Checking and cleaning an SU carburettor

The SU carburettor is straightforward to strip. it’s just one jet (except during a few special types) and really few moving parts. it’s very reliable, but it can become dirty and choked — and a few parts wear out. The most common sort of SU is that the HS. The HIF, overleaf, differs in some ways. they ought to be faraway from the engine for cleaning.

Removing the dashpot

Unscrew the damper and pull it out of the ‘ dashpot ‘ (properly called the piston chamber). it’s going to pull straight out, otherwise you may need to unfasten a circlip to free it. Mark how the dashpot fits on to the most body in order that you’ll replace it because it was. If it’s turned, the piston may stick. Remove the screws holding the dashpot. Lift it off. Pull out the piston carefully, straight upwards. it’s easy to bend the jet needle.

Removing the jet needle

There are dozens of jet-needle sizes: the type number is stamped on the shank of the needle. Remove the needle by loosening the grub screw in the side of the piston. Early carburettors have a rigid needle with a shoulder or groove which must fit level with the bottom face of the piston. Later models have a flexibly mounted needle. This hangs at an angle and must be fitted the right way round. Look before you remove it. There may be an etch mark pointing either towards or away from the butterfly plate.

Alternatively, there could also be a notch which lines up with a groove within the needle guide. Renew the jet needle if it’s bent or looks worn. Wear is tough to assess visually on a centred needle. The needle wears imperceptibly over a period by fuel flowing past it at the mouth of the jet. Wear spots will show as shiny patches on the needle if it’s not centred – particularly at the thick endways a flexibly mounted needle, which actually bears against the lip of the jet on one side. If unsure , renew the needle at major service intervals or after two or three years, consistent with mileage. Or examine it closely against a new one.

Removing the jet assembly

If the carburettor hs a plastic emission-control seal prise it off and throw it away. Disconnect the jet from the choke linkage . On older models you are doing this by removing one self-tapping screw – don’t lose the spacer washer under it. Alternatively, there could also be a circular, toothed spring clip. Lever this off this with a screwdriver. The largest models have a different linkage. there’s a fork with two clevis pins held in situ by split pins. Sometimes the linkage includes a bimetallic disc for adjusting the mixture to suit the temperature. Pull out the split pins and clevis pins.

Unfasten the nut which holds the jet tube on to the float chamber. Pull the tube free. a small rubber sealing ring will probably stay inside the joint. Prise it out with a small screwdriver. Pull the jet out of the carburettor. check out its hole. If it’s worn oval, renew the jet. Prise off the spring clip on the choke linkage. Undo the nut holding the jet tube to the float chamber.

Cleaning and reassembly

Wash all parts in petrol and blow through the jet to clear it – use a foot pump if you have one. To reassemble the jet, slide the metal ferrule into the end of the jet tube. Fit the metal washer over the end, then the rubber seal. Leave a minimum of 5 mm of tube protruding of the end of the seal. Insert the tube into the float chamber. Tighten the nut. When you reassemble the dashpot, note that the piston has a groove in one side and fits in just a method . If the carburettor has a rigid needle it must be centralised within the jet.

Removing the float

Disconnect the fuel pipe from the top of the float chamber. Remove the three screws holding the highest on the float chamber. Lift the top off with care: the float hangs from it and its pivot arm is easily bent, which can upset the critical reserve within the chamber and cause either fuel starvation or flooding of the carburettor (See Adjusting an SU carburettor ).

Check rock bottom of the bowl for sediment and take away any by dislodging it with alittle screwdriver. Wash out the bowl with clean petrol and wipe dry with a lint-free cloth. Replace the top with a new gasket and tighten the screws evenly. Refit the fuel pipe to the chamber top.

Detaching the float and valve

Pull out the float pivot pin with pliers. Mind you do not drop the float as it comes free, or lose the needle valve as it drops out of its housing. Use a spanner to unscrew the housing from the chamber top: there may be dirt inside it.

Look at the needle valve . If a groove has been worn at the pointed tip, it cannot seat properly and should allow the chamber to flood – to overfill with fuel. Renew it. After cleaning and reassembly, check the float height, which governs the extent of fuel within the chamber. Hold the chamber top upside-down and level. The gap between the float and therefore the top should be between 8 in. and in. (3mm and 5mm). If the gap is wrong, you’ll sometimes adjust it by bending the float arm – but only the arm is metal. you can’t adjust an all-plastic float.

Bend a metal arm carefully at its outer end. don’t bend it at the float end, just in case you loosen its fastening to the float, which can cause a leak. If the float has a crack or split, don’t seal it with glue. the load of the float is critical. you want to replace it with a new float.

Removing the choke linkage

Remove the linkage to wash the carburettor. The retaining plate is held on by two screws. Usually there’s a square hole within the fast-idle cam . Turn the cam to reach one among the screws through the opening . Otherwise, remove the cam. With the retaining plate free, pull the choke assembly out, taking care to not damage the seals. Undo four screws to remove the baseplate from the float chamber. don’t lose the rubber sealing ring.

Leave a Reply

Scroll to top