The correct adjustment of the slow-running mixture strength is particularly important on the S.U. carburettor, since the richness of the mixture is determined by slightly raising or lowering the jet in relation to the tapered needle, which is at its lowest point when the engine is idling. Any alteration in the height of the jet will obviously affect the mixture strength over the whole range. This is illustrated, in an exaggerated form, when the jet is lowered to well below its normal position by the mixture control, thus providing an abnormally rich mixture to ensure a quick start.
When the mixture control knob is pushed home, the jet is raised until it comes into contact with the jet-adjusting nut shown in Fig. 27, which provides the adjustment for the slow-running mixture strength.
Remove the air cleaner (which can be conveniently cleaned at the same time, as described on page 40), and first check that the piston is not sticking, as this would render it impossible to attain a satisfactory slow-running setting and would, of course, affect the general performance. The base of the piston, which is visible (with the aid of a mirror) through the air intake of the carburettor, should be lifted with the lifting pin shown in Fig. 27 or the tip of a penknife and released, when it should fall with a slight metallic click. If the piston is obviously sticking or no click is heard, the trouble should be rectified as described under the heading "Curing a Sticking Piston" on page 68.
Assuming that all is well, disconnect the mixture control wire and press the jet firmly into contact with the adjusting nut. Start the engine, allow it to warm up to its normal running temperature and then set the throttle lever stop-screw (shown in Fig. 27) on the carburettor to give a fairly slow idling speed, but not so slow that there is a risk of the engine stalling when the clutch pedal is depressed.
The jet-adjusting nut should now be screwed downwards, turning it one flat on the hexagon at a time, thus enriching the mixture, until the engine begins to "hunt"—a term which describes the rhythmical beat that develops,often accompanied r black smoke from the exhaust. The jet-adjusting not should now beY ;ewed upwards to weaken the mixture, making sure that the jet follows le nut, until the engine is firing evenly. For best acceleration it is des able to have the mixture very slightly on the rich side.
The mixture strength should now be checked by raising the piston very slightly—not more than & in.—with the lifting pin or the tip of a penknife, when the engine sM» ,uld speed up a little and continue to run smoothly. If the engine speed •/,creases appreciably and is maintained when the piston is raised a little it rther—about .} in. in all—the mixture is too rich. If, on the other hand/ -.he engine stalls when the piston is raised by ^ in., the setting is too vw ;tk.
In all proban lity it will be necessary to readjust the idling speed slightly then the correct mixture strength has been obtained and this in turn will call for a very/slight readjustment of the jet-adjusting nut.
The mixed control wire should now be reconnected. Finally, make sure that a ^mall gap exists between the tip of the fast-idle adjusting screw (also shown in Fig. 27) and the cam on the lever that is linked to the base of the jet'.. The exact adjustment of this screw, which regulates the amount
The correct adjustment of the slow-running mixture strength is particularly important on the S.U. carburettor, since the richness of the mixture is determined by slightly raising or lowering the jet in relation to the tapered needle, which is at its lowest point when the engine is idling. Any alteration in the height of the jet will obviously affect the mixture strength over the whole range. This is illustrated, in an exaggerated form, when the jet is lowered to well below its normal position by the mixture control, thus providing an abnormally rich mixture to ensure a quick start.
When the mixture control knob is pushed home, the jet is raised until it comes into contact with the jet-adjusting nut shown in Fig. 27, which provides the adjustment for the slow-running mixture strength.
Remove the air cleaner (which can be conveniently cleaned at the same time, as described on page 40), and first check that the piston is not sticking, as this would render it impossible to attain a satisfactory slow-running setting and would, of course, affect the general performance. The base of the piston, which is visible (with the aid of a mirror) through the air intake of the carburettor, should be lifted with the lifting pin shown in Fig. 27 or the tip of a penknife and released, when it should fall with a slight metallic click. If the piston is obviously sticking or no click is heard, the trouble should be rectified as described under the heading "Curing a Sticking Piston" on page 68.
Assuming that all is well, disconnect the mixture control wire and press the jet firmly into contact with the adjusting nut. Start the engine, allow it to warm up to its normal running temperature and then set the throttle lever stop-screw (shown in Fig. 27) on the carburettor to give a fairly slow idling speed, but not so slow that there is a risk of the engine stalling when the clutch pedal is depressed.
The jet-adjusting nut should now be screwed downwards, turning it one flat on the hexagon at a time, thus enriching the mixture, until the engine begins to "hunt"—a term which describes the rhythmical beat that develops,often accompanied r black smoke from the exhaust. The jet-adjusting not should now beY ;ewed upwards to weaken the mixture, making sure that the jet follows le nut, until the engine is firing evenly. For best acceleration it is des able to have the mixture very slightly on the rich side.
The mixture strength should now be checked by raising the piston very slightly—not more than & in.—with the lifting pin or the tip of a penknife, when the engine sM» ,uld speed up a little and continue to run smoothly. If the engine speed •/,creases appreciably and is maintained when the piston is raised a little it rther—about .} in. in all—the mixture is too rich. If, on the other hand/ -.he engine stalls when the piston is raised by ^ in., the setting is too weak
In all proban lity it will be necessary to readjust the idling speed slightly then the correct mixture strength has been obtained and this in turn will call for a very/slight readjustment of the jet-adjusting nut.
The mixed control wire should now be reconnected. Finally, make sure that a ^mall gap exists between the tip of the fast-idle adjusting screw (also shown in Fig. 27) and the cam on the lever that is linked to the base of the jet'.. The exact adjustment of this screw, which regulates the amount
1, throttle-stop screw
2, jet adjusting nut
3, piston lifting pin
4, fast-idle adjusting screw
by which the engine is speeded up when the mixture control is operated, should be left until the engine is next started from cold, when it should be set so that excessive "hunting" does not occur when the dashboard control is fully pulled out.
When an automatic transmission is fitted, the fast-idle speed should be 1,050 r.p.m. and the normal idling speed 650 r.p.m. This is important to prevent excessive "creep" of the transmission when a gear is selected. It would be best to have these settings checked by a B.L.M.C. dealer with the aid of a rev. counter (tachometer).
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