Carburateur Service Nederland
SU Diagnose Center
Is your engine with SU carburettors running differently from what you want? Tell us what you notice while driving. You get advice on which needle fits your complaint.
- Four short questions about your carburettor, your engine, your needle and what you notice.
- Needle advice from 733 original SU needles, with the explanation.
- A 3D model that shows what your needle does at every engine speed.
- Filling in is free. You unlock your advice and the 3D model with a one-off contribution of 5 euros.
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Tap your type. The type is often on a tag on the carburettor. You can also measure the round opening on the air filter side.
Help: how to recognise your type
HS: the float chamber is a separate bowl on the side.HIF: the float chamber is built into the bottom of the carburettor, the underside is a flat cover.Do you have an older type H or HD? Choose the tile with the same bore.In doubt between two tiles with the same bore? Both use the same needles, the advice stays the same. -
How many carburettors are fitted?
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Type the letters or numbers stamped on your needle, for example FX, AAU or 5.
Help: where is the name of the needle?
The name is stamped into the shank: the thick top part of the needle that sits in the piston.To get there: undo the screws of the round chamber on top of the carburettor, lift the chamber straight up and take out the piston. Undo the small screw on the side of the piston and pull out the needle.Put the piston on a clean cloth and keep the needle straight. The letters are small, a magnifying glass or a photo with your phone helps.I do not know the name of my needle yet
We will look up the standard needle for you. Enter your car and send us the question. -
Tick what you recognise. More than one is fine. For each tick we ask one follow-up question.
In doubt between too lean and too rich? Do the choke test. Drive with a warm engine until the stuttering starts, then pull the choke out a little. If the engine runs smoother, the mixture is too lean. If it runs heavier or black smoke appears, it is too rich.Measure with a warm engine at the revs where the complaint occurs, for example on a rolling road. Low CO means lean, high CO means rich. The input fields now appear with each tick above. -
View it in 3DSee what your needle does at every engine speed, with charts and all dimensions.
Drag to rotate, scroll to zoom
The 3D view starts in a browser with WebGL. The charts and the table below work fully.
Piston lift0
Measuring point1
Annulus area0
Air : fuel (estimate)13,5 : 1
leangoodrich
Your needle and the advised needle
Driving
Mixture across the rev range
Needle profile per measuring point
The thinner the needle, the more room to the jet wall and the more fuel.
All measuring points in numbers
How to read an SU needle
The needle the factory chose for a car is the starting point; a different needle is for an engine with its own needs.
- Points 1 and 2
- Idle and zero point. When changing, choose a needle with the same dimensions at these two points. If the new needle is thicker here, the rest turns out slightly richer than the dimensions suggest; if it is thinner here, slightly leaner.
- Points 3 to 6
- Pick-up in top gear from about 20 to 50 mph, and part-throttle driving.
- Points 7 to 9
- Top speed and full throttle.
- Last points
- On a small carburettor (1 1/4 inch) the last three points are not used: the piston does not reach them.
- Step size
- From lean to rich: try a needle that is about 0.002 inch thinner at that point. From rich to lean: at most 0.001 inch thicker per step.
- Measuring points
- The thickness of a needle is measured at fixed positions. Point 1 is at the top by the shoulder, each following point 1/8 inch (3.2 mm) further towards the tip. The SU booklet calls them stations.
Frequently asked questions
What does an SU needle do?
In an SU carburettor the piston lifts a tapered needle out of the jet. The higher the piston, the thinner the part of the needle in the jet and the more fuel is added. The shape of the needle therefore sets the mixture at every engine speed and throttle position.
How do I know whether my mixture is too lean or too rich?
Too lean shows as light or white spark plugs, popping in the exhaust on the overrun and an engine that runs smoother with a little choke. Too rich shows as black, sooty plugs, a smell of petrol and high fuel consumption. A CO meter measures it precisely.
Where do the needle dimensions come from?
From the original SU literature: 733 needles for the .090, .100, .125 and 3/16 inch jets, fixed and spring loaded. Every table has been checked digit by digit.
Can I order the advised needle from you?
Yes. Copy your request below the advice and send it to us. We supply the needle or rebuild the whole carburettor for you, with worldwide shipping.