As such undoing it (turning it anticlockwise) makes the mixture leaner. An air adjustment screw works by changing the amount of air able to flow through the idle circuit. Both perform the same basic function, but work in different way. Depending on the exact model of Dellorto PHBH this is either an air adjustment screw or a mixture adjustment screw. This is done by adjusting the various jets.Īt very small throttle openings the mixture is adjustable with a screw. If you do any changes to an engine you will often need to change the fuel / air mixture. As fuel is used the level drops, and as the floats drop the needle drop allowing fuel back into the float bowl. As the fuel level rises the floats are lifted up which in turn lifts up the needle, sealing the fuel intake into the float bowl. This is valve is connected to a pair of plastic floats. The mount of fuel flowing into the float bowl is controlled by the needle valve. The needle is tapered so as it rises the amount it restricts the fuel flow changes. The needle jet passes down into the atomiser tube and restricts the fuel flow. From the main jet the fuel passes through small holes into the atomiser tube and then up into airflow. Fuel flows from the float bowl through small holes in the large bolt and then through the main jet. The size of this jet determines the maximum amount of fuel that can pass up into the airflow. The main jet sits in the large brass bolt at the bottom that holds the float bowl in place. The air to fuel ratio of through here is adjustable with a screw.Īs the throttle is opened more fuel is supplied through the main jet, but restricted by the needle jet. At low revs the throttle is basically closed and air is drawn through a small supplementary air passage with a small jet. The needle jet is mounted onto the slide.įuel is allowed into the air from various jets. When you open the throttle the slide is lifted up, opening to allow more air into the engine. The slide is attached to the throttle cable. This means that there is lower pressure in the carb throttle than there is in the float bowl (which is just at normal air pressure) and so fuel is drawn up into the air. This narrowing causes the air to speed up at that point, which results in a drop in pressure. The Dellorto PHBH28 carb is used on the NSR125 and variations of the same carb are also used on the Aprilia RS125 and the Cagiva Mito.Ī carb work by, in simple terms, creating a narrowing of the air passage.
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