Carb Tuning Tips: Improving Mileage, Performance, And Drivability - Chevy Hardcore (2022)

When it comes to carbureted hot rods, it’s often assumed that poor fuel economy, emissions and drivability are compromises we must make in order to gain performance. But in reality, these goals are not conflicting. Who says you have to drown the engine in fuel at part throttle, in order to satisfy the engine’s needs at full throttle? Whether you’re at idle or wide open throttle, efficiency is what you want. We’ll dig into carburetor and ignition adjustments that will help you achieve that, but first let’s take a look at a few other areas that can affect fuel economy.

Volumetric Efficiency

Volumetric Efficiency (VE) is defined as the amount of air (in percent) that your engine ingests at any given time, compared to the engine’s theoretical maximum capacity. Any time we can increase the VE of the engine, it will make more torque at that point. If you can increase torque without having to increase throttle position, performance and mileage will also improve.

This 650 horsepower small block Chevy requires a large, low-restriction air filter to keep up with airflow demands.

On a basic level, an engine is just an air pump. And like any other pump, there is an intake and an exhaust. To minimize pumping losses, there should not be any restrictions on either end. Running an adequately sized low restriction air cleaner and filter ensures that there won’t be any inlet restrictions, but the exhaust side is slightly more complicated. The exhaust manifold or header length and tube diameter can change the resonance tuning of the engine and the VE at any given RPM, but any exhaust component beyond the collector is only a potential for restriction. After the headers, an exhaust system with adequate flow for the power level of your engine should only help to increase VE.

This volumetric efficiency table from a fuel injection map shows the percentage of the cylinder that's filled at different load and RPM points. Increasing VE at cruising speed improves gas mileage.

Camshaft Selection

Choosing a camshaft always involves a compromise between low-end torque and high-end horsepower. The cam specs determine where in the RPM range the highest VE occurs. An increase in duration should provide an increase in VE at higher RPM, but usually comes with a corresponding decrease at lower RPM. Since we’ll likely be cruising at lower engine speeds, the reduction in VE translates into a similar reduction in gas mileage.

(Video) Holley 4150 Carburetor Tuning Secrets. Tune like a professional Redneck!

Intake Selection

The intake manifold plays an important role in volumetric efficiency as well. While a single plane intake manifold with a large plenum volume helps to fill the cylinders more efficiently at high engine speeds, the reduced velocity is sure to cause a reduction in VE at lower speeds. As with the camshaft, intake selection is a compromise between low and high RPM efficiency. A single plane intake therefore has the potential to reduce VE in the lower RPM range. Again, since the engine usually cruises in this lower range, gas mileage will be reduced here as well

EGR Valve

The Exhaust Gas Recirculation valve is commonly removed to clean up the engine bay. While most enthusiasts know that it’s designed to reduce emissions, what is seldom known is that it also increases gas mileage. Since exhaust gases are only recirculated during cruise speeds, the valve has no effect at idle or wide open throttle. When recirculated, the inert exhaust gases takes up some room in the cylinder, leaving less available space for the fresh air/fuel mixture to enter. Two things will now happen. First, the engine makes a little bit less power, which requires the driver to open the throttle slightly more to compensate. This further unrestricts the air intake path, which reduces pumping losses. Second, it reduces the mass of usable air in the cylinder, which decreases engine load and allows for a slightly more advanced ignition timing lead. The combustion chamber temperature is drastically reduced during EGR, which is a big help for high compression street cars.

Torque Converter

Torque converter selection also has a noticeable effect on mileage and performance. The converter has two jobs: transfer engine torque at higher RPM with minimal slippage, and multiply torque at lower RPM by allowing some slippage to take place. We want to keep the engine in it’s power band, between peak torque and peak horsepower, as the car goes down the track. But while cruising around, keeping the torque converter from slipping improves mileage considerably. This is where a converter with an electrically controlled “lockup” clutch really shines. Selecting a torque converter and rear axle ratio that reduce converter slippage yields improved economy.

The 950 CFM Holley HP Series carburetor comes with removable air bleeds and four-corner idle circuitry to make tuning easier.


