Unscrew the brass or plastic cap on top of an SU carburettor and you'll find a small plunger sitting in oil inside the hollow piston rod. That's the damper. It's the simplest part of the carburettor and the one most often neglected. Yet when it runs dry, the car develops a flat spot that owners often blame on the needle, the jet or the ignition.
What the damper does
An SU is a constant-depression carburettor. When you open the throttle, the depression in the engine lifts the piston, which raises the tapered needle out of the jet and lets more fuel through. Without a damper the piston would shoot straight up the moment you opened the throttle. The air flow would jump ahead of the fuel and the mixture would go lean for a split second, which you feel as a hesitation.
The oil damper slows the piston's rise just enough to briefly enrich the mixture as the throttle opens, giving the clean pick-up an SU is known for. It does almost nothing once the piston is steady, which is why a dry damper shows up as a flat spot when you accelerate rather than as poor running at steady speed.
Which oil to use
The SU workshop manual specifies thin oil, preferably SAE 20. Our SU Damper Oil SAE Grade 20 comes in a bottle with a nozzle that fits straight into the piston rod.
Thicker oil slows the piston more and makes the car feel slightly richer and more sluggish on pick-up. Thinner oil does the opposite. The manual lists oil that is too thin as a cause of hesitation, so stick to the correct grade unless you have a specific reason not to.
How much oil: the correct level
It depends on which suction chamber you have.
Standard suction chambers (most H, HD, HS and HIF)
Unscrew the cap and lift out the damper. Top up until the oil is just below the top of the hollow piston rod, then refit the damper and screw the cap down firmly.
Ball-bearing suction chambers, early type
Unscrew the cap and raise the piston and damper to the top of their travel. Fill the recess in the damper retainer with oil, then lower the damper until the cap touches the suction chamber. Repeat until the oil is just visible at the bottom of the retainer recess. Make sure the bearing retainer isn't pushed out of position in the piston rod.
Ball-bearing suction chambers, later type
Unscrew the damper cap and withdraw the damper. Top up to within 6.5mm (¼in) of the top of the hollow piston rod, then refit the damper firmly.
Don't overfill. Excess oil just gets pushed out of the breather hole and makes a mess.
How often to check it
Check the damper oil at every service, and any time the car develops a hesitation on acceleration. On a car in regular use, a quick check every few months is sensible, because the oil slowly creeps past the damper and is drawn into the engine.
Signs your damper needs attention
- A flat spot or hesitation when you press the throttle, particularly from low revs
- Oil level drops quickly between checks, which can point to a worn damper or piston rod
- A cracked or missing cap washer, or a damper that rattles
Worn or damaged dampers and caps are available in our dampers and caps range. If topping up doesn't cure the hesitation, see our SU fault finding chart and check the piston moves freely with the piston drop test.
Pick up a bottle of SU damper oil with your next order. Or get it with the jet and needle valve spanners in our Find My Kit workshop essentials bundle.