The main nozzle is significantly involved in the mixture preparation of the carburettor.
If it is too small, the engine dies under full load and/or runs so lean that the piston and cylinder overheat and cylinder damage would result.
If, on the other hand, it is too large, the mixture is overgreased, the engine runs very unevenly and can only deliver a fraction of its possible power.
Here the ideal compromise has to be found. To determine the right main nozzle, it is first chosen so large from the belly that the target should be an engine stuttering strongly under full load. Of course it needs some experience, but a nozzle is also quickly changed.
If you found such a nozzle you now mount a smaller nozzle until the engine under full load no longer stutters.
This means that you already have an ideal basic nozzle that ensures that the engine does not receive too little fuel under full load, which it also needs to cool the piston and cylinder. A safety optimum represents an engine adjusted in such a way, which throttles completely easily briefly if one closes the gas again under full load.
Arreche, a Spanish manufacturer of carburettors and carburettor accessories, produces after classical type of carburettors of Amal and Mikuni (under license).
Thus hexagonal main nozzles are used in the Arreche carburettors, as they are used in quite similar construction form also in the modern Mikuni TM/TMX carburettors. Among other things, they are incompatible with each other due to the different threads.
The Arreche nozzles are graduated in five steps, e.g. 60, 65, 70,,
(machine-translated)
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