CARBURETTOR Some later Mini models are equipped with special emission carburettors which have plastic seals fitted at the adjustment points to render them 'tamperproof. The purpose of this is to prevent unqualified persons from making adjustments which could increase emissions above a predetermined level, either through lack of understanding or unsuitable measuring equipment.
These seals must be destroyed in order to gain access to the idle speed screw and/or mixture adjusting nut. The idle speed screw is sealed by a small plastic push-fit plug, and the mixture adjusting nut by a two-piece snap-fit plastic cover. With these carburettors it is advised that no attempt be made to remove the sealing plugs or covers or alter the settings of the sealed adjusters.
On carburettors of the conventional type, without seals, the idle adjustment is carried out in the normal manner.
Damper Oil Level The oil level in the carburettor damper reservoir must be maintained approximately 0.5 in (13 mm) above the top of the hollow piston rod .
To check the level, unscrew the damper cap from the top of the suction chamber and withdraw the cap and damper. Top up if necessary with light oil (preferably SAE 20 grade). Push the damper assembly back into position and screw the cap firmly into place.
Failure to maintain the oil at the correct level will cause the piston to flutter and reduce acceleration.
Slow Running Adjustment Before attempting to adjust the carburettor slow-running setting, ensure that all other items relevant to good engine performance are in good condition and/or correctly adjusted (e.g. contact breaker points, ignition timing, spark plugs, valve clearances, etc.). Also check the air cleaner to ensure that the element is clean, and check that the throttle operation is free and unrestricted.
Carburettor tuning should be confined to setting the idling and fast idling speeds and the mixture at idle speed. A reliable tachometer should be used if possible - make use of the car's tachometer if one is fitted.
On vehicles fitted with emission control equipment, refer to the EMISSION CONTROL SYSTEMS section before attempting to carry out any tuning or servicing operations on the carburettor.
Where a vehicle must conform to exhaust emission control regulations, adjustments should only be carried out if the use of a reliable tachometer and exhaust gas analyzer (CO meter) are available.
1. Check the oil level in the piston damper, as detailed above, and top up if necessary.
2. Check that the throttle moves freely without signs of of sticking and returns fully when released.
3. Check that the mixture control returns fully when the choke cable is pushed fully home. Also check that the cable has about 1/16 in (2 mm) free-play before it starts to pull on the lever.
4. Check that a small clearance exists between the end of the fast idle screw and the fast idle cam.
5. Raise the carburettor piston and check that it falls freely onto the carburettor bridge with a distinct metallic click when released. The piston can be raised for this purpose either with the piston lifting pin at the side of the carburettor, or directly with a finger at the air intake. In this latter case the air cleaner must first be removed to gain access. If the piston fails to fall freely, the jet must be centred as detailed in the FUEL SYSTEM section.
6. Connect a suitable tachometer to the engine, if available.
7. On models with automatic transmission, move the selector into the 'N' position and fully apply the handbrake.
8. Start the engine and run it at fast idle until it has attained its normal running temperature. Continue to run it for a further five minutes.
9. Temporarily increase the engine speed to approximately 2,500 rpm, and maintain this speed for about half a minute to clear the inlet manifold of excess fuel.
Repeat this procedure at three minute intervals if the remainder of the adjustments cannot be completed within this period of time.
10. Check the idle speed and adjust if necessary by turning the throttle adjusting screw. The idle speed should be about 750 - 800 rpm for most later models, or 500 - 650 for models up to 1972. Refer to 'Tune-Up Data' at the end of the section for a list of the precise specified figures, if required.
11. Now adjust the idle mixture by turning the jet adjusting nut one flat at a time, up or down, until the fastest idle speed consistent with smooth running is achieved (Point A on graph in ). Turning the nut up will weaken the mixture, and down will enrich it. Now turn the nut up slowly (weakening) until the engine speed just starts to fall (point B on graph). This should give the weakest position for maximum speed.
12. Recheck the idle speed and adjust if necessary to obtain the specified idle speed.
13. The mixture strength can be checked on lifting the piston approximately 1/32 in (0.8 mm) with the lifting pin.
a) If the engine speed momentarily increases very slightly, the mixture is correct.
b) If the engine speed increases, and continues to do so, the mixture is too rich.
c) If the engine speed decreases, the mixture strength is too weak.
14. 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 in the exhaust beat, together with a blackish exhaust, then the mixture is too rich.
15. On models which must conform to emission control regulations, check that the reading is within the specified limits - normally 3.0 to 4.5% CO - using the exhaust gas analyzer. If the reading falls outside the specified limits, reset the jet adjusting nut by the minimum amount necessary to bring the reading just within the limits. If more than half a turn of the adjusting nut is required, the carburettor should be removed for servicing. 16. To check the fast idle speed, pull out the choke knob until the linkage is just about to move the mixture jet and lock the knob in this position. The fast idle speed should be about 1100 - 1200 rpm, or slightly less for pre '72 models. Again refer to 'Tune-Up Data' for the precise specified figures, if required.
If adjustment is necessary, turn the fast idle screw until the correct fast idle speed is obtained. Push the choke knob fully in and check that a clearance exists between the end of the fast idle screw and the fast idle cam.