O.S. Type 4BK, 4D, 5B Carburettor Operating Instructions

About this manual

This manual provides operating instructions for the O.S. Type 4BK, 4D, and 5B carburettors, which feature an automatic mixture control device to ensure a balanced fuel-air mixture at all throttle settings. The document details three adjustable controls: the Needle Valve for full-throttle mixture strength, the Mixture Control Valve for part-throttle and idling speeds, and the Throttle Stop Screw for setting the closed rotor position.

What's inside

• Operating Instructions
• Needle Valve Adjustment
• Mixture Control Valve Adjustment
• Throttle Stop Screw Setting
• Realigning Mixture Control Valve
• Carburettor Cleanliness
• Parts List

Specifications

Frequently asked questions

How do I adjust the mixture control valve if the engine idles unevenly?

If the engine idles unevenly and smokes, it is probable that the idling mixture is too rich. Turn the Mixture Control Valve clockwise 30 to 60 degrees.

What is the corrective action if the engine stops when the throttle is closed?

If the engine stops or idles briefly before stopping, the mixture may be too lean. Turn the Mixture Control Valve about 30 to 60 degrees counter-clockwise.

How can I realign the mixture control valve if it was adjusted too far?

Screw in the Mixture Control Valve until looking into the rotor hole, then gradually unscrew it until 'A' is precisely tangential to the rotor hole.