Carb of the Month: SU OM 1" Bore
The OM SU Carb is a 1" bore carb fitted to prewar Morris Minors, Austin 7s and in pairs on some MGs. It was often fitted to the Morris Cowley as a replacement for the Smiths 5 jet. The casting number on the main body is usually 617T.
Identification: The OM has no front mounting flange, and was often run withot an air filter of any kind. The rear flange has two mounting holes, one either side of the carburetor bore.
It has no fast idle feature, and no piston damper, so only a small amount of oil is needed in the top of the bellhousing. The piston is usually of the weighted type, without a piston spring. The piston assembly from a 1" H1 carb can be fitted, which will give you a sprung piston and a damper rod for improved acceleration.
The throttle plate can be very tricky to install and remove, as it only fits into the bore in the fully open throttle position. There were not any different bore diameters made of this type, although the UB and UBA carbs are very similar to this design.