Malossi PHBG Dellorto 19mm Carburetor for Yamaha Zuma
The job of a carburettor is to supply the engine with air and fuel mixture. While there are many different types of scooter carbs, virtually all operate by the same method. Carb tuning, is the process of adjusting the various fuel and air control mechanisms within the carb to achieve the optimum fuel and air mixture throughout the rev and throttle range.
The job of the main jet is to increase fuelling as throttle is applied. As the slide opens, more air is supplied to the engine. To keep the fuel/air mixture correct, the fuelling must also be increased at the same rate. Installing a larger carb significantly changes the fuel/air mixture, and it must be tuned to suit. While this carb does come with a main jet installed, every scooter is different and will have a slightly different fuelling requirement. You MUST tune the main jet, failure to do so may result in engine damage. Different size main jet kits suit different applications, we have supplied a guide on the selection drop down as to what size kit is generally appropriate for each application.
The job of the pilot jet is to supply the base amount of fuel the engine needs to run (sometimes called the idle jet). This would usually only need to be changed from the factory setting if a pod filter is being run, or if the bike has a racing cylinder installed (bridge port cylinder kit). In situations of sudden, high fuel requirement (eg, rapidly applying throttle from idle), there is a short delay in the fuel supply from the main jet. If the pilot jet is set correctly, its able to make up this shortfall from the main, allowing the engine to begin to rev until the main supply kicks in. If the pilot is set too lean, bogging will occur. If the pilot is set too rich, the engine will not rev.
The job of the needle is to moderate the supply of fuel from the main during medium throttle ranges. These are most often times where you’re trying to maintain a speed limit, on anywhere from 1/4 to 3/4 throttle. The W16 needle supplied in the kit is characteristically very rich – which results in very rough mid throttle range, where the bike will only run cleanly on full throttle. Installing a leaner needle restricts the fuel supply from the main at medium throttle ranges, allowing the fuel/air mixture to be more consistent through the throttle range, and much more pleasant riding experience.