It is important to follow all linkage and lever installation instructions. The number one and two reasons for tuning errors are improper linkage installations and over tightened linkage nut, causing a binding in linkage assembly.



(Video) How To Adjust Fast Idle on Weber 32/36

Start set up by confirming carb base line settings. Do not depend on the factory delivered settings. Check them before the carb is installed.

1. All settings are done with choke disengaged or warmed up so that the choke is fully opened and disengaged. This is done on automatic choke carburetors by first opening the choke butterfly by hand and inserting a wood block or wedge of some kind to hold open while the linkage is cycled (linkage operated through its full movement) to clear the choke cam. (You will hear a metallic click as the cam is released. You can check the fast Idle screw under the choke assembly to confirm that it is not in contact with the choke fast idle cam.)

2. Set the Idle stop screw (speed screw see fig 1) by backing out the Idle speed screw until it is not in contact with the throttle stop lever. Cycle the linkage again to be sure that the linkage comes to close without any assistance. (Checking for linkage bind) Now bring screw back into contact with the lever and continue to open or screwing in 1 turn no more than 11/2 turns.

3. Set the mixture screw (see Fig 1) by first screwing in until the screw stops, bottoms out. DO NOT FORCE OR BIND AS THIS WILL CAUSE DAMAGE TO THE SCREW AND IT’S SEAT IN THE BODY OF CARBURETOR. Back out the screw 2 full turns.

(Video) How to adjust idling on a Weber carb



4a. Start the engine, the engine will run very slowly more like a tractor. As long as the engine stays running idle speed is not important at this point.

4b. The first thing to do is not set up the idle speed, but to set the idle mixture screw to lean best idle setting. First, turn in the mixture screw until the engine dies or runs worse, then back out the screw (recommend turning ¼ to ½ turn at a time). The engine should pick up speed and begin to smooth out. Back out ½ turn more, or until the screw does nothing or runs worse then turn back to the point where it ran its best. Use your ear, not a scope or tuning instruments at this point. You want to tune the engine by sound. Adjust to best, fastest and smoothest running point.

(Video) Weber 32/36 proper setup to my B18; years later!

4c. Now that the mixture screw is at its best running location, you can adjust the idle speed the screw. The screw will be sensitive and should only take ¼ to ½ turns to achieve the idle speed you like.
Check and set idle to your driving preference. Put the car in gear and apply slight load, (AC on) and set the Idle as you like it. Don’t set it too high, as this will cause causes excessive clutch and brake wear. The Idle only needs to be 7 to 900 RPM with light load or AC on.

5. Recheck timing and vacuum hook ups. Recheck mixture screw to lean best idle again. If all is still best and smoothest idle then confirm and note the final settings.

To confirm settings with the engine running. Start by screwing in the mixture screw and count the number of turns it takes to bottom out and note if the engine dies. If Idle Mixture screws are with in ½ turn of base line setting then all is well and have fun. Also check the speed screw and note how many total turns from initial contact. You may have opened (turned in) the speed screw. Your final setting should be under 2 full turns. Reset the screws (back in) to the best final settings (Per your notes) and go on a test drive and have fun. If the settings are other than described then you may want to recalibrate the idle circuit (low speed circuit) to your engines needs. This is done by following the rule of thumb BELOW.

Simple Rules for low speed calibration

(Video) How To Tune Your Weber Carburetor 32/36 (in depth)

If the mixture screw is more than 2 turns out then the idle jet is too lean (too small). When the mixture screw is less than 11/2 then the Idle jet is too rich (too large). These assumptions are based on the fact that the speed screw setting is not opened more than 11/2 turns. If the speed screw has to be opened 2 or more turns then this is also an indication of a lean condition usually requiring greater change. At times it may appear to be showing signs of richness or flooding it is really a lean condition. See pictures and notes in the tech 2 article supplied in the kit instructions, view and please understand the need to keep throttle plate as near to closed as possible so as not to prematurely expose the transition holes. This is what causes the visible rich condition, and confirms the need to increase the jet size. JET KITS are available if needed.

