Carburettor dismantling and assembly

NOTE: Don't fiddle with Dell'Orto carburettors unless you have to. Every time one is taken down it loses a fraction of its precision fit and in practice you should use new gaskets. It will cost you money you could otherwise save. Also remember that they are generally very reliable and accurate instruments. In the Maintenance section fuel filter and float chamber cleaning has been described; this section will cover jet changing, needle changing and general cleaning. For the purposes of this section there are only two types of carburettor: there's the VHB type with 'square' slide, and the PHF type with round slide and pump jet. Most SFCs were fitted with the PHB instrument; this is effectively a PHF but without the pump jet, and can be discussed with the PHF.

Before tackling anything on your carburettors, fi/st ask yourself if it is strictly necessary. Then take a look at the breakdown illustration of your type and then try to assess whether you should remove the instrument from the machine. Lots of tasks can be undertaken with the instruments still mounted on the engine. These include: main jet changing, needle changing, throttle cable changing. Sometimes it helps just to loosen the inlet clamp to enable you to twist the body of the instrument around on its stub for better access.

The VHB is simple and conventional. The float chamber is fixed to the underside of the carburettor body by a brass bolt. Remove the brass bolt and the float chamber comes away; a special rubber seal sits in a groove on its mating face. This seal must be intact and fit the groove properly. New ones usually seat, old ones don't. Use a little Vaseline to hold it in place when replacing. Inside the float chamber is a twin float which pivots on a bar and thereby operates the float needle. The float needle has to seat in a drilling. Make sure the floats are not punctured (each is marked with a weight for accurate replacement) and that the float needle is not damaged or worn. Shake the float and listen for petrol slopping about. See the illustration which shows how to obtain the correct float height. The main jet sits in the other end of the brass nut which fixes the float chamber. It can be unscrewed with a proper screwdriver. It too should be marked with a number.

Three further 'jets' sit up inside the body of the carburettor. There's the idle jet, up in the centre, the short one; the choke jet and the emulsion tube, the long one. The choke jet is the one with the little rubber O ring. Starting from the other end of the carburettor, remove the slide cover by undoing the two screws which fix it to the body. Carefully pull out the slide, which runs in a special channel. You can see how the throttle cable's nipple fits into the slide body and how the spring and needle and its clamp fit. Note in what groove the needle should fit.

That leaves the rest of the choke mechanism which is hidden under the choke cover fixed by one screw. It hides a spring and small slide. Finally there are the idle screw (that's the large external knurled screw with a spring underneath it) and the mixture screw (beneath it and much smaller), outside the body.

'To keep the carburettor in good working order, you should occasionally carry out the following operations: clean the carburettor, washing and drying all the parts including the body and especially the drillings and jets, etc. Check the components for wear, in particular the slide, needle, idle jet and emulsion tube. Make sure the float needle does seal against its seat. Replace any worn or doubtful parts with new ones of the same size as before.' Use only petrol to clean the parts and do not poke jets with wire or pins; nylon bristles are suitable. Compressed air helps.

Petrol Carburetor Jet Pin

The VHB carburettor used on the GTL - cr~e- f tnents are similar

The PHF is more complicated, mainly because of its pump jet. (The pump jet squirts neat petrol into the venturi of the carburettor for instant throttle reponse).

From a dismantling point of view follow the sequence given for the VHB and for the moment ignore the diaphragm located just above the inlet tract. Once the float chamber is removed, remove the float and the float needle in the standard way. Hidden up the body of the carburettor you will now find the float needle drilling, which is removable and the main jet (in the centre this time, not in the brass chamber fixing nut) sitting in the atomiser. The atomiser is on top of the emulsion tube. All three should come out in order, changing the size of the screwdriver as you go. Then there's an 'inlet' valve and the choke jet. The former is the fat one' while the latter is the one with the O ring.

Again, starting at the other end, undo the two screws which locate the slide cover onto the body. Very carefully pull out the slide. On the PHF fitted to the Twins, the slide cover hinges a special plastic 'leg' located in a groove cut in the body and pivoting on the cover. This 'leg' is activated by the slide when the throttle is opened and this in turn activates the diaphragm to then pump neat petrol into the venturi. Further disassembly of the slide cover is possible.

On the Triple the slide cover is more complex and quite a different shape. It still has the 'plastic leg' but the cover incorporates a cap through which the 'throttle' spindle passes. Over the spindle, and inside the cap, is a rocker arm, which activates a short cable at its other end to pull up the slide. Note how the bits fit together — they all disassemble.

Needle removal is more difficult because of the presure of the slide return spring. The throttle cable passes through the slide and lodges in a drilling while the needle fits in a special slip 'over' the throtde cable. Once you see it, it will be obvious. All that is left is to dismantle the diaphragm, and remove the idle and mixture needles, a task carried out in the same way as described under the VHB sub-section. Take care with the diaphragm for it is easily damaged. It sits under its cover under which is a compressed spring. Take care. Its removal and replacement is straightforward. Remember that it is set at the factory and there is no good reason to alter it on most machines.

For cleaning and reassembly advice, read the VHB sub-section.

NOTE: For the best results when you need to replace one or more particular components on one instrument, replace them on the others, too.

NOTE: The SFC equipment PHB carburettor is dismantled and assembled in an exactly similar way to the PHF except that you should ignore the sub-section on the vacuum diaphragm as this is not fitted to the PHB. For those SFCs equipped with the obsolete Amal 36mm Concentrics here is a brief sub-section. The float chamber is fixed by two screws — the float sits in it and comes away with it, as does the float needle. The main jet sits in its aluminium holder which is also removable. The needle jet sits up in there too. In the body of the carburettor are located the mixture screw and the throttle stop screw — both these are removable from the outside. The mixture screw is the one with the finely shaped needle affixed to it. The slide cover is fixed by two screws. Remove these and the slide complete with choke comes out. The needle is easily removed once the clip is loose after the return spring has been compressed. Reassembly is a straightforward reverse sequence.

Amal's Concentric was only fitted to a few SFCs. The factory only ordered 200 pairs, but this number was not fitted to production line machines

The PHB carburettor fitted to the majority of the SFCs

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