jets flow the fuel through them, since your motor is now a 240 it is moving a lot more air and needs the fuel to mix with it, you can only go so far with a mixture screw so the larger jet flows more fuel, it is a really easy fix, all you do is pull of the carb and take of the bottom caled the bowl(4 screws holding it on), then in side be carefull not to hit or bend any thing, but u will see a brass fitting in the middle with a hol on it, that is your main jet, unscrew it an put in the new one, then reassemble the bowl and your done, u will need to adjust your mixture accordingly