Page 29 - Demo

Page 29.Alvis Three Litre Shop Manual - T.A. 21 and T.C. 21be adjusted the mixture should be adjusted by screwing the jet adjusting screws up or down equal amounts, pushing the jets hard up against them until satisfactory running is obtained. Cap nuts beneath the carburettors conceal the adjusting screws. As these are adjusted the engine will probably run faster, and it may therefore be necessary to unscrew the throttle adjusting screws a little by equal amounts in order to reduce the speed.If the mixture is correct on both carburettors lifting the piston of one of them with a penknife blade will make the engine beat become irregular from excessive weakness. If lifting the piston on one carburettor stops the engine, and lifting that of the other does not, this indicates the mixture on the first carburettor is weaker than the second and therefore the first one should be enriched by unscrewing the jet adjusting screw.When the mixture is correct from both carburettors the exhaust beat should be regular and even. If it is irregular, with a splashy type of misfire and a colourless exhaust, the mixture is too weak. If there is a regular rhythmical type of misfire in the exhaust beat, together with a blackish exhaust, then the mixture is too rich. When the correct mixture and synchronisation has been obtained tighten the throttle spindle connection clamping bolts.Tuuniingg aand Adjuustmtmentt off Thermmostatic tatic CaarbuburettttorAs it can generally be assumed that the tapered form of the needle, the strength of the spring, and the diameter of the disc have already been appropriately chosen, tuning is generally confined to the adjustment of the stop screw. It will be appreciated that the main purpose of this adjustment is to limit the downward movement of the needle, the head of which, at the lower extremity of its travel abuts against the upper surface of the stop screw. The final downward movement of this needle determines the degree of enrichment provided, with the thermostatic carburettor in operation under idling condition.An approximate guide to its correct adjustment in this respect is provided by energising the solenoid when the engine has already attained its normal running temperature. The stop screw should then be so adjusted that the mixture is distinctly, although not excessively, rich; that is to say, until the exhaust gases are seen to be discernibly black in colour, but just short of the point where the engine commences to run with noticeable irregularity.Anti-clockwise rotation of the stop screw will, of course, raise the needle under these conditions, and increase the mixture strength, whilst rotation in the contrary direction will have the opposite effect. In order to energise the solenoid under conditions when the thermostatic switch will normally have broken the circuit, it is merely necessary to short circuit the terminal of the thermostatic switch directly to earth, or if this is not readily accessible, to make a connection between the appropriate terminal of the pair to earth by means of a separate wire. The only troubles likely to occur with the correctly adjusted carburettor are:1. Piston sticking2. Flooding from float chamber or mouth of jet3. Dirt or water in carburettor.Piistton SSticki tickinggRemove the suction chamber, withdraw the piston and thoroughly clean both parts with petrol and a clean cloth. Apply a few drops of light oil to the piston rod, preferably diluted with paraffin if any signs of rust or corrosion are noticed on the rod. Replace the piston in the suction chamber and test for rotational and sliding freedom. Any direct local contact between these two parts attributable to some indentation of the suction chamber may be rectified by carefully removing any high spots which may show up on the suction chamber bore by means of a hand scraper. On no account should any attempt be made to enlarge generally the bore of the suction chamber, or to reduce the diameter of the enlarged part of the piston, as the maintenance of limited clearance between these parts is absolutely essential.