Carburettor Tuning Tips For Drag Racing

The carburettor mixes the air and fuel and sends the proper amounts through the intake manifold to the engine at the right time. There are numerous adjustments possible to affect how the carburettor operates and this in turn the performance of the engine. There are different opinions on how to tune the carburettor to get the optimum power and performance …. it is in fact a bit of an art form on its own. This article covers few tips for carburettor tuning with a focus on drag racing where excellent performance is require in and from idle to wide throttle.

Install a bigger correctly sized carburettor

The logic is that more the air and fuel mix that the engine can be supplied through the carburettor, the more power can be produced by the engine. Carburettor size is measured in cubic feet per minute (CFM) of air. The larger the CFM number, more the air that the carburettor can handle.  A stock carburettor has an air flow of 500 – 600 CFM. For a drag car to achieve sub-ten second passes, its carburettor CFM needs to be double that of the stock one.

It is equally important to have the correct sized carburettor. It enables the tuning process with the appropriate air speed through its venturis. The right size for the carburettor should not be based on just the horsepower and should include factors such as cubic centimetre of the engine, camshaft duration, intake manifold type, transmission type, etc.

Setting the correct fuel pressure

After selecting the carburettor another important step in the tuning process is making sure of the correct fuel pressure followed by setting the float adjustment correctly. Low fuel pressure impacts the float and results in problems during start-up. High fuel pressure can make an engine run lean.

Resolving a lean condition

A lean carburettor adjustment is caused by low fuel proportionately in the air/fuel mix and results in poor engine performance and even engine damage. This can be determined by reading the spark plugs or using an air/fuel ratio monitoring system. A jet change may not be the required solution and for an overall well-tuned engine it could be resolved with air bleeds.

Jet selection

With a bigger carburettor, a higher airflow is made possible to meet the demands for drag racing engines. However, changes are required for the carburettor to flow sufficient fuel to keep up with the increased air flow and prevent the engine running lean.

Performance carburettors are equipped with replaceable jets. The jets do most of the fuel metering and larger jets can be installed for more fuel to match the higher air flow for the engine to produce more power. The main jet controls the fuel at wide open throttle and the balance between the main and pilot jets controls the carburettor off-idle response. Care should be taken when making these changes as larger than ideal jets are not best suited for the engine nor may it be the solution for a rich idle condition.

Initial ignition timing

The ignition system ignites the correct air/fuel ratio at the correct time. Ignition timing is affected by various factors such as fuel type, fuel quantity injected, fuel injection timing, spark timing, combustion chamber shape, engine temperature, compression ratio, etc. The carburettor tuning cannot be the best unless the ignition timing is correct. The initial ignition timing provides a clean off idle and crisp throttle response.

While machines and tools are available for carburettor tuning, superior quality tuning comes from years of experience including an understanding of the car, race and track conditions. It covers a range of simple and complex work, including idle mixture adjustments, high speed mixture adjustments, replacing and resizing jets, choke linkage adjustments, air bleeds, changing venturis, metering blocks and more. If you are unsure, we encourage you to use the services of carburettor tuning specialist, and we are a highly experienced team of specialists.