The main jet is considerably involved in the mixture preparation of the carburetor.
If it is too small, the engine dies off under full load and/or runs so leanly that the piston including cylinder overheats and a cylinder damage would be the consequence.
If it is too large, the mixture is too rich, the engine runs very unevenly and can only deliver a fraction of its possible power.
Here it is important to find the ideal compromise. To determine the correct main jet, it is first of all selected from the belly so large that the goal is an engine that stutters heavily under full load. Of course it needs some experience, but a nozzle is also quickly changed.
Once you have found such a nozzle you mount a smaller nozzle until the engine does not stutter under full load any more.
This way you have already an ideal basic lubrication which 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 slide valve design, where the main jet is positioned directly in the float chamber (no SI carburettors), use as:
secondary jet 4mm / 5mm
main jet 5mm / 6mm
jets.
The following carburetors use the 5mm nozzle type as a secondary nozzle:
PHBH, PHBL, VHSA, VHSB, VHSC
The following carburetors use the 5mm nozzle type as a main nozzle:
PHBD, PHBG, SHA, SHBC
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