The carburetor itself has perhaps the most pronounced effect on fuel consumption. Since each carb comes pre-calibrated for a certain cubic inch and RPM range, the best place to start is with the right size carburetor. You can calculate the required CFM using the following formula:

Theoretical Carburetor CFM = (Engine CID * Maximum RPM) / 3456

This formula assumes 100% volumetric efficiency at redline, but street motors are more likely to fall in the range of 80 – 95%, depending on head/cam/intake/exhaust selection. Therefore, we should factor that into the equation by using an additional formula:


Required CFM = Theoretical CFM * Estimated Engine VE%

Now that we know the required amount of airflow, the next decision is whether to use a vacuum secondary carburetor or double-pumper design. For drag-race-only applications, a double-pumper will help the car launch harder at the track, but if the vehicle is primarily used on the street, the vacuum secondary is often a much better choice. Lack of secondary accelerator pump shot and a delayed secondary opening will increase fuel economy. The vacuum secondary carb’s fuel calibration is usually more efficient as well. In either case, make sure the idle and main air bleeds located on the top of the carburetor remain clean and unrestricted.


Now that we have selected the right carburetor, we can dig into tuning it. The most accurate method to tune a carburetor is with a wideband air/fuel ratio meter. They use an oxygen sensor in the exhaust stream that can accurately gauge the air/fuel ratio of the engine at any given time, which allows you much more precise adjustment. Before beginning, make sure the floats are adjusted properly, as this can affect the air/fuel ratio. On a Holley, the fuel level should be at the bottom edge of the sight plug hole.

Even race cars can have decent idle quality. This low 10 second car idles very well, despite having a large roller cam.

Next, warm up the engine and adjust the idle mixture screws until the highest sustainable vacuum reading and engine RPM are achieved. Adjust both screws in even increments, beginning with two turns out. At idle, you want an air/fuel ratio of around 14.7:1 (lambda 1.00) for standard gasoline. Some engines with larger camshafts may need additional enrichment, due to the dilution of the intake charge. In this case, you can go as rich as 13.5:1 (lambda .92).

Idle Feed Restrictors (circled) limit the total amount of fuel available to the idle and transfer slot circuits.

Make sure that your idle speed adjustment screw is set low enough that the primary throttle plate doesn’t uncover more than .060-inch of the transfer slot, as viewed from the bottom side of the throttle body. If the transfer slot is uncovered, it will richen up the mixture considerably at idle. Some engines with large camshafts require more air at idle than is provided with this idle speed position. If this is the case, drill a small hole in each of the primary throttle plates. If you still can’t get engine speed high enough to idle, repeat with the secondary throttle plates. You can also enlarge the holes some if necessary. In the end, the throttle blade must close far enough to prevent the off-idle and main jet circuits from engaging prematurely. Once you have the idle speed screw adjusted correctly, recheck the idle mixture screws.

Transfer slots (circled), viewed from bottom of carburetor, must be nearly concealed by closing the throttle plates. When opened too far at idle, a rich condition will result.

(Video) 8.12-Edelbrock AVS2 Fine Tuning

Transfer Slot

At speeds just above idle, the tranfer slot now begins to kick in. This circuit feeds the motor between idle and roughly 2000 – 2500 RPM. With a steady throttle position, the engine should want an air/fuel ratio of 14.7:1 (lambda 1.00). Again, some engines with larger camshafts may need slight enrichment here as well, but perhaps not as much as at idle. Idle feed restrictors control the amount of fuel through this circuit, while idle air bleeds control the amount of time this circuit remains on. If it stays on too long, it can overlap with the main jet circuit, causing an over-rich condition.

Main Jetting

Contrary to popular belief, the main jets are designed to control the air/fuel ratio of the cruise circuit, not wide open throttle. This covers an RPM range from around 2000 – 2500 and up, starting up as the transfer slot begins to fade out. Again, test in steady state by holding a steady throttle position. Record the air/fuel ratio in 500 RPM increments starting at 2000 RPM, and work upward until you feel comfortable. Shoot for an air/fuel ratio of 14.7:1 (lambda 1.00) by changing main jets. Leaning the engine out further increases NOx emissions and temperatures, without producing much better results.

Bottom-side view of a Holley venturi. Fuel is pulled from the fuel bowls, through the main jets and power valve channel restrictions, by a low pressure zone created by the booster venturi.

