Rotary Engineering

Frequently Asked Questions (FAQ)
Any questions or suggestions should be sent to faq@rotaryengineering.com

Carburetor Tuning Basics - This subject is the most important topic on how to make horsepower. Majority of what I reference in the following paragraphs are directly related to carburetors with adjustable circuitry to modify fuel delivery curve with Emulsion Tubes, Air Jets, Main Fuel Jets and Chokes or Main Carburetor bodies. Along with these replaceable parts comes Throttle plate upgrades, and Throttle bore modifications to increase Air Flow delivery. If you are using a carburetor that uses these features to control fuel curve delivery you on the right track to obtaining your performance goals.

Weber Carburetors (IDF, IDA, DCO, DCOE)
Our Custom built Holley Carburetors utilize Weber Powerplates which incorporate Weber circuitry into a Standard Holley Carburetor.
Mikuni
Dellorto

When air flows through the main venturi a vacuum condition is created which draws fuel through the main fuel jets. The main fuel jet controls the fuel flow throughout the operating range. A larger main fuel jet will result in delivery of more fuel making the mixture richer. A smaller main fuel jet will result in delivery of less fuel making the mixture leaner.

The Air Jets control the mixture mostly at mid to high rpm (off idle circuit). The air jets are best described as a controlled air leak into the main circuit. The air jets work by "bleeding off" vacuum which draws fuel through the main fuel jets. A larger air jet will result in making the mixture leaner at mid to high rpm by bleeding off the vacuum signal. A smaller air jet will result in a richer mit to high rpm mixture by bleeding off less of the vacuum signal.

Emulsion tubes control the metered fuel and air introduced into the Carburetor. When air enters the emulsion tube through the air jet and fuel enters through the the main fuel jet this condition emulsifies the fuel delivered into the carburetor. The emulsion tube has a series of small holes from top to bottom which regulate the fuel mixture. These holes allow air and fuel to enter the main circuit and emulsify fuel. Low speed engine conditions or engines at idle do not require use of the emulsion tube or the main circuit. As engine speed increases the fuel level in the float bowl drops uncovering these holes and allowing air from the air jets to enter the main circuit resulting in a lean mixture. As the engine speed increases the fuel level in the float bowl continues to drop. This uncovers even more of the holes in the emulsion tube, which makes the air jet have a greater effect on the low to high rpm fuel delivery mixture.

Different Carburetor manufactures have different recommended fuel pressures settings. Changing fuel pressure will impact the amount of allowable float drop. If the fuel pressure is increased there will be less float drop and the mixture will be richer. On Mazda Rotary applications this can be a very beneficial characteristic since rotaries are well known for consuming large amounts of fuel and are notorious for running lean at High RPMS. All of the carbs listed above have standard recommended settings for the float level and float drop. Ask you local carb manufacture/re seller for standard recommended float level/float drop carb settings.