Sign up now to join the JEGS email newsletter and be the first to learn about new products, special deals and e-mail only offers!
A well-designed fuel and air delivery system is indispensable for optimizing the performance of any powerplant, whether using a self tuning fuel injection system or a carburetor. Neglecting these crucial areas can render all the efforts put into building a robust engine futile. In this article, we will explore essential tips for carburetor maintenance as well as carburetor tuning and fuel system care, focusing on air delivery, fuel delivery, and various aspects of carburetor adjustment. By understanding the significance of these components and implementing the recommended practices, you can unleash the full potential of your engine, ensuring efficient fuel combustion and maximizing horsepower output. So, let's delve into the world of carburetors, fuel systems, and the intricate art of fine-tuning for peak performance.
Air Delivery
To maximize horsepower, it is crucial to ensure that the carburetor receives the coolest and most dense air possible. Minimizing restrictions in the inlet path, or even pressurizing the air, is highly desirable. The denser the air, the greater the amount that can be drawn into the cylinders, enabling the engine to burn more fuel and generate increased power. To achieve this, we recommend installing a hood scoop or an external air intake, wherever regulations permit. Air drawn from under the hood tends to be heated by the engine and headers, resulting in a reduction in power output. Even a 10-degree Fahrenheit decrease in temperature can translate to a roughly one percent gain in power.
Maintaining a minimum clearance of three inches between the top of the venturis and the hood scoop is essential. For racing engines producing over 500 HP, it is preferable to use the tallest possible air cleaner element, with a four-inch element being the preferred choice. When employing a hood scoop or external air intake, it is highly advisable to seal the carburetor to it. This prevents air from flowing over the top of the carburetor and bypassing the inlet tract, which can lead to fuel siphoning and result in the engine running lean. Windshield snorkels are particularly prone to siphoning if not properly sealed. Air pan kits designed to seal the carburetor to the scoop are available, or they can be custom fabricated. Whenever feasible, it is recommended to use an air bell or radiused intake to enhance airflow into the carburetor.
The benefits of installing a sealed scoop are notable in drag race cars, with potential improvements of 0.3 seconds in elapsed times (ETs) and a top speed increase of up to seven miles per hour. However, it should be noted that if the scoop is too short or the fuel delivery system is inadequate, sealing off the scoop may not yield improved ETs. Oval track cars similarly benefit from careful consideration of the inlet tract design. Depending on the track length, optimizing the air intake system can typically lead to lap time improvements ranging from 0.1 to 0.5 seconds.
Fuel Delivery
Many racers unknowingly encounter fuel delivery issues in their race cars, even if the vehicles do not exhibit any noticeable problems such as misfires or unusual noises. Modern state-of-the-art engines have significantly higher power output compared to those of the past decade. The production of horsepower relies on the efficient conversion of fuel into energy. The more pounds of fuel an engine can effectively burn per hour, the greater horsepower it can generate. Therefore, even if your car appears to be running smoothly, it may not be receiving the necessary amount of fuel to achieve maximum power.
In oval track applications where a mechanical fuel pump is specified, it is preferable to use a Belt Drive or Hex Drive Fuel Pump. These pumps offer superior fuel delivery volume compared to other mechanical pumps, while maintaining low fuel pressure at low engine speeds. This characteristic helps prevent spark plug fouling caused by excessive fuel.
For drag race cars, employing an Electric Fuel Pump is the most effective solution to prevent fuel starvation. If a car feels sluggish or loses power mid-track but then regains performance in a higher gear, it is likely experiencing intermittent fuel starvation. This occurs because the carburetor bowls are initially filled at the starting line, allowing the car to launch forcefully. However, as the car accelerates rapidly in the lower gears, the engine quickly reaches higher RPM, intensifying its demand for fuel. Consequently, the fuel level in the float bowls drops, leading to fuel starvation and a subsequent loss of power. In high gear, where engine speed increases more gradually, the bowls have time to refill, restoring proper fuel supply.