Power Valve Channel Restrictions

At throttle positions over half throttle, the reduction in manifold vacuum should cause the power valve to open. This uncovers two small fuel passages in the metering block called PVCRs (Power Valve Channel Restrictions). These passages provide additional fuel for power enrichment during heavy load situations. Since optimal main jetting was already determined, any additional fuel increase or decrease must result from a change in PVCR size. At WOT, shoot for an air/fuel ratio of around 12.5:1 (lambda .85) for naturally aspirated engines. The front side of the carburetor can be tuned separately from the back by disconnecting the secondary linkage and wiring it shut, so it cannot get sucked open. One word of caution about doing this – the engine will make more vacuum at WOT than usual. Temporarily install a power valve with a rating higher than the amount of vacuum your engine now produces at WOT. You may find as much as 5 – 6 inches of vacuum at full throttle when only running on the primaries.

(Video) Edelbrock Carb Live Tech and Q&A

The Power Valve Channel Restrictions (arrows) are blocked off by the power valve, until manifold vacuum drops below the power valve rating.

While tuning the air/fuel ratio at WOT, spark advance should be temporarily retarded to several degrees lower than your anticipated total timing. This is done to keep things safe while dialing in the air/fuel. We’re only tuning the fuel curve at this point, so maximum power production is not yet a concern. Ignition advance will be tuned separately at a later point.

Power Valve

Power valves should be sized to open after half throttle. If your engine produces, say, 16 inches of vacuum at cruise speed, then giving it half throttle should drop engine vacuum to around 8 inches. In this case, a power valve of around 6.5 should work. Generally speaking, around 2 inches less than your minimum vacuum at idle should also work out about right for a hot cam. Once the primaries are tuned in and the power valve size is selected, hook up the secondary linkage and run the engine at WOT again. Now any deviation from the desired air/fuel ratio at WOT can be adjusted by changing secondary main jets.

Wideband data collected from a full-throttle pass shows this engine tune-up is right on the money.

Accelerator Pump

Now that we have tuned in the idle, transfer slot, main jets, PVCRs, and power valve, we can focus our attention on transient fueling. On double-pumper carbs, the secondary linkage should be disconnected again, while the primary accelerator pump is tuned by changing pump cam position, pump cam, and squirter size. Target a transient air/fuel ratio that is slightly richer than your current target air/fuel ratio. For part throttle, this might be around 13.8:1 (lambda .94). At full throttle, try to maintain your target WOT ratio of 12.5:1 (lambda .85) without any momentary lean spikes. With the primary pump now tuned, hook up the secondaries and repeat the procedure for the back.

Spark Advance

After getting the air/fuel ratio correctly dialed in, we can now move on to the real power builder – ignition timing. Beginning with around 8 degrees lower total timing than expected, run the car on the dyno or at the drag strip and note the average power or MPH results. Traction issues must be removed from this equation in order to obtain meaningful data. Let the car cool down for a little while, and then run it again, adding 2 degrees of total timing. Note the results, and repeat the procedure until the engine does not pick up any more power or the car does not pick up any more MPH. What we’re looking for are the big increases. If the results are the same or worse, back up to the last setting.

During these tests, spark knock MUST be completely avoided at all times. You can listen for knock by ear, or with “det cans” or chassis ears hooked to the motor. An aftermarket knock sensor setup can also be used, and spark plugs should be read for signs of detonation along the way. We want as much timing as possible, as long as it adds power or MPH without knocking. An additional margin of safety is also a good idea, in case you run across a bad tank of gas in the future, or better weather than what you tuned for.

Tuned to Perfection

(Video) How To Adjust The Idle Mixture Screws On An Edelbrock Carburetor

Now that you have picked out the right parts that work together as a combination, tuned the carburetor for the proper air/fuel ratio in all scenarios, and found the optimal spark advance settings, the engine should perform really well. The byproduct of this newfound efficiency is an increase in gas mileage, drivability, and most likely a reduction in emissions as well. Apparently, you can have your cake and eat it too.


