The piston is the heart of an SU carburettor. It rises and falls with engine demand, carrying the needle in and out of the jet. If it sticks, even slightly, the mixture goes wrong. The SU workshop manual lists a sticking piston as the first cause of erratic running, stalling at idle, lack of power and high fuel consumption.
Step 1: the lifting pin check
Most SU carburettors have a small spring-loaded lifting pin on the side of the body, just below the suction chamber (on some models you lift the piston with a finger through the air intake instead).
- Remove the air cleaner.
- Raise the piston with the lifting pin and let it go.
- The piston should fall freely and land on the bridge of the carburettor with a distinct click.
If it falls slowly, sticks part way or doesn't click, something is wrong.
Step 2: find out whether it's the jet or the piston
Lower the jet fully and try again. If the piston now falls freely, but only drags with the jet in its normal position, the jet is out of centre and the needle is rubbing on it.
Centring the jet (H, HD and HS types)
- Disconnect the jet linkage. Remove the jet, take out the jet locking spring, refit the adjusting nut and screw it fully up.
- Refit the jet, keeping the jet head in its correct position relative to the lever.
- Slacken the jet locking nut until the jet assembly is free to move.
- Remove the damper and press the piston down onto the bridge (a pencil on top of the piston rod works well). The needle will centre the jet.
- Tighten the locking nut, keeping the jet hard up against the adjusting nut, then check the piston drop again.
- Reassemble and top up the damper with SAE 20 damper oil.
A bent needle will also drag in the jet however carefully you centre it. If the needle is visibly bent, replace it. Our needle pages show the full profile of every needle so you can match yours.
Step 3: clean the piston and suction chamber
If the piston still sticks with the jet lowered, the problem is in the piston and suction chamber.
- Mark the chamber's position, unscrew it and lift it off without tilting, then lift out the spring and piston.
- Clean the inside of the chamber and the piston rod guide with petrol or methylated spirit, and wipe dry. Never use abrasives — the piston and chamber are a matched pair with a very small clearance.
- Check for scoring or damage. A dented or scored chamber won't come right with cleaning.
- Reassemble without oil first and check that the piston moves freely, then refill the damper.
Step 4: the timed piston drop test
If you still can't find the cause, the workshop manual gives a timed test for standard (non-ball-bearing) suction chambers on HS and HIF carburettors. It checks the clearance between the piston and chamber.
- Temporarily plug the transfer holes in the piston.
- Fit the piston into the chamber without its spring.
- Fit a nut and screw with a large flat washer into one of the chamber fixing holes, so the washer overlaps the chamber bore.
- Fit the damper and washer.
- With the piston fully home, turn the assembly over so the chamber falls away until the piston rests on the washer.
- Time how long the chamber takes to fall the full length of the piston travel.
For carburettors from 38.0mm (1½in) to 47.6mm (1⅞in) bore, it should take 5 to 7 seconds. If it's outside this, check for oil, dirt and damage. If it's still out after re-checking, the suction chamber and piston need replacing as an assembly.
On ball-bearing chambers, check instead that all the balls are present (two rows of six on HS types). Then hold the assembly horizontal without the damper and spring, and spin the piston. It should spin freely without any tendency to stick.
Replacement pistons and suction chambers
Pistons and chambers are matched, so they're replaced as an assembly. Browse our pistons and suction chambers, or use Find My Kit to see the correct assembly for your carburettor. Check that the keyway and neck style match yours before ordering.
Still not right? Our fault finding chart covers the other common causes. Or ask us.