The Fuel Can Test
Numerous articles and books have been written to explain the intricacies of fuel systems, yet many racers, both inexperienced and experienced, lack a comprehensive understanding of fuel flow and its relationship to horsepower. The generation of torque and horsepower necessitates the precise combination of air and fuel. To produce one horsepower for one hour, approximately 0.5 pounds of gasoline is required. A simple experiment with a single-cylinder engine, such as a lawnmower, under a one-horsepower load for an hour would reveal a fuel tank that is approximately 0.5 pounds lighter. This equation translates to fuel flow, expressed as Brake Specific Fuel Consumption (B.S.F.C.) on a dyno sheet. Highly-tuned racing engines can achieve impressive B.S.F.C. figures of around 0.40, meaning 0.4 pounds of fuel per horsepower per hour. It's worth noting that alcohol fuel has a different formula, requiring approximately one pound of fuel per horsepower per hour, which often necessitates the use of a belt-drive pump.
Consider the fuel demands of an engine: a 600-horsepower engine requires 300 pounds of gasoline per hour, while an 800-horsepower engine needs 400 pounds per hour, according to the same formula. It's important to remember that these quantities of fuel need to pass through the needles, seats, and fuel pressure regulator. Additionally, the fuel delivery system must contend with formidable 'G' forces, which can potentially stall fuel flow in the line. This insight also explains why both oversized and undersized fuel lines can be detrimental, as they can hinder proper fuel delivery. This brings us to an area that is often poorly understood.
One might assume that a single carburetor is easier to feed than two, but that's not the case. In an engine with a tunnel-ram layout, both the needle and seat area and the float bowl capacity are doubled. Conversely, a single four-barrel carburetor, which is more prevalent today, faces a more challenging task in maintaining proper fuel levels in the bowls. For instance, a 700-horsepower tunnel-ram engine requires 350 pounds of fuel per hour, which is slightly over 85 pounds per float bowl. On the other hand, a 700-horsepower engine running a single four-barrel carburetor needs 175 pounds per float bowl. A 1200-horsepower Pro Stock engine, with its maximum demand of 600 pounds of fuel, requires 150 pounds per bowl.
What happens when fuel delivery is inadequate? The engine may experience misfires or damage its components, and overall performance may fall short of expectations. Upgrading components such as camshafts, racing carburetors, or cylinder heads, without considering the strain on the fuel delivery system, can further exacerbate these issues. Carburetors cannot achieve the ideal air/fuel mixture unless the fuel system can maintain the correct float bowl levels. Insufficient fuel levels may not cause immediate misfires or piston damage, but they restrict fuel flow and lead to reduced performance. It is not uncommon to see improvements of 0.1 to 0.4 seconds in elapsed times (E.T.) after upgrading the fuel system with a single four-barrel carburetor. In extreme cases, E.T. reductions of up to one second have been observed.
Can a fuel system that is too large harm performance? No, it ensures that your setup can reach its full potential as the needles and seats shut when the float bowls are full. Conversely, a marginal fuel system can be affected by fluctuations in battery voltage, leading to inconsistent fuel flow and compromised float levels in the carburetors, which hampers consistency. So, how can you determine if your fuel volume is adequate?
You can test your fuel system by using a one-gallon gas can (avoid using molded plastic gas containers, marked super-jugs, or antifreeze jugs as they may provide inaccurate readings). Open the top of the tin can and connect the two or four carburetor fuel lines from your regulator. Switch on the system and carefully measure the time it takes to fill the can. A high 10-second car should be able to pump one gallon in 25 seconds or less. A 9-second car should achieve this in 20 seconds or less, while an 8-second car can accomplish it in 15 seconds or less. For vehicles running in the 7-second range, filling the can in under 12 seconds is the goal. It is crucial to observe two strict rules during the test: keep a fire extinguisher nearby and perform the test with someone else present. How can you ensure you are maximizing your car's performance? Conduct the gas can test even if your car is running well - there is nothing to lose and everything to gain, including improved E.T. and increased consistency. Whenever your car's performance is lacking, always start with the gas can test as it is one of the most cost-effective diagnostic tools available. Remember that valve springs, ignition systems, torque converters, and even engines have been replaced unnecessarily, when the real culprit was a faulty fuel system.
Fuel Filters
To ensure optimal performance, it is crucial to use racing-specific fuel filters. Installing a fuel filter is highly recommended as long as it does not impede fuel flow. It is best to place the fuel filter in the line before the fuel pump. This arrangement effectively filters the fuel, safeguarding the fuel pump and the entire system from any harmful contaminants.