How do you adjust a carburetor with better gas mileage? ›

Turn the idle settings screw so that rpm reaches about 3000 rpm. Now tune the air/fuel screw to make the mixture leanest as possible. Please refer to the above whether you must turn the screw clockwise or anti clockwise. As you make the mixture leaner, slowly the engine RPM decreases…

How do I know if my carburetor is too rich or lean? ›

If the mixture is too lean, it will leave no color, while a rich mixture will cause the fuel ring to become more prominent. Over-rich mixtures will give the plug a sooty appearance.

How can I increase drastically mpg? ›

Our driving habits have a significant impact on fuel efficiency.
  1. Go Easy on the Pedal. Speeding, braking, and rapid acceleration waste gas. ...
  2. Slow Down. ...
  3. Leave Extras at Home. ...
  4. Use Cruise Control (When Appropriate) ...
  5. Turn off the Car. ...
  6. Check Tire Pressure. ...
  7. Replace Spark Plugs. ...
  8. Check the Alignment.

How can I improve my fuel efficiency and performance? ›

Your Personal Action Plan
  1. Drive for maximum fuel efficiency. Accelerate gently. Maintain a steady speed. Anticipate traffic. Avoid high speeds. Coast to decelerate.
  2. Adopt other tips and tricks. Avoid unnecessary idling. Measure tire pressure monthly. Use a manual transmission properly. Avoid carrying unnecessary weight.
7 Dec 2021

What is the best air fuel ratio for a carburetor? ›

The ideal chemical air-fuel ratio, in which all the fuel is oxidized, is 14.7:1, that is, 14.7 parts air to 1 part fuel. A “rich” condition means more fuel is being used, an air-fuel ratio less than 14.7, while a “lean” condition means less fuel is being used, an air-fuel ratio higher than 14.7.

How do you make a carburetor run leaner? ›

Regardless of whether or not the engine is running too rich or too lean, bring it down to a very lean mixture by turning both screws a quarter-turn at a time, counter-clockwise, then slowly bringing them back up to an equal and smooth mixture.

Does backfire mean rich or lean? ›

Lean Air/Fuel Mixture

Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn't have enough gasoline can cause a backfire, too. A "lean" mixture is one that doesn't have enough fuel, and too much air.

Is high idle lean or rich? ›

The most common symptoms of lean jetting in a dirt bike are:

High idle. Hanging idle.

Is it better to run rich or lean? ›

Q: Is it better to run the too lean or rich engines? An engine running slightly rich will give more power, but running lean will cause catastrophic engine damage. Running too rich can also cause severe damage to your engine.

What mods improve gas mileage? ›

Top Power and Mileage Mods You Can Do in a Day
  • Cold-Air Intake. ...
  • After-Cat Exhaust. ...
  • Headers. ...
  • Distributor Upgrade/Ignition Timing. ...
  • Carburetor Swap. ...
  • Intake Manifold. ...
  • Ignition Coil/Plug Gap. ...
  • Fuel Injection.
2 Apr 2016

Does wd40 increase gas mileage? ›

WD-40 definitely will not improve gas mileage.

How can I improve my gas mileage by 70 percent? ›

Here's what to do:
  1. Never drive above 45 mph. Yes, this includes highways. ...
  2. Remove passenger side mirror. ...
  3. Avoid braking and rapid acceleration. ...
  4. Turn off engine at red lights. ...
  5. Windows up/AC off. ...
  6. Try to stay relaxed. ...
  7. Legally draft when possible. ...
  8. Overinflate tires by 10 percent.
13 Jun 2013

Will a tuner help with gas mileage? ›

The short answer is “yes,'' but a longer explanation is in order. While manufacturers of aftermarket performance products claim tuners can add 3-4 mpg, the actual savings, if any, largely depends on how and where you drive. By definition, performance products are designed to increase engine output.

What speed is best for gas mileage? ›

Every car gets better gas mileage at 50 miles per hour than at 70.

Which mixture ratio is the richest? ›

What is a Rich and Lean Mixture? Simply put, a rich air-fuel mixture contains less air than the stoichiometric ratio, whereas a lean mixture contains more air than the stoichiometric ratio. Thus, an example of a rich air-fuel mixture is 9:1, and an example of a lean mixture is 17:1.