EXAMPLE With the speed screw set at no more than two (2) turns in after contact with the stop lever; and the best idle occurring with the mixture screw set at 3 turns from bottom, indicates the need for a larger Idle jet. Achieving the best idle at under 2 turns indicates the need for a smaller idle jet.

The secret to understanding the critical nature of the carburetor set up and the advantages of a WEBER over other carburetors is the idle circuit. Referred to as the low speed circuit by Weber, this circuit is responsible for 80% of the driving operation. This is the reason that the Weber should give a fuel economy improvement over most factory carbs along with significant performance gains. In the worst case you should not see a significant fuel economy loss over stock, while improving HP & Drivability.

The Weber Carburetor is a sequentially timed device to the motor like the distributor. Time taken in the setup will provide more fun later.

(Video) How to tune Weber carburettors and set up on a rolling road

If you should need to call REDLINE WEBER for technical assistance we will need to know your final settings to help. Technical assistance is free for the first 60 days of purchase. Units in service over 60 days may be assessed a service fee. All charges will be noted up front after a brief consultation to determine any possible defect. If the carb is out of the warranty period and no defect is determined we will estimate the possible cost of tech support or recommend literature available that may help.



Contact REDLINE WEBER at 1 800 733 2277 ex 7457 Monday thru Friday 8:00 to 5:00 Pacific Standard Time

Non warranty Tech support is on a fee for services basis with minimum charge of 35.00 per problem resolution. With tech support calibration and tuning parts will be available at discount pricing as well as access to technical support documents and mailings.


How do you adjust the idle on a Weber carburetor? ›

Weber Carb Lean Idle Adjustment Method: (at home)

Run the engine until it is warmed up and the choke is fully opened and disengaged. Rotate the Idle Speed screw counter-clockwise, until tachometer reads about 600-700 rpm's (as low as it can idle, with the engine still running).

How do you adjust a carburetor to run lean? ›

Adjust both screws equally and find the right mixture.

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.

What should idle screw be set at? ›

Before starting the engine, use a small screwdriver to check the position of each idle mixture screw. Slowly turn each idle mixture screw clockwise (in) and count the number of turns until it lightly seats. Generally this will be anywhere from 1 to 2 turns out from fully seated.

How do you lean out a carb at idle? ›

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.

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

Q: How Do You Tell if a Carburetor Is Rich or Lean? A: One way to tell for sure is by "reading" the spark plugs. If the plug tip is white, the mixture is lean. If it's brown or black, it's rich.

How do you adjust engine idle speed and mixture? ›

Set idle speed adjusting screw, clockwise to increase rpm, counter-clockwise to decrease rpm. Idle rpm range should be 950 to 1050 rpms. Adjust idle mixture by turning idle mixture screw slowly clockwise until the engine runs poorly.

Does running lean mean too much air? ›

An engine running lean means that there's an excessive amount of air in the air-fuel ratio. As we mentioned before, most engines demand a specific balance of air and fuel to conduct a perfect combustion cycle. This balance or target is called stoichiometry.

What causes lean at idle? ›

Fuel System Troubles

An obstruction in the fuel filter or a failed fuel pump can reduce fuel pressure and hinder fuel delivery. Low fuel pressure significantly reduces the rate of flow in the fuel injectors that produces the lean condition.

What causes lean condition at idle? ›

Because your engine is running on less fuel than it should, it is running “lean.” Your engine running lean is always a symptom of another problem - it could be a dirty MAF sensor, a damaged oxygen sensor, or damaged fuel injectors.

What is the perfect idle RPM? ›

The idle speed should feel consistent without skipping or slipping. In most of today's cars, an idle speed of 600 to 1000 RPMs is average. If your car is idling rough, though, it won't feel smooth. The RPMs will jump up and down, for example, or they'll fall below 600 RPM (or whatever is typical for your vehicle).