Controlling Fuel Pressure Settings
For a gasoline carburetor, it is recommended to set the fuel pressure within the range of 6 to 8 psi. However, an alcohol carburetor operates under different circumstances and has distinct requirements. An Alky carburetor typically needs fuel pressure of 4 to 5 psi at idle and 9 to 12 psi at wide open throttle. It is essential to remember that fuel pressure cannot replace fuel volume. If the fuel bowls are not adequately filled, the pressure becomes insignificant. In reality, fuel pressure merely serves as an indication of the level of restriction within the fuel system.
Regulators and Bypasses
In most electrical fuel pump systems, it is necessary to incorporate a fuel pressure regulator. For many applications, a single regulator is sufficient. However, in high horsepower engines, it is advisable to use two regulators to prevent excessive fuel restriction and ensure adequate fuel volume. In the case of mechanical fuel pumps and certain electrical pumps, employing a bypass system is preferable over a regulator. When consistent fuel pressure is required from an electrical or mechanical pump, it is recommended to use a diaphragm bypass without an idle bleed. On the other hand, a belt-driven fuel pump, whether using gasoline or alcohol, necessitates a diaphragm bypass with an idle bleed. In situations where higher pressure mechanical fuel pumps are employed, a throttle bypass is needed to supply the variable fuel pressure required by the carburetor.
Setting the Idle Mixture & Idle Speed
Ensure that the engine reaches its optimal operating temperatures. Gently close the idle mixture screws, then unscrew them by 1-1/2 turns as a starting point. While the engine is running, gradually adjust the mixture screws inward or outward to achieve the best idle quality. Repeat this process twice, with the first adjustment being a coarse tuning and the second adjustment serving as a final fine-tuning.
If it proves challenging to properly adjust the idle quality or if the idle mixture screws cannot be unscrewed sufficiently for appropriate adjustment, it may be necessary to have the idle circuits modified, as the cam may not generate enough idle vacuum. One common reason for limited adjustability in the idle mixture screws is excessive opening of the primary butterflies, which hampers the control of idle quality through the mixture screws. When setting the idle speed, ensure that the primary and secondary butterfly openings are adjusted equally. However, the transfer slot should have no more than 0.040" exposed when the carburetor is removed and turned upside down. Secondary adjustment is performed using a screw accessible from the underside of the carburetor. Remember that the secondary throttles should only be slightly cracked open at idle. Even on carburetors without adjustable secondary idle mixture, the position of the secondary throttle at idle can be adjusted. Depending on the cam design, it may be necessary to modify the throttle adjustment by opening or closing it. Finding the optimal setting for idle requires some trial and error. However, here is a suggested starting point: For engines idling at or above 1000 rpm, begin with the primary and secondary butterflies open to the same degree. For engines idling below 1000 rpm, start with the primary butterflies open approximately 0.020" and position the secondary butterflies at the bottom of the transfer slot. From these initial settings, proceed to adjust the idle adjustment screws until a steady and smooth idle is achieved.
Adjusting the Carb Jetting
Whether you have a Demon carburetor or a Holley carburetor, the jetting should be reasonably close if the carburetor is used in its intended application. To achieve maximum performance, adjust the jet size by increasing or decreasing it by two numbers (both primary and secondary) as necessary. If the performance continues to improve, keep adjusting the jet size in the same direction. However, there will come a point where the ET or lap times will start to worsen, indicating that the mixture has moved past the optimal air/fuel ratio. At that stage, make adjustments by moving one jet size at a time in the opposite direction until the optimal performance is restored.
Always prioritize jetting for performance rather than relying solely on spark plug color. Most high-energy ignitions leave minimal residue on the spark plugs. In the case of a drag car, the plugs may remain bone white, rendering spark plug readings ineffective. For oval track cars, the plugs may exhibit some coloration, although it takes longer with a high-energy ignition. If the car tends to run slightly hot, increasing the jet size by one or two sizes can alleviate the lean condition without sacrificing performance. The extra fuel will be burned off by the ignition system, contributing to engine cooling. However, if increasing the jet size results in the engine appearing leaner, it indicates a potential fuel system issue. When using a conventional ignition, it is still recommended to jet for optimal performance, but the plugs will take on a tan color after a short period of operation.
