#TechnicalTuesday - Fine Tuning Float Type Carb

written by Maurizio Voltini.

1) DISASSEMBLY

First of all, remove the carburettor from the engine. Loosen the screws of the bands that seal the sleeve to induction duct, unscrew the top cap and slide the gas cable pawl out of the hosing in the guillotine.

To adjust carburetion, open the carburettor and change some of the elements. There are 2 ways that you can get in to do this. From underneath by removing the tank, and from above by removing the big cap. Unscrew the big screw cap in the lower part of the tank. It doesn’t lock the tank to the body, it forms a sort of sump from which the fast running jet draws petrol. Use a socket wrench or a 21 mm combined.



Take out the 3 socket head screws using the appropriate 4mm spanner. Be careful not to ruin the washer, you can use it again only if it is in perfect conditions.
Now you can see the jets. The one with the biggest diameter is the fast running one; next to it you find the slow running jet, and next to that the starting jet. Each one can be taken out with the help of a screwdriver. It is important to use one that is the right size for the cut in the jet screw-head. Being made of brass, these tend to get ruined easily if the right tools aren’t used. 



Above the fast running jet, you have the base and sprayer; on the slow running jet there is the diffuser (fuel nozzle) of the relative circuit, while the start jet is by itself. All the jets and slow running fuel nozzle can be taken out using a screwdriver. The fast running jet has a base that must be mounted again with “arks” facing inside the carburettor, a bit like a hat upside down.

A special spanner is required to remove the fuel nozzle of the fast running jet: it’s a fine number 9 socket wrench (the size itself is already unusual).
Unscrew the nut at the top by hand. Inside there is the guillotine gas valve return spring, while you can see the upper body of the valve itself, which is easy to remove, in the cylindrical body.



Using a 8mm “T” wrench, unscrew the nipple, the special screw with cut that houses the end of the gas tube and holds the conical needle, which is in turn held in place by a circular clip. Take out the clip using a pair of fine pointed pliers. Be very careful not to ruin the fine washer that enables you to vary position by half a notch.



The bottom part of the guillotine is bevelled; the rear part ends before the front. A lower rear border (contained bevel) draws bigger petrol load. Therefore you have enriched fuel titre. Therefore the shape of the gas valve is another parameter that influences carburetion and delivery. 



Going back to the lower part, using fine pointed pliers, gently take out the float pin. The float comes away with the fuel valve petrol flow needle, which isn’t and must not be confused for the conical pin. This valve is made up of brass seat, removable and available in different diameters, complete with needle with rubber lined point. When the engine is off, the float goes up, the needle shuts off the flow of petrol going towards the Venturi duct and prevents the engine from flooding.

When the engine is on, the level in the tank lowers and the needle valve allows the fuel to pass through. The diameter must be suitable for the engine’s needs. The point of the needle valve is subject to wear and usually a groove forms in the area where it rests in its seat. To remove this, use a number 9 “T” wrench, the same that has been used before. 



Last of all take out the screw that holds the start circuit lever, the idle adjustment screw and the engine rev screw. 

2) ON TRACK ADJUSTMENTS

Before carrying out any adjustments, try to set only the delivery arc that you think needs adjusting, and more important, work on one thing at a time with engine warmed up. In idle or at low revs, if the mixture is rich, the engine sound deep and muffled, revs increase slowly and there could be signs of flooding; if the mixture is too lean, the idle speed tends to increase. Setting is right when the response to throttle opening is good. When you have the wrong air mixture screw opening, too open (big jet) or too closed (small jet), you get idle jet not working correctly. Adjust air mixture screw and idle jet, but also fuel nozzle.
If passing, during intermediate speeds, in progression phases, you can feel a clean delivery, although weak, this has to be enriched; you can mount a conical needle on a lower notch that determines a higher final position that means a greater petrol flow with the same opening. The opposite applies if the engine is slower in response, if it hesitates or knocks. Also the (fast running) fuel nozzle has to be adjusted, but differences are usually the barest minimum, 1 or 2 points at most and this is according to external temperatures. 

Also, it could happen that at the end of the straight, at full speed, the engine doesn’t sound right, it perhaps starts to “knock”, this is a sure sign that fuel mixture is too rich, so perhaps you need to mount a jet with a smaller diameter. But if you mount one where the diameter is too small, you could get an engine seizure and won’t get maximum performance from the engine. 



3) CHECK

The colour at the bottom of the insulation of the central electrode of the spark plug and at the centre of the piston crown, which should be a light hazelnut brown (or tobacco) after several laps, is proof that carburetion is set properly. As we get away from these 2 reference points, it should gradually get darker.

IMPORTANT TO KNOW

True that there are lots of parameters, but it is in fact this that gives rise to a fine adjusted carburetion, because each limit is responsible for a delivery arc. Therefore, you need some basic operation for setting each engine, to adapt it to air temperature and the different type of track on which you are racing. Then again, some operations are necessary and the ones most frequently carried out are: changing the fast running jets, sometimes also changing the sprayer; moving or changing the conical pin; turning the airscrew in idle. At this point, don’t forget that in the VHSH30 the screw is before the guillotine and therefore, it adjusts only the air. The PHBE30, changed by regulations, but still commonly used, has the screw after the gas valve and therefore, it acts on air/fuel mixture. Tighten screw to enrich fuel in the first case, and loosen it for the contrary. Adjustment always starts with screw completely closed, not too tight though or you risk ruining the air-seal washer at the bottom. As for adjusting idle, one often hears, internal and external, the first meaning diffuser and the second meaning the idle jet. If in doubt, it is better to have a mixture that is slightly richer than too lean because the latter could cause engine seizure.

Basic carburetion for some 125 cc gear class engines (Dell’Orto VHSH 30)