Instructions for Amal Carburetter Type 19

HOW THE CARBURETTER WORKS

The carburetter atomises the correct amount of petrol with air which is drawn into the engine. A float chamber maintains a constant level of fuel at the jets and cuts off the fuel supply when the engine stops. For cold starting a tickler (21) for flooding is fitted. A throttle valve (5), operated by a cable from the handlebar controls the volume of mixture and, there fore, the engine power. At all openings of the throttle the mixture is automatically correct.

At small throttle openings, as for starting, idling and slow running, a petrol/air mixture is drawn from the pilot jet system, fuel being fed via the passage in the float chamber to the pilot jet (9), where it mixes with air entering the pilot air passage (16), the resultant mixture then passing out of the pilot outlet (8) and by-pass (7), where it mixes with air passing through the main air choke.

Further opening of the throttle enables fuel passing through the main jet (11) and needle jet (10) to be mixed with air from the primary air passage (13) and bleed holes (12) in the needle jet, then out into the main air choke where it is further mixed with the main incoming air stream.

STARTING INSTRUCTIONS

STARTING WHEN ENGINE IS COLD.

Turn on fuel supply, depress tickler sharply three or four times to flood float chamber, open throttle slightly and start engine. When engine starts ease back the throttle.

STARTING WHEN ENGINE IS WARM.

Open throttle slightly and start engine. It should not normally be necessary to flood the float chamber when starting a warm engine.

GENERAL.

Experience will show when it is necessary to flood the carburetter and also the best setting of the throttle valve. If the carburetter has been over-flooded, which would result in a wet engine and over-rich starting mixture -fully open the throttle valve, give the engine several turns to clear the richness, then start again with the throttle valve slightly open. If the engine refuses to start after the instructions given have been carried out and the carburetter is suspect, read and carry out the instructions given in the section headed "Maintenance."

MAINTENANCE OF CARBURETTER

CABLE CONTROLS.

Ensure there is a minimum of backlash in the cable when the throttle valve is in the closed position. Adjustment is provided by a cable adjuster (1) on mixing chamber top. Check that the throttle valve will fully open and that handlebar movement does not operate the throttle.

FITTING CARBURETTER.

Erratic slow running is often caused by air leaks. Check that there is none at the point of attachment of the carburetter to the engine. The carburetter should be a good push fit on the induction pipe and the outlet clip pin (20) tightened; if the carburetter is removed, when refitting put a spot of oil on the induction pipe and push carburetter on with a screwing motion make sure carburetter is fully home before tightening outlet clip pin.

PETROL FEED.

To ensure that fuel is reaching the carburetter, turn off fuel tap, check that filter gauze (28) is clean by removing banjo bolt (27) and banjo (26) (with fuel tap combined on certain models). Before replacement momentarily turn on tap to see that fuel gushes out freely. Petrol pipes with vertical loops cause air locks and should be avoided.

FLOAT CHAMBER.

Continual flooding of the float chamber is caused by the float needle not shutting off correctly due to foreign matter being on either the float needle (24) or its seating (25). Access to the float chamber for cleaning is by removing the three float chamber screws (18) and withdrawing the float chamber, together with its float needle and joint washer, down off the mixing chamber. Take care not to damage the joint faces. Removal of the float is by inverting the float bowl. Flush the float chamber and needle seating and wipe the float needle clean; do not attempt to clean by scraping. On old machines failure of the float needle to shut off may be due to wear on the needle or its seating. Replacements should be fitted. When replacing the float chamber assembly care should be taken to ensure that the throttle needle (4) locates in the needle jet and the float needle (24) is replaced point uppermost.

PILOT JET (9) AND NEEDLE JET (10).

These are removable after dismantling the float chamber following the instructions given under float chamber. Wash in clean petrol and blow through the passages; do not prod or reamer jets. On old machines, if wear is evident on the needle jet a replacement should be fitted.

MAIN JET (11).

To clean, remove the needle jet with main jet screwed in its base, from the float chamber. Clean assembly in petrol and blow through with air. If it is necessary to remove main jet a screwdriver should be used.

THROTTLE NEEDLE (4) AND THROTTLE VALVE (5).

Check that the throttle valve with its throttle needle moves freely up and down when the control is operated. On old machines check that no undue wear has taken place between the throttle valve and the bore in which it operates, and on the portion of the throttle needle that operates in the needle jet. If wear has taken place replacement parts should be fitted.

