oval track car, the plugs will color, but the process takes longer with a high-energy ignition. If the car runs a little too hot, jetting up one or two sizes will alleviate the lean condition without hurting performance, because the ignition will burn off the extra fuel and cool the engine. However, if jet size is increased but the engine appears to be running leaner, a fuel system problem is indicated. If a conventional ignition is used, jetting for best performance is still the way to go, but the plugs will take on a tan color after a short time.


Generally an engine will pop, miss or surge if it's lean, although an excessively rich condition can cause the same problems. As a rule, cool, dense air requires larger jets, while hot thin air requires smaller jets. Also, whenever a carburetor spacer is added or removed, a camshaft, cylinder head or intake manifold change is made; it will be necessary to retune the carburetor to regain maximum performance.


Another problem that will cause a stumble or hesitation is when the secondary jets become uncovered, and no amount of accelerator pump tuning will cure it. This occurs when a car leaves so hard that G-force loading on the fuel in the secondary bowl causes it to be pushed up against the rear wall, uncovering the jets. BG Fuel Systems Stainless Steel Jet Extensions will correct this situation. Whenever jet extensions are installed in a carburetor equipped with a secondary power valve, the power valve must be removed and the hole plugged. Jet size must then be increased accordingly.

Pump Circuitry

Even when the jetting is within the ball park, this is no guarantee of optimal performance. Performance will be poor if the accelerator pump nozzle (squirter) diameter is incorrect. If a car is sluggish during initial acceleration and a puff of black smoke blows out of the headers when a drag car leaves the starting line, or when an oval track car comes off a corner, the accelerator pump nozzle diameter may be too large. Fuel spilling out of the vent tubes is another possible cause. The latter problem is easily cured by running a rubber hose from one vent tube to another, with a slot cut at the top of the hose. As with jetting, determining the best squirter diameter is accomplished by trial-and-error testing. Simply adjust size up or down for best performance.

Accelerator Pump Lever Adjustment

Do not forget about the accelerator pump lever adjustment. For oval track use, the pump lever should be adjusted so there is no play in the pump linkage when he throttle is closed. This will assure that there will be no lean stumble when the carburetor comes off idle. Tuning an accelerator pump for maximum performance off the corner often involves reducing, rather than increasing, the pump volume and discharge rate.

Drag racing calls for a slightly different approach. For the hardest starting line launch with a foot brake, the pump lever override spring should be adjusted so that fuel starts to discharge through the nozzle at an engine speed lower than launch RPM. If a car leaves the starting line at 5000 RPM, the pump shot should begin at 4700-4800; an 1800 RPM launch calls for the accelerator pump shot starting at 1500. The key is to have no slop in the accelerator pump system at starting line RPM, so that the pump shot is not "used up" below that RPM. Although adjusting the accelerator pump as described will create a lot of slop in the pump linkage at idle, (and may produce a stumble when driving in the pits) a car will leave harder. Drag cars equipped with a stick shift or transbrake, where starting line launch is accomplished with the carburetor wide open, requires adjusting like an oval track application.

Float Level Adjustment

When assembling a carburetor or reinstalling the fuel bowls, you need to adjust the float to be approximately 0.450" from the top of the bowl (in line with the bowl screw bosses when the bowl is upside down). This "dry" setting is to get you in the ball park. Recheck the float level with the engine running and the sight plugs out. Fuel should just wet the outside of the bowl as the fuel seeps out. On the secondary side, where the sight plug is lower, fuel level should be slightly higher. On Demon carburetors, the large, patented sight glass windows allow float level setting without having fuel escape from the float bowl. You can adjust the float level relative to the three cast-in marks next to the float window.

