Here’s the front of one of my old SUs…I’m getting ready to pack them up so I can get my core refund. That’s the piston that you’re looking at through the front of the body. The jet is attached to that little black knob at the bottom of the pot and the mixture is adjusted by turning the hex nut, which simply changes the height of the jet in relation to the level of fuel in the float bowl (the little cylinder to the left). When the jet is lower, the mixture gets richer. When you activate the choke, it just pulls down the whole jet assembly.
The SUs in my previous post were rebuilt and reconditioned by Z-Therapy. Not only do they do a beautiful job cleaning and polishing the carb bodies, but they also do some fancy machining to fix a design flaw in butterfly shaft journals. These would wear out, creating a vacuum leak that screws up the mixture. ZTherapy also has a great video on installing and tuning SUs.
Ztherapy is so named because Datsun licensed the SU design for use on their 1600 and 240Z sports cars, creating a fairly large market both from racers and wannabe wrenches like me. Unfortunately, the Japanese versions of the carbs were manufactured with metric dimensions and their English counterparts with Imperial dimensions. The parts are almost identical, but not interchangeable.
Posted by Rich Roat on November 20, 2006