Arreche carburettor main jet size 145 for all Arreche carbs from 17.5 - 24mm. Generally an increase of 10-15% in the main jet size is a good option when fitting a race pipe. Arreche carbs come with two jets for tuning purposes. Check your plug to see if your mixture is too lean or too rich. Light brown = optimal Dark to black = too rich grey to white = too lean, which can lead to seizures.