The 1" OM SU Carb9th of November, 2019
The OM is a 1" bore SU carb fitted to prewar Morris Minors, Austin 7s and in pairs on some MGs. In rare setups triple OM Carbs are fitted. 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 carburet`tor bore.
It has no fast idle feature when the choke lever is pulled, 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.
The standard piston lid was often a black bakerlite material, but many enthusiasts replace this with a brass equivalent.
Photos of OM SU Carb
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