Re-assembling & How to trace faults
|Sheet 1||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|
RE-ASSEMBLING after dismantling. See that the washer on the bottom of the jet block is in good condition, otherwise fuel will leak across its face causing rich erratic running. if the washer is faulty it should be replaced by a new one. When replacing the throttle see that the jet needle goes into the centre hole in the jet block and once in, check the throttle works freely when the mixing chamber cap (2) is screwed down firmly by spring clip (18).
When re-assembling the float see that the narrow leg portion of its hinge is uppermost, as this operates the float needle. Care should be taken to see that the joint faces of the side cover and body are not damaged or warped and that the joint washer is in good condition, otherwise difficulty will be experienced in making a petrol tight joint.
TO TRACE FAULTS
|There are only TWO possible faults in carburation; either RICHNESS of mixture or WEAKNESS of mixture, so in case of trouble, decide which is the cause, by;|
|1. Examining the petrol feed.||Verify
jets and passages are clear.
Verify the banjo gauze is clear and allows ample flow.
Verify there is no flooding.
|2. Looking for air leaks.||
the connection to the engine.
|3. Defective or worn parts.||Slack
throttle or worn needle jet.
|4. TEST WITH THE AIR VALVE,||to see if by richening the mixture, the results are better or worse.|
smoke in exhaust.
Petrol spraying out of the carb.
Four strokes, eight stroking.
Heavy, lumpy running.
Heavy petrol consumption.
Erratic slow running.
Engine goes better if;
Verify correctness of fuel feed, stop air leaks, check over ignition and valve operation and timing. Now at throttle position shown on sheet 7 , fig 5, test to see if the mixture is rich or weak. This is done by partially closing the air valve, and if the engine runs better weakness is indicated, but if the engine runs worse richness is indicated.
|TO CURE RICHNESS||TO CURE WEAKNESS|
NOTE: It is not correct to cure a rich mixture at half throttle by fitting a smaller main jet because the main jet may be correct for power at full throttle, the correct way is to lower the needle.
CHANGING FROM STANDARD PETROLS TO SPECIAL FUELS, such as alcohol mixtures will, with the same setting in the carburetter, certainly cause weakness of mixture and possible damage from overheating.
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