Dashpot Oil (mossmotors.com)

Carb (SUs, Strombergs, etc.) Dashpots
Poor acceleration and “sputtering” during acceleration may be due to a low oil level in the carburetor dashpots. Automatic transmission fluid works well in some carbs, but not in others. The old recommendation of “the same oil as used in the engine” is a good place to start. If this gives too lean a mixture on acceleration, try a slightly heavier oil; if too rich, then a lighter oil is indicated. Fill to within 1/4 inch of the top of the hollow air piston rod. Do not overfill!