In the SI carburettor, the main jet is located in the jet block under the mixer tube and main air correction jet. It is responsible for making a certain amount of petrol available for premixing. With all tuning measures, no matter how small, it is necessary for the main jets to be raised, as they mainly impact on the middle and upper rev range. Their effect is linked very closely with the other components of the middle SI jet block and for comprehensive tuning, it is necessary for all of the components involved to be adjusted accordingly. Please note that we exclusively use DELL'ORTO jets. Those who've tuned carburettors and found the actual jet size does not correspond to the number engraved on the side, know what we're talking about. If 105 is written on it, then there'd better be 105 in it! Anything else is a cheap trick.
The size of the jets increases on the basis of one, two and three stages. The jets get increasingly richer. Simply attach the corresponding size to part no 4026. The jets from 142 upwards are redesigned specially under the SERIE PRO logo as DELL'ORTO is only able to supply jet up to 140. The SI 105-128 and 130-140 jetting kits each contain 5 and/or 10 jets and are an ideal basic programme for any tuner.
Conclusion: A selection of main jets is absolutely compulsory for every scooter driver.
SIP-TIP: For optimum tuning in the middle and upper rev range order a corresponding mixer tube (e.g. BE4 art no. 40406000) at the same time.
SIP community user 'Chiemgauvespa' on part no 40260000: "DELL’ORTO Power: Very exact jet sizes which make it easy to tune the carburettor well."