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How To Use The Lifting Pin On Su Carbs

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#1 KernowCooper



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Posted 19 September 2014 - 10:55 AM

Setting of H type carburetters


Centralising the Jet, Base Setting and Using The Lifting Pin


After the correct operating temperature has been attained, close the throttle completely by unscrewing the throttle adjusting screw until it is just clear of its stop. Open it by screwing down this screw 1 1/2 turns.

The piston and suction chamber should now be removed and the mixture control wire disconnected. Screw the jet adjusting nut until the jet is flush with the bridge of the carburetter, or 'full up' if this position is not obtainable. Replace the piston and suction chamber assembly and check that the piston falls freely on to the bridge of the carburetter. Turn down the jet adjusting nut two complete turns (12 flats). Re-start the engine and adjust the throttle adjusting screw to give the desired idling speed as indicated by the ignition warning light. Turn the jet adjusting nut until me fastest idling speed is obtained consistent with even firing. During this adjustment it is necessary to ensure that the jet is pressed upwards and is in contact with its adjusting nut. As the mixture is adjusted the engine will probably run faster; it may therefore be necessary to unscrew the throttle adjusting screw a little in order to reduce the speed.



Using The Lifting Pin, or Some SUs have none so use a small screwdriver to lift the slide


Check the mixture strength by lifting the carburetter piston (by means of the lift pin situated on the side of the carburetter body) by approximately 1/32 in., when if

(1) The engine speed increases and continues to run faster, the mixture is too rich.

(2) The engine speed immediately decreases, the mixture is too weak.

(3) The engine speed momentarily increases very slightly, the mixture is correct.


When the mixture is correct the exhaust note should be regular and even. If it is irregular, with a splashy type of misfire and colourless exhaust, the mixture is too weak. If there is a regular or rhythmical type of misfire, together with a blackish exhaust, then the mixture is too rich. Reconnect the mixture control wire with approximately 1/16 in free movement before it starts to pull on me jet lever. Set the mixture control knob on the dash panel to its maximum movement without moving the carburetter jet (about 5/8 in.) and adjust the fast idling cam screw to give an engine speed of between 800-1000 rev/min (when hot). When setting the slow running of a vehicle, consideration of the type of power unit fitted should be made.


Engines with large valve overlap will require a fast idle and it will be difficult to obtain an extremely smooth tickover.

Edited by KernowCooper, 19 September 2014 - 11:07 AM.

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