Triumph Repair: 1959 tr3, constant velocity, true air

Question
I know it may be out of your field but my question is, how dose the choke on the su card. work? Seams to just advance the throttle. Will take your advice and look for the Robert Bently manual u recomended. Thanks P.S. is there a tr3 expert?

Answer
Hi Keith,
The "choke" on all SU carburetors is not a choke like on a fixed jet carburetor. It lowers the jet which lowers it in relation to the metering needle thus allowing a richer mixture. There is a cam on the linkage that does open the throttle at the same time. Even though the SU carburetor seems simple in design with very few parts, it is designed to keep the air flow over the jet at the same velocity no matter how much volume of air is going through it. The vacuum piston and it's metering needle controls the mixture all through the RPM and load range.
The adjustment of this "Constant Velocity" carburetor is process covered in the Bentley manual and I also cover my version of adjustment on my web site at http://mg-tri-jag.net/tech16.htm
The SU is a carburetor with true air flow controlled metering of fuel and is a step between a conventional carburetor and fuel injection. Once you learn how to adjust the SUs, you will not want any other kind of carburetor.
Howard