Checking and cleaning a fixed-jet carburettor

Here you can get how to Checking and cleaning a fixed-jet carburettor.

Fixed-jet carburettors are easily blocked by small dirt particles and chemical residues from fuel , because some of the jets are very small. Dismantle and clean them about once a year. Jets wear out after a couple of years. In some cases you’ll replace them with new ones of the proper size from a carburettor service kit. On other types the jets aren’t removable, and you want to obtain a new or exchange replacement carburettor. Consult the car workshop manual to seek out if the jets are renewable.

Replace all gaskets after every cleaning. you’ll also need new rubber diaphragms for the accelerator pump and therefore the economy device which is usually an integral a part of it. Take extra care to not allow any fluff or dirt to enter this sort of carburettor as you clean it. Dismantle it on a clean tray in order that none of its many small parts – like balls and comes in one-way valves are lost if they fall out.

Make notes and drawings as you’re employed in order that you’ll replace all parts exactly as they were. Count and note the amount of turns needed to get rid of any adjuster screws, and wind them back to the same setting when refitting. Remove the carburettor from the car (See Removing a carburettor for cleaning ), keeping it upright to avoid spilling fuel. Clean the surface to prevent dirt entering. Use a lint-free cloth in the least times. there’s no got to remove minor surface discoloration.

Wash all parts except rubber diaphragms in petrol. Keep diaphragms clean and dry. After reassembling a carburettor, oil the external moving parts with thin machine oil of the sort used on sewing machines.

Removing the top

Disconnect the linkage between the throttle and choke lever arms. this might be secured with circlips which you’ll ease off with a screwdriver; or by split pins which you straighten, pull out and discard. Always use new split pins when reassembling, or circlips if they’re damaged during removal.


Remove the screws holding the top and bottom of the carburettor together. lift off the top and take away the gasket. If the gasket is stuck to the flange , carefully prise it off with a sharp knife and clean the face of the flange. There is probably a ball valve in the accelerator-pump circuit in the bottom of the carburettor. Slowly turn the bottom over to pour away any fuel left in it.

Be ready to catch the ball, and maybe a weight which rests thereon , as these fall out. Note where they belong. Gently wipe out any loose dirt within the bottom of the float chamber with a clean rag soaked in petrol. lookout to not rub dirt into any jets or passages inside the chamber.

Removing the diaphragm

Be careful to not nick or tear the rubber diaphragm of the accelerator pump as you remove it. The diaphragm may stick to the retaining cover or to the carburettor body. That also applies to the diaphragm of any economy device.

Remove the screws of the diaphragm cover, take off the cover, then gently detach the diaphragm. there’s likely to be a spring behind it: don’t lose this. Inside there are other removable parts, like a piston. Check the diaphragm minutely for holes or cracks. If holes are found, replace the diaphragm. A service kit for the carburettor should include gaskets and diaphragm.

Removing the float and valve

Remove the float to clean the needle valve which it controls. The float is delicate and simply damaged, so lookout . Usually you merely push out the pivot pin and lift the float off. Unscrew the needle valve, wash it and blow through it with a foot pump to clear it. If the needle or jet are worn, replace the entire valve. A worn valve may cause flooding – but there are two other possible causes.

One may be a leak within the float, which makes it sink. Shake the float: any fuel in it means that it’s leaking and should be renewed. The other cause of flooding is an incorrect float level. The pivot pin is held in two mounting posts, one among which is split. Free the pivot pin by inserting a really small screwdriver within the split and gently levering it open. Push the pin out as soon as it can be moved freely. The post is an alloy casting and is brittle, so lookout because it will break easily.

Checking the float height and the needle-valve assembly

The correct height setting varies between carburettors, but the setting for your particular one should tend during a service manual for the car. Otherwise, consult your local dealer. The setting is measured in several ways, consistent with the sort of carburettor. On most of these which have the float pivoted under the carburettor top, invert the top and insert the shank of a suitably sized drilling bit between float and top.

On others, position the top in order that the float depresses the needle valve, then measure the space between the underface of the carburettor top and the bottom of the float – usually with the gasket fitted. This measurement should be made with a 150 mm steel rule. With certain carburettors – notably some Solex types – the measurement is to the marked centreline of the float, or its underside. Adjust the peak by bending the tag on the float arm, which rests against the needle valve. Usually there are two tags – the opposite one stops the float from falling too far, and its setting doesn’t need to be exact.

Do not bend the float arm where it’s attached to the float – in doing so you’ll damage the float and cause it to leak. A leaking float should not be repaired, for the load is critical. If you seal it by adding a blob of solder or epoxy glue, the load of the float and therefore the fuel intake is affected.

Keeping the jets clean

There are four sorts of jet, though a carburettor may have more than one ofll|one amongst|one in every of”> one among each – for example in a twin-choke carburettor. Each of the carburettor jet caters for a different part of the engines speed range. you’ll to some extent diagnose a fault during a particular jet by bad performance at the engine speed it governs.

The main jet is that the largest. It supplies fuel at full throttle openings and high engine speeds. it’s usually set either within the bottom of the float chamber or in an extension of the carburettor top which reaches down into the chamber. The idle jet supplies a small trickle of fuel for idling. it’s very narrow and easily blocked. Between idling speed and high speed, progression jets supply more fuel as needed. The progression system also includes an emulsion tube – a mixing device that’s longer than the jet and has holes within the walls.

The accelerator pump also has a pump jet. Trouble during this jet causes hesitant acceleration – but this might equally flow from to another fault within the pump, like a holed diaphragm. Usually jets and emulsion tubes are removable and have a screw squeeze its head. They’re made from soft brass and easily damaged, so use a screwdriver that matches the slot exactly.

If the jet doesn’t have a screw slot, it’s not removable. Clean it in place, employing a foot pump to blow air though it. Never blow through a assembled carburettor – if you are doing you’ll rupture the diaphragms, cause the float to collapse or do other damage. Weber carburettor have an air-correction jet fitted into the emulsion tube, and a main set during a jet holder which is inside the body of the carburettor.

Clean jets removed from a carburettor by washing them in petrol, and use a foot pump to blow them clear, but don’t poke them with wire. don’t even use a soft brush : it’d leave a bristle inside which might block the jet. If you’ve got recurring problems with the jets blocking, check the filter gauze at the carburettor fuel intake, if fitted.There could also be a hole within the gauze, which is letting dirt through.

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