The throttle valve complete with throttle needle and attached to the cable can be withdrawn from the carburetter after removing the two screws securing the mixing chamber top to the mixing chamber. To separate the throttle valve and throttle needle from the cable, release the cable at the control end and push the inner cable forward in the throttle valve until the nipple (17) in the throttle valve clears its hole, then withdraw the cable through the slot in the throttle valve, the nipple passing through the hole at the extreme end of the slot.

When replacing this throttle assembly see that the slot in the throttle valve locates on the leg of the mixing chamber top, that the throttle needle clip is correctly located and the throttle needle is entering the needle jet, before attempting to push the assembly home.

ADJUSTMENT OR TUNING OF CARBURETTER

A certain amount of adjustment is provided for on the carburetter to ensure that a correct mixture is obtained. The correct mixture is one that is neither too rich nor too weak. See that there are no faults as outlined in Maintenance" as these would affect the correct functioning and adjust- ment of the carburetter. Check that the ignition, timing, etc., is functioning correctly.

Carburetters as supplied by the makers for fitting to specific machines should under normal conditions only require adjustment of the throttle stop adjusting screw (19), pilot air adjusting screw (15) and the position of the throttle needle (4) to ensure best starting, idling, slow and general running with maximum fuel economy. See "Adjustment of Carburetter."

For special conditions or adaptations, or where it is suspected that the carburetter may have had an unsuitable throttle valve or main jet sub- stituted, as these parts vary according to engine requirements, it will be necessary to completely re-tune the carburetter. See "Complete Tuning of Carburetter.

ADJUSTMENT OF CARBURETTER

First, if possible, run engine until warm, then shut off. Screw in the pilot air adjusting screw (15) as far as it will go without strain, then unscrew it in an anti-clockwise direction approximately 1 and a half turns. Unscrew the throttle stop adjusting screw (19) so that the throttle valve can fully close, then with the throttle slightly open start the engine and throttle down to a fast idling speed.

Now, first set the throttle stop adjusting screw to hold this position, and then unscrew it to allow the throttle valve to be further closed and the engine to slow down until it begins to falter, then screw the pilot air adjusting screw in or out until the engine runs regularly and faster.

Then further unscrew the throttle stop adjusting screw until the closing of the throttle valve again makes the engine run slower and just begin to falter, then again adjust the pilot air adjusting screw to get best slow running.

If, after this second adjustment, the engine is still running coo fast, carry out the same procedure a third time.

After each adjustment of the throttle stop adjusting screw and pilot air adjusting screw, test that the engine does not falter or cut out when the throttle is opened fairly quickly if the engine does falter or cut out, the adjustment has been set for too slow running, resulting in an over-weak pilot mixture.

Now try the throttle needle (4) in as low a position as possible-viz., with the clip (6) in a groove as near the top of the throttle needle as possible; if when the engine is under load or pulling, acceleration is poor, or there is spitting back in the carburetter, raise the needle by two grooves. If very much better try lowering the needle by one groove and leave where it is oest.

In general, if the engine runs heavily and lumpy the mixture is too rich and will be the cause of excessive petrol consumption, but if there is spitting-back in the carburetter, poor acceleration or over-heating, the mixture is too weak.

After the best position of the throttle needle has been found again check over the slow running.

COMPLETE TUNING OF CARBURETTER

Check as follows and in the order given.

Ist-Size of Main Jet.

With the engine pulling or under load, open up to full throttle, if at full throttle the engine runs heavily, the main jet (11) is too large, resulting in a rich mixture. If at full throttle, by slightly closing the throttle, the engine seems to have better power, the main jet is too small, resulting in a weak mixture.

Examination of the sparking plug will also help to determine whether the mixture is correct. If the plug body at its end has a cool appearance, the mixture is correct; if sooty, the mixture is too rich; if however, there are signs of intense heating, the mixture is too weak.

2nd-Idling and Slow Running.

Adjust the throttle stop adjusting screw (19) and pilot air adjusting screw (15) as instructed in "Adjustment of Carburetter.

3rd-Amount of Throttle Valve Cut-away.

If there is objectionable spitting from the carburetter when the throttle is opened up from the idling or slow running position, which signifies weakness, first try slightly richening the pilot mixture by screwing in the pilot air adjusting screw (15) sufficiently, but if this is not effective, screw it back again and fit a throttle valve with a smaller cut-away (14).

If the engine jerks under load at this throttle position and there is no spitting, either the throttle needle is in a much too high position, or a throttle valve with a larger cut-away (14) is required to cure richness.

4th-Position of Throttle Needle.

Adjust position of throttle needle (4) as instructed in "Adjustment of Carburetter."

5th-Idling and Slow Running.

Go over the idling and slow running adjustment for final touches.

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