Power Valve Tuning

The power valves function is to supply extra fuel for wide- open throttle, high load conditions. When manifold vacuum falls below the vacuum level stamped on the power valve, the power valve opens and enriches the main circuit by about six to ten jet sizes. This occurs under high engine loads such as full throttle. The power valve number should be at least 1.5-2.0 in/hg of vacuum under engine idle vacuum. If an engine produces 8.0-9.0 in/hg of vacuum, a 6.5 power valve would be a good first selection. Using a power valve with a rating lower than this will delay the enrichment and can cause hesitation. However, on an oval track car with restricted carburetor rules, using a power valve with a lower rating can sometimes help performance coming off corners. Any drag race carburetor with a secondary power valve must be turned sideways to avoid fuel starvation. The power valve is higher than the jets and is the first to be uncovered as fuel is pushed to the rear of the float bowl. Since there is no way to put an extension on the power valve, the carburetor must be turned sideways to eliminate fuel starvation.

Throttle Plates and Linkage

Maximum performance cannot be achieved unless the carburetor is wide open when the throttle is pushed to the floor. Consequently, throttle opening should be checked regularly, immediately following any major changes in the engine compartment. Maximum airflow cannot be achieved if the throttle plates do not reach the wide-open position, or if it is pulled past wide open. The throttle linkage should operate smoothly without binding and should be free of obstructions that may cause the carburetor to stick open. An auxiliary return spring MUST always be installed to insure that the throttle positively closes. It may be necessary to install a stop to prevent the carburetor from being opened too far and to prevent damage to the linkage. Take your time and set the linkage up right! Tuning linkage on sideways mounted tunnel ram carbs will take longer than a single 4 barrel.

Climate Changes and Performance

Changes in air density due to changes in temperature, barometric pressure, and humidity will have a direct impact on engine performance. As the air density changes, adjustments to the fuel mixture are often necessary. These factors can change from afternoon to evening. Everyone knows the engine will make more power at night when the air cools. The cooler, denser air carries more oxygen per cubic foot and thus support the capacity to burn more fuel. The weather conditions may not change enough from afternoon to evening to require mixture adjustments, but adjustments will certainly be necessary as the seasons change. Traveling to tracks in different climates or at different altitudes may also cause a need for fuel mixture adjustments. These factors cannot be overlooked when tuning for best performance.

Tuning With Spacers

Spacers and plenum dividers provide an easy way to change the configuration and the characteristics of the intake tract and the relationship it has with the carburetor. Adding a plenum divider to an open plenum manifold can help keep the left to right fuel distribution balanced for oval track applications This is especially helpful on alcohol engines.

Using spacers between the carburetor and intake manifold can produce dramatic results. The use of a four-hole spacer can improve low end to mid-range by helping the carburetor draw and atomize fuel. An open center spacer increases the plenum area and can benefit the mid-range and upper rpm power. It is not uncommon to see combinations of spacer types or stacking of similar type spacers being used.

The actual results from any spacer or combination of spacers can only be measured during a test and tune session on the specific engine combination being run. This information can be a very useful tool when tuning to find the best horsepower or to change the power characteristics to suit a specific track condition.

Routine Maintenance

For consistent performance, a carburetor must be kept clean. Spray the air bleeds with carburetor spray or WD-40 every week. Air bleeds become clogged from dirt and dyes in the fuel. Clogged or dirty air bleeds can cause a stumble or a high- speed miss in an otherwise perfect carburetor. If you are putting the car up for the winter, use this little trick. Spin the engine over with the ignition off and the throttle open and spray of WD-40 liberally down the venturi will leave a fine mist of protection on the valve seats and cylinders to prevent rust. Finally, your carburetor should be rebuilt at least once a year and more frequently if it's operated in dirty conditions.

Alcohol Tips

Applications running alcohol require additional maintenance beyond that of a gasoline fuel system. Alcohol, being extremely corrosive, should not be left in the fuel system or carburetor for an extended period of time. Proper care includes draining and flushing the entire fuel system, usually with gasoline. The most common method is to drain the system and add gasoline to the fuel cell allowing the pump to draw fuel through the lines and to the carburetor. Alcohol carburetors are much richer than gasoline, so when the engine begins to idle and die, the system is pretty well flushed. If your sanctioning body allows the use of fuel additives, always use an additive that lubricates the fuel system for protection while racing.

Article courtesy of Barry Grant

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