Adjusting an SU carburettor

Tools you might need

An SU carburettor- paul-ryan.info Stub for pipe fromcrankcase ventilator Liftingpin Fuel-intakepipe Locking nut Throttle-to-jetchoke linkage Petrol feed to jet Jet adjuster nut Piston Dashpot screw-in top

An SU carburettor

The SU carburettor is to be found on many British-built cars. It needs a periodic check to see if it is in tune.

Unlike other types of carburettor, which have fixed jets, the SU has only one jet, so the mixture setting affects the engine throughout its speed range.

Some SUs also have exhaust emission-control devices to comply with anti-pollution laws. Adjustment of these is described in Adjusting an emission-control carburettor.

Before tuning the carburettor, carry out all the checks in Checking and cleaning an SU carburettor, and bring the engine up to normal working temperature.

Also check the level of oil in the dashpot - the domed piston chamber at the top of the carburettor.

The oil is there to slow the movement of the piston. The delay enriches the mixture briefly when the throttle is opened suddenly. SAE 20 engine oil is the correct grade to use when you top up.

On older SU carburettors, which have a vent hole in the screw-in plastic top of the dashpot, the oil level should be i in. (13 mm) above the top of the piston tube.

On dustproof SUs, which have a hole in the dashpot neck instead of the top, the oil should be i in. (13 mm) below the piston top.

If the engine is slow to respond to sudden throttle openings, or responds fluffily, correcting the dashpot oil level may cure the problem, so that you do not need to retune.

Air cleaner- paul-ryan.info

Air cleaner

The air cleaner in the front of the carburettor may have to be removed, but leave it on if possible.

However, if correcting the oil level does not resolve the problem, you may need to clean the carburettor (See Checking and cleaning an SU carburettor) before tuning it.

Whenever possible, make adjustments with the air cleaner in place removing it alters the partial vacuum inside the carburettor.

On cars where the carburettor is not very accessible, you may have to remove the air cleaner to reach some parts, then carry out further fine adjustments to obtain correct engine running later on.

Checking the mixture

- paul-ryan.info

Hook a finger under the lifting pin to raise the piston.

Use the piston lifting pin at the side of the carburettor to check the fuel-air mixture.

With the engine running at working temperature, hook your finger under the pin and raise the piston about 1/32in. (1 mm).

If, while the piston is raised, the engine speed increases briefly then returns to normal, the mixture is correct. If it rises and stays high, the mixture is too rich. If it falls and the engine tends to die, the mixture is too weak.

To correct the mixture, move the jet adjuster nut one hexagon flat at a time.

Screwing it up - anti-clockwise as seen from above - makes the mixture weaker.

Screwing it down - clockwise makes the mixture richer.

Each time you move the adjuster, wait for about ten seconds, then check again with the lifting pin to see whether the mixture is now right.

Take care that the engine temperature does not rise above normal - which it will if you take too long.

Such a rise will result in a false, overweak mixture setting which will show when the temperature returns to normal.

Before and after adjustment, check that the jet needle is central in the jet.

Stop the engine, use the damper to lift the piston to the top of its travel and let it drop.

It should fall smoothly with a sharp click. If it does not, the jet is out of line and you must centralise it.

An off-centre jet may also score or bend the tapered needle. Do not attempt to clean up or straighten a badly scored or bent needle. It must be replaced (See Checking and cleaning an SU carburettor).

Centralising the jet

- paul-ryan.info

With the air cleaner off, the piston can be lifted with a screwdriver. The jet should be screwed up as high as the bridge if possible.

Remove the air cleaner and the dashpot screw-in top and damper. Use a screwdriver to raise the piston.

Turn the jet adjuster nut up as far as it will go, or until the jet is level with the bridge inside the carburettor.

Slacken the large locking nut above the adjuster nut on the jet where it enters the carburettor body.

- paul-ryan.info

Hold the piston down with a pencil while you tighten the locking nut.

Now use a pencil or soft metal rod, such as a stick of solder, to push the piston right down. Hold it down and tighten the locking nut.

Check that the piston drops with a click - if it does not, repeat the centralising process.

Screw the jet adjuster nut down two full turns, which should bring it near enough to the correct setting for the engine to be able to run.

Refit the air cleaner and reset the mixture. Give the piston a final click check before you screw back the dashpot screw-in top and damper.

We also have this article in Spanish

The ultimate video course

We take this car to pieces and then build it again, explaining how every single part works.

By the time you finish watching this, you'll understand everything inside a car.

Watch us take a Mazda MX5 Miata to pieces, and then build it back together again into a modern working car.

  • 3+ hours of HD video available now. Production complete by July 2018.
  • Every part explained in detail.
  • We've created the most detailed 3D model ever produced so we can show you everything working.
  • Over 20 hours of footage — see the contents.
  • Buy now and get our 150 page PDF for free.

This is the most in-depth course on car mechanics ever produced. Buy now and save 75%.

Buy today for $20
Normal price $80.

Read more essential guides

Adjusting a Stromberg carburettor

The Stromberg CD carburettor, like the SU (See Adjusting an SU carburettor), is a constant-depre...

How the fuel system works - fixed-jet carburettors

The fixed-jet carburettor resembles the simpler variable-jet type (See How variable-jet carburet...

Preparing for carburettor adjustment

The carburettor is often suspected of causing an engine to run roughly, when the real source of t...

Checking and cleaning an SU carburettor

The SU carburettor is simple to strip. It has only one jet (except in a few special types) and...

Checking and cleaning a fixed-jet carburettor

Fixed-jet carburettors are easily blocked by small dirt particles and chemical residues from fue...



узнать больше