The animated drawing above gives a simplified picture of what is going on in your SU carburetor when you "floor it". Note that the piston (orange in the drawing) is a bit behind the butterfly valve (gold) which is directly connected to the accelerator pedal. This is the function of the bakelite (or brass) 'dipstick' that screws into the top of the 'pot', it interacts with the oil in the center of the piston, slowing the rise but not affecting the fall of the piston. The restricted air flowing over the jet (blue) moves faster and picks up more fuel for an accelerator pump effect. The heavier the oil, the more pronounced the effect. The piston is pulled up by vacuum (or pushed up by ambient air pressure, really) from the plenum which overcomes spring pressure, and (slowly) the oil valve, to raise the piston. As the piston rises so does the tapered needle (purple) which allows more fuel to flow from an increasing annular ring between the jet and needle. There are a LOT of SU needle variations out there along with programs to help you pick the right one, several entries in the links section of this site address this in depth. The large nuts on the bottom of the SU simply raise and lower the jet assembly for leaner or richer mixture, so getting the float level right is very important in correctly tuning the SU.

I hope the graphic makes clear how many fairly precise sliding "fits" the SU carburetor depends on for good performance. Sub standard air filtration will cause damage over time, to your engine and your carburetors, as well as robbing you of fuel efficiency in an increasingly expensive market.

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