What's a good air-fuel ratio for a carbureted motor at idle? ›

Most general-purpose aftermarket performance carburetors are designed to deliver around a 12.5:1 air/fuel ratio to avoid lean surge conditions. Most mild street engines can tolerate part-throttle air/fuel ratios of 13.5:1 all the way up to as high as 15.0:1.

What is the ideal air-fuel ratio for a spark ignition engine? ›

The theoretical air–fuel ratio mixture for a gasoline fueled engine, for complete gasoline fuel combustion, the stoichiometric air–fuel ratio is about 14.7:1. In order to completely burn 1 kg of gasoline fuel, the combustion process needs 14.7 kg of air.

How do I know if my pilot jet is too rich? ›

If the pilot circuit is rich, the engine will have a rough idle or may not return to idle without "blipping" the throttle. You may also notice a distinct smell of raw fuel and some eye irritation from the exhaust.

How do I know if my carburetor is too big? ›

An engine with a carb that is too big will put out less Torque and Horsepower . It will be difficult to drive due to poor low-end torque. If you drag race your car, an oversized carb will produce slow 60 ft. times.

Which way do you turn a mixture screw to lean? ›

On most carburetors, turning the mixture screw in (clockwise) leans the mixture, while counterclockwise (out) enriches the mixture. Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a -turn and evaluate the results.

What does a rich engine sound like? ›

Re: what do rich and lean sound like? A rich condition will result in excessive smoke from the silencer, the plug will often carbon foul and the engine will produce a sputtering/crackling sound. A lean condition will result in slow throttle response, you twist the throttle but the power delivery is lethargic and flat.

What air fuel ratio is too rich? ›

For maximum reliability at full power, air fuel ratios from 10.5-12.5:1 are considered best, depending on the engine. Richer than around 10.5:1, you start to get noticeable black smoke from the exhaust and the car can struggle to run properly without misfiring.

What air fuel ratio is too lean? ›

In a real engine, a 14.7:1 air-fuel ratio is a little too lean. To compensate for incomplete combustion, and to reduce NOx emissions, modern automobiles are tuned to run more rich, sometimes dipping as low as 12:1 or even richer during high-load situations.

How do I know if my spark plugs are rich or lean? ›

If the engine is running to lean, the spark plug can look greyish white. If the engine is running to rich the spark plug will look black, sometimes a glossy black if it's really rich.

What are the symptoms of a rich fuel mixture? ›

Seven Signs Your Air and Fuel Mixture Is Too Rich
  • Check Engine Warning. If your vehicle's exhaust has too much gas in it, your check engine light will come on. ...
  • Strange Odors. ...
  • Poor Fuel Economy. ...
  • Engine Performance Problems. ...
  • Emissions Test Failure. ...
  • Engine Idle Trouble. ...
  • Part Damage.
15 Dec 2020

What does rich exhaust smell like? ›

A Gasoline Smell in the Exhaust

If the exhaust coming from your vehicle smells like gasoline, it's often due to an overly rich air/fuel ratio. This means that either too much fuel or too little air is getting into your vehicle's combustion chambers.

What happens if engine gets too much air? ›

What happens if the engine gets too much air? Too much air translates to the engine running with a lean air-fuel mixture. This lean mixture eventually results in the engine overheating. An overheated engine will have poor performance leading poor motorcycle experience.

How do you know if car is running rich? ›

Car Running Rich Symptoms
  1. Check Engine Light.
  2. Pungent Smell.
  3. Excess Fuel Consumption.
  4. Poor Engine Performance.
  5. High Carbon Monoxide Emissions.
  6. Foul Spark Plugs.
  7. Black Smoke From the Exhaust Pipe.

Will running lean damage spark plugs? ›

The most prominent engine running lean symptoms include lower power output and stalling. It can also lead to trouble starting the engine, issues with the spark plugs and an illuminated Check Engine Light.

Does throttle body spacer increase MPG? ›

The spacer is bolted on the back of the throttle body, downstream of the main airflow. This is strictly an aftermarket enhancement used to increase the fuel economy, torque and horsepower.

How can I improve my v8 MPG? ›

Accelerate slowly but steadily from a stop, such as at a traffic light. Large V-8 engines can speed up in a hurry, but when you run with the engine wide open, it blows through your gas quickly. According to, this trick can increase your vehicle's fuel efficiency by up to 35 percent.