How many turns out should the idle screw be? ›

Tip: The factory position for most air fuel mixture screws is usually between 1.5 and 2.5 turns out from being screwed all the way in. If you ever want to start fresh, turn the screw clockwise until it is lightly seated, then back it out about 2 turns. Then you can make adjustments from this position.

What idle is too high? ›

High RPM – Idle speed is measured in revolutions per minute (RPM). For most 2 liter and below sized engines the vehicles, the standard idle speed should be around 750 RPM (when the engine is warm). If your car's RPM is above 900 with a warm engine while idling, it is likely using too much fuel.

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 happens if carburetor float level is too high? ›

In an extreme case, if the floats are set too high, fuel will overflow via drillings inside the carb body. In addition, fuel may flow into the engine unrestricted, which, if the engine is not running, can cause hydraulic lock – that is, as the piston rises on the compression stroke it cannot compress the fuel.

What is excessively rich or lean idling? ›

Running rich only occurs if there is an issue with your engine's combustion process. The cause of this is your engine receiving too much fuel. Conversely, “running lean” refers to your car receiving too much air and not enough fuel.

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.

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.

How do you know if a mixture is too lean? ›

Symptoms of a lean fuel mixture:
  1. Check engine light on – One of the first signs of a lean fuel mixture is the check engine light turning on. ...
  2. Reduced acceleration – A lean fuel mixture can cause a noticeable loss in engine power.

Why does idling require a rich mixture? ›

Thus, the engine requires a rich mixture (more fuel than a stoichiometric mixture) for starting at idling to compensate for the charge dilution due to exhaust gases from the combustion chamber. Exhaust gas dilution prevents efficient combustion by reducing the contact between the fuel and air particles.

How do you know if your carburetor is running 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 do I relearn idle speed? ›

Follow these steps to complete an Idle Relearn:

Put gear shifter in "P" (Park), turn off all accessories. Run the engine until it reaches normal operating temperature. Allow engine to idle for at least one minute. Turn the air conditioning on and allow the engine to idle for at least one minute.

How do I fix a system that is too lean? ›

What repairs can fix the P0171 code?
  1. Replacing the fuel pump.
  2. Replacing the fuel filter.
  3. Replacing the fuel pressure regulator.
  4. Replacing the powertrain control module.
  5. Replacing one or more injectors.
  6. Replacing one or more oxygen sensors.
  7. Replacing the mass air flow sensor.
  8. Repairing a vacuum leak.

Is it better to run lean or rich? ›

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 is the best air-fuel ratio? ›

The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel that burns all fuel with no excess air. For gasoline fuel, the stoichiometric air–fuel mixture is about 14.7:1 i.e. for every one gram of fuel, 14.7 grams of air are required.

Does a lean engine run hotter? ›

Adding in extra fuel prevents detonation and just generally makes the engine happier. So, yes, if you lean out your engine, going from 12:1 to around 14:1, your engine will run a bit hotter.

Can running too lean damage an engine? ›

A petrol (gasoline engine) that runs too lean will create a lot of heat, this heat can damage the engine, too lean will eventually burn a hole in a piston.

What happens if you run lean for too long? ›

If a car is driven on a lean-running engine for about a week, it could cause some long-term damage to the engine. The car may start running poorly and have a lower fuel economy.

Will a vacuum leak cause a rich or lean condition? ›

A vacuum leak will cause the engine to run lean. In other words, there will be more air entering the engine than is needed to balance out the air/fuel ratio. This can result in the following: Engine stalling.

What RPM is too low for idle? ›

While there's some variance depending on the kind of car you're driving and its engine, most modern cars should comfortably idle somewhere between 600 and 1000 RPMs. If you notice your car idling outside this range, you might want to have it looked at by a mechanic.

How do you set up a Weber carburetor? ›

First, turn in the mixture screw until the engine dies or runs worse, then back out the screw (recommend turning ¼ to ½ turn at a time). The engine should pick up speed and begin to smooth out. Back out ½ turn more, or until the screw does nothing or runs worse then turn back to the point where it ran its best.

