General Hints & Tips
|Illustrations of Carburettor types 375,376,389 & 689||How it works & part names||General hints & tips|
|Re-assemling & How to trace faults||Parts to tune up with||How to tune up||Tuning twin engines with twin Carburettor|
STARTING, engine hot. Do not flood the carburetter, Then open the throttle about one eighth of its travel and kick-start. If the carburetter has been flooded and won't start because the mixture is too rich - open the throttle wide and give the engine several turns to clear the richness, then start again with the throttle one eighth open, and air lever wide open. Generally speaking it is not advisable to flood at all when an engine is hot.
STARTING, general. By experiment, find out if and when it is necessary to flood, also note the best position for the air lever and the throttle for the easiest starting.
CABLE CONTROLS. See that there is a minimum of backlash when the controls are set back and that any movement of the handlebar does not cause the throttle to open; this is done by the adjusters on the top of the carburetter. See that the throttle shuts down freely.
PETROL FEED, verification. Later models are fitted with a filter gauze at the inlet to the float chamber. To remove the filter gauze, unscrew the banjo bolt (22), the banjo can then be removed and the filter gauze removed from the needle seating. Ensure that the filter gauze is not damaged and free from dirt. Before replacing the banjo, turn on the petrol tap momentarily and see that the fuel gushes out. Avoid petrol pipes with vertical loops as they cause air locks. Flooding may be due to a worn or bent needle or a leaky float, but nearly all flooding with new machines is due to impurities (grit, fluff etc.) in the tank, so clean out the float chamber periodically until the trouble ceases. If the trouble persists, the tank might be drained, swilled out, etc.
FIXING CARBURETTER AND AIR LEAKS. Erratic slow running is often caused by air leaks, so verify there are none at the point of attachment to the cylinder or inlet pipe, check by means of an oil can and eliminate by new washers and the equal tightening up of the flange nuts. Most models have an "O" ring provision machined into the flange, make sure that this is undamaged and replace if necessary.Also in old machines look out for air leaks caused by a worn throttle or worn inlet valve guides.
BANGING IN EXHAUST may be caused by too weak a pilot mixture when the throttle is closed, it may also be caused by too rich a pilot mixture and an air leak in the exhaust system, the reason in either case is that the mixture has not fired in the cylinder and has fired in the hot silencer. If the banging happens when the throttle is fairly wide open, the trouble will be ignition not carburation.
BAD PETROL CONSUMPTION of a new machine may be due to flooding, caused by impurities from the petrol tank lodging on the float needle seat and so prevent its valve from closing. If the machine has had several years use, flooding may be caused by a worn float needle valve. Also bad petrol consumption will be apparent if the throttle needle jet "15" (see fig. 2 sheet 2 ) has worn; it may be remedied or improved by lowering the needle in the throttle, but if it cannot be then the only remedy is to get a new needle jet.
AIR FILTERS. These may affect the jet setting, so if one is fitted afterwards to the carburetter the main jet may have to be smaller. If a carburetter is set with an air filter and the engine is to run without it, take care not to overheat the engine due to too weak a mixture; testing with the air valve (sheet 5 ) will indicate if a larger main jet and higher needle position are required.
EFFECT OF ALTITUDE ON CARBURETTER. Increased altitude tends to produce a rich mixture. The greater the altitude, the smaller the main jet required. Carburetters ex-works are suitable for altitudes upto 3,000 feet approximately. Carburetters used constantly at altitudes 3,000 to 6,000 feet should have a reduction in the main jet size of 5%, and thereafter for every 3,000 feet in excess of 6,000 feet altitude, further reductions of 4% should be made.
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