Does high flow exhaust increase gas mileage? ›

The best way to explain fuel efficiency is by giving an example. The math is rather straightforward. A typical exhaust improves a vehicle's power output by 2 to 5%. Let us say your vehicle get roughly 16 miles per gallon and by adding a new exhaust system sees a gain in fuel economy of approximately 3%.

What does toothpaste in the gas tank do? ›

It's safe to conclude there are no benefits, fuel saving or otherwise, to putting toothpaste into your gas tank. These obnoxious ads, as mentioned, are simply click bait to a scam product.

Do magnets on fuel lines work? ›

Interesting that the EPA itself have tested about 100 fuel saving devices including fuel line magnets and concluded most have no effect at all. Some did actually work (not magnets) but the improvement was only slight. Quite a few had negative effects and some could damage an engine and may even be dangerous.

What can I put in my gas tank to save gas? ›

If you're getting ready to store your car for winter, or just don't plan on driving it for several months, add a fuel stabilizer to the gas tank before storing the car. That way, it will be that much easier to start your car when you're ready to take it out of storage! Fuel stabilizer is easy to use.

Does seafoam increase gas mileage? ›

When it comes to fuel economy, Sea Foam cleans harmful fuel residues and deposits that cause dirty or clogged fuel injectors, helping restore injector spray patterns and recover lost mileage. It's safe and effective in any gasoline or diesel engine.

Does cold air intake increase mpg? ›

How Much MPG Does A Cold Air Intake Add? Cold air intakes may increase fuel economy by 3-5 MPG, but only if they produce cooler air than your vehicle's original intake.

How do you max out mileage on a car? ›

7 ways to get better fuel economy from your car
  1. Be gentle on the throttle. While driving, use the least amount of throttle possible. ...
  2. Anticipate and drive. Look at the road ahead and observe the flow of traffic. ...
  3. Switch off. ...
  4. Shed weight and drag. ...
  5. Maintain your car. ...
  6. Increase tyre pressure. ...
  7. Fill your car with good quality fuel.
19 Jun 2022

How much can a tune increase mpg? ›

Try to purchase your gas at the coolest time of day.

After that, you are just wasting fuel. Regular tune ups can save an average of 4%. Replacing a dirty air filter can increase gas mileage up to 10%. Make sure tires are pressurized to the maximum limit provided by the manufacturer.

How do you make a carbureted engine more fuel-efficient? ›

Lack of secondary accelerator pump shot and a delayed secondary opening will increase fuel economy. The vacuum secondary carb's fuel calibration is usually more efficient as well. In either case, make sure the idle and main air bleeds located on the top of the carburetor remain clean and unrestricted.

Do Iridium spark plugs improve gas mileage? ›

Improve the fuel economy with iridium spark plugs

Switching to iridium spark plugs can considerably improve the mileage of the gasoline vehicles on the highway. This change comes from the fact that you now have a better ignition profile to ensure the efficient burning of the fuel.

Does removing catalytic converter increase fuel consumption? ›

Removing a catalytic converter will not improve MPG. However, replacing an old or clogged one will. Catalytic converters do not affect gas mileage unless they are not working correctly, so removing one will not make a difference unless it wasn't working correctly, to begin with.

Which brand of gasoline gives the best mileage? ›

By far, Chevron has the best quality gas in the United States. Their Techron blend of additives has been found to reduce carbon deposits and keep your engine running clean for thousands of miles. In addition, Chevron gas stations are easy to find.

Does higher octane give better mileage? ›

So is Higher Octane Fuel More Efficient? In a word, no. On its own paying for premium gasoline does not make your car run better or get greater gas mileage. Giving your car the fuel it requires to run smoothly and efficiently, without damage to the engine, does make a difference in your fuel mileage.

Which gear uses the most fuel? ›

All cars are designed to start at the lowest gear, as that's where you get the most power for acceleration, but driving at the highest gear will increase fuel economy. Fuel consumption will increase when you stay long at lower gears. Driving slow at the highest gear will also increase fuel consumption.