Does idle affect timing? ›

Yes. In the case of both valve timing and distributer timing (in older cars with a spark distributer) changing the timing will affect the idle speed at the same throttle setting, because it will affect the efficiency of the engine at that throttle setting.

Does high idle use more gas? ›

Vehicles get ZERO miles-per-gallon while idling. Larger vehicles tend to waste more fuel than smaller ones. Idling can also cause greater engine wear-and-tear over time, resulting in higher maintenance costs. MYTH: Frequent restarting is harder on the engine and wastes more fuel than idling.

What causes rough idle? ›

Causes of a rough idle. Many different problems could result in a rough idle for your car or truck, including: dirty fuel injectors, clogged air filters, bad spark plugs, and a variety of exhaust system issues. Let's go over each of these causes in more detail and learn how they can be remedied.

How do I reset my high idle? ›

The idle will relearn if you put the vehicle in drive and allow the idle to come down with all accessories off for about two to three minutes. Then turn on your a/c with blower in high position for three minutes. This should fix it.

What is ideal idle AFR? ›


With 14.6 being the industry standard stoich AFR value, we typically like to see these conditions run slightly richer.

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 I set my carburetor to use less fuel? ›

Once the engine has arrived at operating temperature, go back to the carburetor and adjust the air fuel mixture screw, or screws. Tightening the screw increases the amount of fuel, while loosening decreases the amount of fuel.

Does float level affect idle? ›

Does float level affect idle? Yes, the idle can be affected by the float level if it's too high or too low. When it's too high, the idle may be slightly lower because the fuel mixture is richer, and vise-versa.

What happens if carburetor gets too much air? ›

An engine “runs lean” when the balance of fuel and air gets thrown off. Typically, there's a 12:1 or 15:1 ratio of air to fuel, and when there's too much air or not enough fuel, it causes sneezing or popping sounds in the intake.

What are the symptoms of a carburetor running rich? ›

Rich Mixture Symptoms -

1 - Engine acceleration is flat, uneven, not crisp. 2 - The throttle needs to be opened continuously to maintain consistent acceleration. 3 - The engine performs poorly when the weather gets warmer, or the engine works better in cold conditions. 4 - Black smoke coming out of the exhaust.

How do you tell if an engine is rich or lean burn? ›

Rich-burn engines generally operate with (λ) equal to 0.995. Lean Burn: Lean-burn engines operate with an AFR that has a lower concentration of fuel to air, making it a “fuel-lean” mixture. Lean-burn engines operate with (λ) anywhere between 1.5 and 2.2.

What is H and L on carburetor adjustment? ›

Start by locating the three adjustment screws. They are typically labeled L (low speed jet), H (high speed jet), and I (idler jet).

Which way do I turn idle screw to idle? ›

Tighten the screw clockwise to increase the idle speed.

If your idle speed is too low, use a Phillips head screwdriver to turn the idle screw clockwise. If the coil is vertical and the screw is at the bottom, you need to turn the screw counterclockwise if you're looking at it from above.

How do you adjust idle air screws? ›

Turn the screw counterclockwise and clockwise 1/2 a turn from the middle position and listen to the sound of the idle. Set the screw in the position where the engine's idle sounds most even and smooth to balance the fuel mixture.

What will cause a carburetor to idle high? ›

Possible Causes of High Idle Speed

In vehicles that have carburetors, a bad accelerator pump or power circuit may be to blame. The engine may be overheating, and the cooling system might need repair. The fuel pressure regulator may be operating at too low of a pressure and might need to be replaced.

What does the L screw do? ›

The L-limit stops the derailleur from moving inward (toward the spokes). The limit screw does not make the derailleur move, pulling the inner wire makes the derailleur move. The L-screw allows the pulley wheels to shift the chain to the innermost sprocket and yet not shift off the sprocket into the spokes.

Does pilot screw affect idle? ›

Does the pilot jet affect idle? Yes, the pilot jet definitely affects the idle. You don't need to adjust the idle screw until you have properly adjusted the pilot jet circuit and air screw or fuel screw.


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