Topping up the dashpot oil level

Guide Carburettor dashpot oil top up on SU and Stromberg carburettors.

Guide

Tools needed

• Correct oil for make
• Rag to clean
• Grips if lid (screw top) is very tight
• Torch or light to check level if bad lighting

Time: 15 mins
Difficulty: very easy

The SU type carburettor has a tapered needle that slides in an adjustable jet to increase or decrease the petrol flow. The needle is attached to a piston assembly which slides and down the suction chamber.As the throttle buttefly opens, air is draw in through the piston assembly from the suction chamber the piston, the piston rises lifts the tapered needle out of the jet, and increases the oil-filled damper assembly in the top of the suction chamber and limits the speed at which the piston rises.
If there is no oil in the damper, the piston will rise quickly causing engine hesitation and stalling. check the damper oil-level regularly and top up if necessary.

Unscrew the cap on top of the carburettor suction chamber Damper cap lift and withdraw the damper unit.

Pour in light oil SAE 20 on SU carburettors, and a special Zenith dashpot oil on a Stromberg CD unit.

Fill the suction piston guide rod to within 1/4 in (6.35mm). of the top of the rod. Gently replace the Damper Guide rod damper, push it slowly down and screw the cap fully home.

Check the action of the piston by lifting the air valve through the air intake opening. It should resist the lift and rise slowly under pressure if the oil is at the correct level and the damper in good condition. If not, fit a new damper assembly.