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.
Thus one already has an ideal basic nozzle that ensures that the engine does not get too little fuel under full load, which it also needs to cool the piston and cylinder. The Dell'Orto carburettors in conventional slider design, where the main nozzle is positioned directly in the float chamber (i.e. no SI carburettors), use as:
auxiliary nozzle 4mm / 5mm
main nozzle 5mm / 6mm
nozzles.
Following carburettors use the 5mm nozzle type as secondary nozzle:
PHBH, PHBL, VHSA, VHSB, VHSC
Following carburettors use the 5mm nozzle type as main nozzle:
PHBD, PHBG, SHA, SHBC
(machine-translated)
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