How can I reduce fuel consumption? ›

10 ways to reduce fuel consumption
  1. Keep tires pumped up. Tires that are underinflated have a higher rolling resistance on the road. ...
  2. Lose the weight in your boot. ...
  3. Drive with AC. ...
  4. Don't go too fast or too slow. ...
  5. Remain steady when accelerating. ...
  6. Avoid braking aggressively. ...
  7. Cruise in top gear. ...
  8. Practice predictive driving.

Which is more fuel efficient carburetor or fuel injection? ›

So, which is better? Fuel injection has an upper hand when it comes to performance, fuel efficiency, and better throttle response. While the FI system may cost you more, the long-term aspects of the FI system are better than a carburetor.

How do you adjust a carburetor? ›

Turn the high speed or main jet screw clockwise until the engine begins to slow. Then, turn the screw the other way until the engine begins to slow. Turn the screw back to the midpoint. Once adjusted, check engine acceleration by moving the throttle from idle to fast.

How do you adjust a carb screw? ›

Set the idle screw so that the engine is idling at a normal RPM. Take a small screwdriver and start turning one bleeder screw on one side of the carb inward until the idle either drops or rises. If it drops, you are going the wrong way, so turn it the other way until the you hear the engine's idle rise back up again.

Does a full tank use more fuel? ›

A full tank of fuel weighs more, driving round with additional weight in a car or van will reduce fuel economy.

What speed is most fuel-efficient? ›

7. The Energy Saving Trust says that the most efficient speed you can travel in a car in terms of achieving the best fuel economy is 55-65mph. Any faster, though, and the fuel efficiency decreases rapidly. For example, driving at 85mph uses 40% more fuel than at 70mph (oh, and it's illegal too).

What was the last car to use a carburetor? ›

In the U.S. market, the last cars using carburetors were: 1990 (General public) : Oldsmobile Custom Cruiser, Buick Estate Wagon, Cadillac Brougham, Honda Prelude (Base Model), Subaru Justy. 1991 (Police) : Ford Crown Victoria Police Interceptor with the 5.8 L (351 cu in) V8 engine.

What is the best type of fuel injection? ›

The sequential fuel injection system eliminates the only disadvantage of MPFI and is the most widely used fuel injection system today.

Which carburetor from the following is used mostly for high speed engine? ›

The downdraught carburetor is placed at a higher level than the inlet manifold and as a result, in this type of carburetor, the air and the mixture flow a downward course. In such carburetors, the mixing tube and throat are largely resulting the engine speed to be high even when the vehicle is running at a high speed.

How do I know if my pilot jet is too rich? ›

If the pilot circuit is rich, the engine will have a rough idle or may not return to idle without "blipping" the throttle. You may also notice a distinct smell of raw fuel and some eye irritation from the exhaust.

What does the air screw do on a carb? ›

The air screw is the “fine-tune” adjustment for the pilot jet (sometimes called the slow jet). The screw adjusts how much air is allowed through the pilot jet. This is important for jetting to get the proper air-fuel mixture.

How do you adjust the idle mixture? ›

Adjust idle mixture by turning idle mixture screw slowly clockwise until the engine runs poorly. Note position. Slowly turn the screw counter-clockwise until it starts to stumble.

What happens when you turn the idle mixture screw in and out? ›

Loosening the screw strengthens the air and fuel mixture and increases the amount of fuel flowing to the engine. Loosening the screw is also called making the fuel mixture richer, which increases the RPMs at which the engine idles.

How do you adjust air fuel mixture screws in the carburetor? ›

Pilot Air/Fuel Screw Adjustment Explained - Single Carb - Part 1

What does the idle adjustment screw do? ›

The goal of adjusting the idle-mixture screws is to achieve the highest possible idle vacuum at a set idle speed. If the idle speed increases after you adjust the idle-mixture screws (which is likely), be sure to adjust the idle speed back to the base speed.


1. How To Make The Edelbrock Carb REALLY Work For You
(Uncle Tony's Garage)
2. Holley & Edelbrock Carbs - Adjust this FIRST (Carb Tuning 101)
3. Fine Engine Tuning With A Vacuum Gauge
(Uncle Tony's Garage)
4. The Ultimate Edelbrock Carburetor Tuning Guide
(Muscle Car Solutions)
5. SEMA 2017: Fidanza Flywheels Improve Street Performance
(Power + Performance)
6. Back on the Wagon
(Finding Fast)

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