SU Carburettor Float Level Settings

If your SU carburettor floods, runs rich at idle or starves at high speed, the float level is one of the first things to check. The float sets the fuel level in the float chamber, and that level sets how much fuel reaches the jet before the needle has done any metering at all. Get it wrong and no amount of mixture adjustment will make the carburettor run properly.

On every SU carburettor the fuel level is set by the float hinge lever, not by packing washers under the needle valve or bending the float itself. The figures below are the factory settings for each float chamber type.

Float level settings at a glance

Float chamber / float type Setting How it is measured
T1 float chamber (1⅞in outside diameter) — H type 7/16in test bar Lid inverted, lever resting on needle; bar should just slide between lever radius and lid face
T2 float chamber (2¼in outside diameter) — H and HD types 7/16in test bar As above
T4 float chamber (3in outside diameter) 5/8in test bar As above
HS type with brass float 5/16in test bar As above
HS type, early nylon float with steel needle 0.125in – 0.187in gap Gap between lever and lid face
HS type, Delrin (spring-loaded) needle with new-geometry float and lid 0.125in – 0.187in gap Float arm resting on the needle plunger without depressing it
HS type, Delrin needle with later all-nylon float 0.062in – 0.187in gap As above. This float is not adjustable
HIF type (integral float chamber) 1.0mm ± 0.5mm (0.04in ± 0.02in) below the float chamber face Body inverted so the needle is held shut by the weight of the float only

How to check the float level on H, HD and HS carburettors

  1. Remove the float chamber lid and turn it upside down so the hinge lever rests on the float needle under its own weight. Don't press on it.
  2. For bar-type settings, slide a drill shank or rod of the correct diameter (7/16in, 5/8in or 5/16in) between the radius of the hinge lever and the machined face of the lid. It should just pass, without lifting the lever.
  3. For gap-type settings, measure between the lever and the lid face with feeler gauges or a drill shank of the minimum and maximum size.
  4. If the setting is out, bend the hinge lever at the crank where the straight and curved portions meet — never bend the flat pad that touches the needle. Make small adjustments and re-check.
  5. Refit the lid with a new gasket and check for leaks once the system has pressure.

The later all-nylon float used with the Delrin needle is not adjustable. If it is outside the 0.062in – 0.187in range, check the needle valve and replace whatever is worn.

How to set the float on an HIF carburettor

The HIF has its float chamber built into the body, under a bottom cover. To check it:

  1. Remove the float chamber cover and turn the carburettor upside down so the needle valve is held shut by the weight of the float alone.
  2. Check that the marked point on the float sits 1.0mm ± 0.5mm (0.04in ± 0.02in) below the face of the float chamber.
  3. Adjust by carefully bending the brass pad that bears on the needle, then make sure the float still pivots freely on its spindle.

Check the needle valve first

A worn needle valve will give you a false reading and will flood however carefully the float is set. If the needle tip has a groove worn into it or the seat is marked, replace the needle valve and seat as a pair before setting the float. We stock the common sizes, including the VZX 1100 (0.070in bore) and WZX 1102 (0.096in bore) — check your carburettor's specification for the correct bore. The SUT 1 needle valve seat spanner makes removing the seat much easier.

Floats that sink or stick

Old brass floats can develop pinholes and slowly fill with fuel, and early nylon floats can absorb fuel and lose buoyancy. Either way the fuel level creeps up and the carburettor floods. Shake the float near your ear — if you hear liquid inside, replace it. Our StayUp float kits use a closed-cell float that cannot fill with fuel:

Symptoms of an incorrect float level

  • Too high: fuel weeping from the jet or overflow, a strong petrol smell, black smoke, rich running at idle, wet plugs, hard hot starting.
  • Too low: hesitation or fuel starvation at high speed or under hard acceleration, and a mixture you can't get rich enough at idle.

Not sure which float chamber or needle valve you have? Use Find My Kit to see every part for your car, or send us your carburettor spec number and we'll confirm it. For the full procedures for every SU type, the SU Workshop Manual (ALT 9525) is the definitive reference.