It is generally desirable to decarbonise the engine and to grind in the valves lightly on a new or recently overhauled engine after it has covered the first 2,000 miles. It is not, however, necessary or even advisable to repeat the process at anything like such frequent intervals, and often a very considerable mileage can be covered before it is necessary to decarbonise again.
The usual indications that the work is needed are a falling off in power and a tendency for the engine to pink excessively on accelerating and when pulling hard.
As the utmost care is needed to prevent the entry of dirt or foreign matter it is desirable to wash down the engine thoroughly with clean petrol or benzol before commencing any dismantling. Work should be carried out in surroundings as free as possible from dust and after removal of the head (as described below) the cover should be replaced on the bevel housing and clean rag or paper should be tied over the open ends of the bevel bush housing and vertical shaft cover.
Before the head can be removed the petrol tank has to be taken off. To do this turn off both petrol taps and remove the petrol pipe. Take out the bolt from the nose of the saddle and lift the front. Remove the three tank bolts. One through the frame at the back and the other two underneath the tank at the front. See that the rubber buffers are not mixed so that they may be replaced in their correct places. Now lift off the tank.
Unscrew the ring holding the fixing chamber top to the mixing chamber of the carburetter and pull out the slides. Undo the exhaust pipe nut, disconnect the exhaust lifter cable from the lever on the head and remove the "Y" shaped oil drain pipe by loosening the union nuts which hold it to the head and the lower half of the pipe (see Fig. 14). Take out the sparking plug, and remove cylinder head steady strap.
Loosen the bottom and top gland nuts and push them respectively up and down the vertical shaft cover well clear of the threads. Prise out the spring circlip ring from the groove in the vertical shaft cover just above the bottom bevel bush housing, and slide it up the outside of the cover. This will allow the vertical shaft cover to slide down into the bottom bevel bush housing and the top coupling will be exposed. Remove the four nuts which hold the cover to the top bevel housing and pull off the cover. It is now that care is needed to set the engine to the correct position for the removal of the head. Rotate the crankshaft by means of the foot starter until the marks on the top bevels are in register. This will be made clear by reference to the illustration Fig. 15. It may be necessary to turn the engine anything up to twenty-one turns before the correct registering of the marks will be obtained according to the position in which the engine came to rest.
The slot in the top of the vertical shaft will be in line with the crankshaft and the piston will be at the top of compression stroke when the marks register.
The cylinder head bolts may now be unscrewed by using the box spanner provided and the head lifted off the barrel. Take care not to damage the copper cylinder head gasket. It is best to take it off and hang it up until needed during reassembly. DO NOT TURN THE CRANKSHAFT WHILE THE HEAD IS OFF OR THE TIMING WILL BE UPSET.
It is unnecessary to remove the cylinder barrel, and we do not advise disturbing it at all unless it has to be taken off for attention to the piston or piston rings. This is not likely to be needed except after a considerable mileage has been run. The cylinder piston and rings should be disturbed as little as possible. Frequent removal and reassembly sometimes cause prematurely heavy oil consumption.