The variable speed carburettor is of a conventional air-bleed style. The arm rotates a transversely drilled drum within the body, to restrict the air intake flow. The fuel/air ratio at high speed is adjusted with a fuel metering needle valve and controlled during transition to low speed with an adjustable air bleed inlet. I made and trialed several prototypes before finalizing the design. I had to achieve a balance of maximum power, fuel/air suction and reliable transition between low and high speed. The optimum hole sizes and positions were determined by trial with the first pre-production carburetors.
The body and actuating arm are injection moulded from a 40% fiberglass reinforced polyphenylene sulphide resin. (PPS) This is a process and a material of choice, not expediency, and they allow a shape and accuracy that make for efficient operation. A mould was needed and although high speed production and minimum material use were not considerations, the process required that it was made to a good level of toolmaking technique. Following moulding, the body must still undergo twelve separate machining operations before it is complete.
The rotating drum is machined from steel bar and case hardened before being ground to an accurate size to give an airtight seal and extended life. In total, there are seventeen components to the carburettor, each of which is made by moulding, turning, milling and grinding similarly to the major engine parts.
The following links are videos in AVI format. To download, please right click on the link and choose 'Save Target As' from the popup menu.
You can select to enlarge any of the thumbnails or to play the clips and then choose from the menu at left to read more of the manufacturing detail.