Problem-Solving
In most cases, an engine will exhibit symptoms such as popping, missing, or surging when it is running lean, although an overly rich condition can also lead to similar issues. As a general guideline, when dealing with cool and dense air, larger jets are needed, whereas hot and thin air calls for smaller jets. Additionally, whenever modifications are made to the engine setup, such as adding or removing a carburetor spacer, changing the camshaft, cylinder head, or intake manifold, it becomes necessary to readjust and fine-tune the carburetor in order to restore optimal performance.
Extensions
A stumble or hesitation can arise from another issue where the secondary jets become exposed, and adjusting the accelerator pump won't resolve the problem. This situation occurs when a vehicle launches aggressively, subjecting the fuel in the secondary bowl to intense G-force, which pushes it towards the rear wall and uncovers the jets. To rectify this, jet extensions can be utilized. However, when installing jet extensions in a carburetor that has a secondary power valve, it is necessary to remove the power valve and seal the opening. Subsequently, the jet size should be increased accordingly.
Pump Circuitry
Even if the jetting is approximately correct, it does not guarantee optimal performance. Poor performance can occur if the diameter of the accelerator pump nozzle (squirter) is incorrect. When a car exhibits sluggishness during initial acceleration and emits a cloud of black smoke from the headers upon launch in drag racing or when exiting a corner in oval track racing, it suggests that the accelerator pump nozzle diameter may be too large. Another potential issue is fuel spilling out of the vent tubes. This can be remedied by connecting a rubber hose between two vent tubes, with a slot cut at the top of the hose. Similar to jetting, determining the ideal squirter diameter requires trial-and-error testing. Adjusting the size up or down will help achieve the best performance.
Accelerator Pump Lever Adjustment
It is important not to overlook the adjustment of the accelerator pump lever. In oval track racing, the pump lever should be set to eliminate any play in the pump linkage when the throttle is closed. This ensures a smooth transition from idle without any lean stumble. Fine-tuning the accelerator pump for optimal corner performance often involves reducing the pump volume and discharge rate instead of increasing it. On the other hand, drag racing requires a slightly different approach.
To achieve the most powerful starting line launch with a foot brake, the pump lever override spring should be adjusted so that fuel begins to discharge through the nozzle at an engine speed slightly lower than the launch RPM. For example, if a car leaves the starting line at 5000 RPM, the pump shot should start at around 4700-4800 RPM. Similarly, an 1800 RPM launch would call for the accelerator pump shot to begin at 1500 RPM. The goal is to have minimal play in the accelerator pump system at the starting line RPM, ensuring that the pump shot is not wasted below that RPM.
It is worth noting that adjusting the accelerator pump as described may introduce some play in the pump linkage at idle, which can result in a stumble when driving in the pits. However, this adjustment allows the car to launch more aggressively. For drag cars equipped with a manual transmission or transbrake, where the starting line launch is executed with the carburetor fully open, a similar adjustment method should be employed, resembling that of an oval track application.
Float Level Adjustment
When assembling or reinstalling a carburetor, it is important to properly adjust the carburetor float position. The recommended float setting for a Holley-style carburetor is approximately 0.450" from the top of the bowl, which aligns with the bowl screw bosses when the bowl is upside down. This initial adjustment, known as the "dry" setting, is a starting point.
To ensure accuracy, it is necessary to recheck the float level while the engine is running and with the sight plugs removed. The fuel level should be such that it lightly wets the outside of the bowl as it seeps out. On the secondary side of the carburetor, where the sight plug is positioned lower, the fuel level should be slightly higher.
For Demon carburetors, the presence of large, patented sight glass windows simplifies the process of setting the float level. You can make adjustments relative to the three cast-in marks located next to the float window, allowing for precise float level calibration without the risk of fuel escaping from the float bowl.
Power Valve Tuning
The purpose of power valves is to provide additional fuel during high-load, wide-open throttle situations. When the manifold vacuum drops below the specified level indicated on the power valve, typically stamped on it, the power valve opens and enriches the main circuit by approximately six to ten jet sizes. This activation occurs under conditions such as full throttle when the engine is experiencing high loads.
To select an appropriate power valve, it is recommended that the power valve number corresponds to a vacuum level of at least 1.5-2.0 in/Hg below the engine's idle vacuum. For instance, if the engine generates 8.0-9.0 in/Hg of vacuum at idle, a 6.5 power valve would be a suitable initial choice. Using a power valve with a lower rating than this may cause hesitation and delay in fuel enrichment. However, in the case of oval track cars operating under restricted carburetor rules, utilizing a lower-rated power valve can sometimes enhance performance when coming off corners.
