How To Adjust An Emission Control Carburetor

How To Adjust An Emission Control Carburetor?

Many countries have been asked to take measures limiting the emission of harmful gasses such as carbon monoxide (CO) with the advent of anti-pollution laws. Since cars play a key role in discharging these gasses and giving rise to global warming, many car manufacturers have started to design carburetors with an inbuilt emission control system to comply with these laws. However, problems can arise if these carburetors start acting up. This is why all drivers need to learn how to adjust an emission control carburetor.

Thus, to teach you the art of knowing how to adjust an emission control carburetor, we have curated for you the perfect guide. We will cover all the bases, whether it’s dodging false settings in a variable venturi carburetor or enriching the mixture in a SU-type HIF.

What is an emission control system?

Systems designed to limit or reduce the production and discharge of harmful gasses from the combustion engine are emission control systems. Noxious gasses are emitted from four primary sources: the fuel tanks, crankcase, carburetor, and engine exhaust.

The fuel tank and carburetor of a car emit hydrocarbons from gasoline continuously. When discharged into the environment, these hydrocarbons add to environmental pollution and harm the ozone layers.

Thus, many vehicles are employed with an emission-control carburetor, part of a car’s emission control system. It limits the discharge of hydrocarbons and other harmful gasses from the carburetor and fuel tank.

How to adjust an emission control carburetor

 A mere amateur can also know how to adjust an emission control carburetor in less than an hour. Emission control carburetors come in different types and settings. On some, the idle speed is adjustable, while in others, you can easily tamper with the mixture via a regulating screw.

Let’s discuss the four basic types of emission control carburetors:

  • Variable venturi carburetor
  • Stromberg CD2SE and CDSE
  • SU type HIF
  • Fixed- Jet Carburetor

1.     Variable venturi carburetor

What is it?

A carburetor consisting of components such as a suction passage that helps the airflow, a valve intended to create slidable movements and act as a variable venturi, etc., is known as a variable venturi carburetor. The venturi’s purpose is to increase the velocity of air. Thus, this low pressure yet the high speed of air pushes fuel to the airstream via a nozzle placed at the venturi’s center.


The seals in a Variable venturi carburetor are easily removable. Thus, its misplacement disbalances the default factory setting of the mixture. Before starting the adjustment process, you must always check what you can or cannot readjust.

Firstly, rule out all possibilities causing bad running. Then, look for the set screw and try turning them while keeping the air cleaner and air filter intact. Ensure not to burn yourself while doing so, as the will be dangerously hot.  

When you work on the adjustment, make sure that the car’s engine is at a normal running temperature. If the engine gets hotter than usual, it will produce false settings. If the readjustment of the carburetor doesn’t improve your car’s engine’s performance, then the last resort is to remove the carburetor and clean it.

2.     Stromberg CD2SE and CDSE

What is it?

The Stormberg constant depression (CD) carburetor works on the principle of constant vacuum. This compact carburetor is dustproof, and a concentric float chamber surrounds it. It is constructed using die-cast aluminum castings, making it very lightweight compared to other carburetors.


Stromberg CD has two emission control models: CD2SE and CDSE. The majority of these carburetors allow no room for needle or jet adjustment. However, the speed screw is adjustable and a trimmer screw that helps adjust the fuel flow to a little extent.

The speed screw should only be adjusted using gas analysis equipment. Some CDSE models have an adjustable needle height. In addition, provided no additional tampering is done, the jet heigh of many CDSE models is also adjustable.

3.     SU type HIF

What is it?

Designed specially to meet the exhaust emission control system’s requirements of your car’s exhaust, the Horizontal Integral Float Chamber (HIF) carburetor is a special carburetor. The HIF carburetor comes with variable CDs. This helps them attain the exact induction mixture and helps ensure that the toxic elements don’t exceed the statutory requirements.


Previously, putting nothing but a dab of paint on the adjusting screws adjusted such carburetors. However, plastic shrouds are placed around the jet and idling speed adjustors on the contemporary one.

The HIF model is a tad different from other SU models as it has a float chamber under its carburetor’s body while other models have one beside it.  

Meant to adjust the jet’s height, at the side of HIF’s body, there is a screw. You’ll find this screw placed beneath an aluminum plug. This plug is easily removable using a thin screwdriver.

To enrichen the mixture, turn the screw clockwise. However, if you want to weaken the mixture, turn it anti-clockwise. Some HIFs might also have an aluminum plug instead of a plastic shroud.

4.     Fixed- Jet Carburetor

What is it?

A fixed jet carburetor uses multiple jets to mix fuel and air. These jets regulate the amount of air and fuel they pass through. Like a variable jet-type carburetor, it is a venturi, which allows the air to flow towards the engine.


Placed over the volume screw in a fixed jet carburetor is an easily removable cap. Once you uncover the screw, it adjusts normally. Sometimes required will be a special screw head and a matching tool.

The throttle stop and the volume screw regulate the idling speed. However, there is also an extra screw that regulates the airflow and consequently alters the idling speed.

The bypass idle speed screw is the only screw that needs adjustment. In addition, it is the only one that can be accessed without the seal’s removal.


Who would’ve thought basic tools such as a screwdriver, an AFR gauge, and a vacuum gauge could be pivotal in maintaining a healthy car engine? So, pick them up and get to work. Knowing how to adjust an emission control carburetor is a life-altering skill and has the power to make your car run faster by ten folds. Just be wary of car fans and hot surfaces, and you’re good to go!