Changing carburrettor jets (from old site)

<TABLE cellSpacing=0 cellPadding=0 width="100%" border=0><TBODY><TR vAlign=top><TD>Changing carburettor jets

contributed by Karl Darbyshire aka Karlos
When increasing the boost pressure above standard, the engine will run lean (pink) due to insufficient fueling. To overcome this factor the carb jets can be replaced to enable the correct air and petrol mixture through all the operations of the carb.

The two jets to be replaced in the carburetor are the air correcting and main jet.

The Main Jet

The main jet controls the amount of fuel entering the emulsion tube at low revs. The larger the jet the more fuel passes through the emulsion tube making the fuel mixture richer at the low end of the rev scale. The diameter of the jet sizes relates to millimeters and range from the standard 120 to 130, 135, 140, 150, 160. (eg 150 jet = 1.5mm wide hole)

Air Correcting Jet

The air corrector controls the amount of air entering the emulsion tube and weakens the mixture at the top end of the rev scale. To stop the car running lean at the top end the air correctors diameter is reduced. These jets range from 120, 100 and 90

Typical carb jet configuration, whilst running 16 psi with a strapped or alloy intercooler are:-

135 Main Jet.
90 Air Correcting Jet.

Twelve Easy Steps

1. Remove all boost, water and petrol hoses connecting to the carb.
2. Remove the carb from the inlet manifold. The carb is attached by two 12mm nuts and washers, and are situated at each side of the carb.
3. Place the carb on a bench and remove the excess fuel, buy pulling the idle spring.
4. Remove the 6 screws securing the carb top, be careful of the float.
5. In the surrounding of the venturi there is a small air correcting jet. Remove the old jet and refit the new one.
6. To remove the main jet, remove the 12mm brass bolt that is situated on the lower front side of the carb.
7. When removed look through the hole and you will find a main jet.
8. Place a medium size flat blade screw driver through the hole into the float chamber and remove the old jet.
9. Refit new jet.
10. Refit carb top and secure carb to the inlet manifold.
11. Refit all hoses, renew any perished hose clips.
12. For best results visit a rolling road to set the correct CO value.

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