In drag race carburetors equipped with a secondary power valve, it is necessary to orient the carburetor sideways to prevent fuel starvation. This is because the power valve is positioned higher than the jets, and it becomes the first component to be uncovered as fuel is pushed towards the rear of the float bowl. As there is no way to add an extension to the power valve, the carburetor must be turned sideways to ensure an adequate fuel supply and prevent starvation.
Throttle Plates and Linkage
To attain maximum performance, it is crucial for the carburetor to fully open when the throttle is pressed down completely. Therefore, it is important to regularly check the throttle opening, particularly after any significant changes in the engine compartment. Optimal airflow cannot be achieved if the throttle plates fail to reach the wide-open position or if they are pulled beyond that point. The throttle linkage should operate smoothly without any binding and should not have any obstructions that could cause the carburetor to become stuck in the open position.
To ensure proper functionality, an auxiliary return spring must always be installed to guarantee the throttle closes securely. In some cases, it may be necessary to add a stop to prevent the carburetor from being opened excessively, which could lead to damage to the linkage. It is essential to take the time and properly set up the linkage for optimal performance. Keep in mind that tuning the linkage on sideways mounted tunnel ram carbs may require more time compared to a single 4-barrel setup.
Climate Changes and Performance
Variations in air density caused by fluctuations in temperature, barometric pressure, and humidity directly influence the performance of the engine. When air density changes, it often necessitates adjustments to the fuel mixture. These factors can even shift throughout the day, with noticeable differences between afternoon and evening conditions. It is widely known that engines tend to generate more power at night when the air cools. The cooler air, being denser, contains a higher concentration of oxygen per unit volume, enabling it to support greater fuel combustion.
While the weather conditions might not exhibit significant changes from afternoon to evening, adjustments to the fuel mixture will undoubtedly be required as the seasons transition. Traveling to tracks located in different climates or at varying altitudes can also prompt the need for fuel mixture adjustments. These factors should never be disregarded when aiming to achieve optimal performance through tuning.
Tuning With Spacers
Spacers and plenum dividers offer a convenient means of altering the configuration and properties of the intake tract, as well as its relationship with the carburetor. Introducing a plenum divider to an open plenum manifold can effectively maintain balanced fuel distribution from left to right, particularly beneficial for oval track applications, especially when using alcohol engines.
Incorporating spacers between the carburetor and intake manifold can yield significant outcomes. A four-hole spacer, for instance, can enhance low-end to mid-range performance by aiding the carburetor in drawing and atomizing fuel. On the other hand, an open center spacer increases the plenum area, delivering advantages in the mid-range and upper RPM power range. It is not uncommon to witness combinations of different spacer types or the stacking of similar spacers for specific requirements.
The true impact of any spacer or combination of spacers can only be accurately assessed through dedicated test and tune sessions conducted on the specific engine setup. Such information serves as a valuable tool for fine-tuning, enabling the discovery of optimal horsepower or the modification of power characteristics to suit specific track conditions.
Routine Maintenance
To maintain consistent performance, it is crucial to keep the carburetor clean. Regularly spray the air bleeds with carburetor spray or WD-40 to prevent clogging. Dirt and fuel dyes can accumulate in the air bleeds, leading to stumbling or high-speed misfires even in an otherwise well-functioning carburetor. When storing the car for winter, a helpful tip is to turn the engine over with the ignition off and the throttle open, then generously spray WD-40 down the venturi. This creates a protective mist on the valve seats and cylinders, safeguarding them against rust. Lastly, it is recommended to rebuild the carburetor at least once a year, and more frequently if it operates in dirty conditions with the proper carb rebuild kit.
Alcohol Tips
Applications that utilize alcohol as fuel require additional maintenance compared to gasoline fuel systems. Alcohol is highly corrosive, so it is important not to leave it in the fuel system or carburetor for an extended period of time. Proper care involves draining and flushing the entire fuel system, typically using gasoline. A common approach is to drain the system and introduce gasoline into the fuel cell, allowing the pump to draw fuel through the lines and into the carburetor. Alcohol carburetors run much richer than their gasoline counterparts, so as the engine starts to idle and stall, the system is effectively flushed. If your governing body permits the use of fuel additives, it is advisable to utilize an additive that provides lubrication to protect the fuel system during races.
