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    After a few thousand kilometers usage the carbon build up in the engine will cause general falling off in power, accompanied by increased fuel consumption and starting trouble. De-carbonizing will normally be necessary every 8000 kilometers. This can be carried out without removing the engine from the frame. The mileage between de-carbonizing will vary from machine to machine depending on the type of usage. A machine used for frequent short journeys will require more attention that one that is used for long distance touring.

    1. Removal of the Petrol Tank

    Close the petrol tap.  Disconnect the petrol hose from petrol tap end.

    Remove the two studs which holds the petrol tank to the frame and pull the petrol tank upwards.

    2.Removal of the Cylinder head

    Remove the engine steady eye bolt.

    Disconnect the high tension lead from the spark plug. Remove rocker oil pipe.

    Remove the exhaust pipe and silencer.

    Remove the air filter by undoing the bolts on the side of the air filter body.

    Push the carburetor back clear off the nuts after removing the fixing nuts.

    Remove the rocker box covers.

    Remove the de-compressor cable from the lever end of the handlebar (LH side).

    Remove the rocker arms and bearings completely after removing four 3/16" nuts on each.

    Lift out the push rods both inlet and exhaust.

    Remove the six cylinder head nuts and washers

    Lift the cylinder head off the barrel, tapping it gently beneath the exhaust and inlet ports 
    with a wooden mallet.  Do not tap the fins.

    3.Removal of Cylinder and Piston :

    Slacken the two clamp nuts on the top of the crankcase neck. Remove the 1/4" nut above the tappet chest and lift the barrel.

    Remove the circlip retaining the gudgeon pin on the timing side of piston taking care not to drop the circlip into the crank case.

    Extract the gudgeon pin using special tool PED 2015 (with adaptor if necessary) so that the piston and the pin can be replaced the same way around, ie. split skirt to the front.

    During the operation put a piece of clean rag in the top of the crankcase to prevent foreign matter getting in. Finally cover the crankcase with a clean cloth to prevent ingress of dust and dirt.

    4.Removal of Valves :

    To remove valves from the cylinder head, first lift off the end caps from the valve stems. If these have stuck they can be removed with a screwdriver. Using compressing tool PED 2018 ST compress the valves springs from the valve tip. Slacken back the compressing tools and release the springs. Withdraw the valve and place its springs, top spring collar, bottom collar, the end cap and split conical collars together in order that they may be reassembled with the valve from which they were removed. If the valve will not slide easily through the valve guide, remove any slight burrs on the end of the valve stem with a carborundum stone or by using a fine jeweler's file to remove any sharp edge or burr. If the burrs are not removed and the valve is forced out, the valve guide may be damaged.


    5.Decarbonizing the Cylinder head - Combustion Chamber:

    Remove carbon from the valves, ports and combustion chamber by scraping. Take care not to cause any damage to the valve faces or valve seat inserts. Scrape gently to avoid scoring the cylinder head. DO NOT, under any circumstances, use caustic soda or potash for the removal of carbon from aluminium alloy. Remove the piston rings carefully. For cleaning the grooves in the piston, a piece of broken piston ring thrust into a wooden handle and filed to a chisel point can be used.



    6.Piston and Rings

    If the piston rings are in good condition they can be put back, taking care to fit them in their original grooves and the same way up. If the rings show brown or black patches, or if their gaps are more than specified service limits (Page 79), when in position in the barrel, new rings should be fitted. The correct gap for new rings are given in the technical specifications (pages 6 & 8 ) for 350 and 500cc. The gap should be measured in the least worn part of the cylinder, which will be found at the top or bottom of the bore.

    Only for 350cc:

    The original size of the cylinder bore is 2.751" (69.875mm). If the wear at any point in the bore exceeds 0.008" the cylinder should be rebored to 0.020" and an oversize piston fitted. (It should be rebored to 0.40" after a further 0.08" wear). Piston sizes available are .020" and .040" oversize. The original side clearance between the piston rings and grooves is 003". If the grooves show a wear of .005" the piston should be replaced.

    7. Big end bearing inspection:

    Examine the condition of the big end while the piston is removed. About 0.10" - 0.20" end float is permissible and it will be possible to rock the connecting rod slightly. The big end has a floating bush with an original clearance of approximately .003" However, if a DEFINITE up and down play can be felt engine should be stripped further to have the big-end renewed.



    8. Valves, Valve Guides and springs

    Wear on the valve stems can be seen on examination and if a definite step has formed, the valves should be renewed. Before replacing the valves, they must be ground on to their seats If good faces are not formed with reasonable amount of grinding, the seats must be cut with a cutter (included angle 90o) and the valve refaced in a universal grinder. Do not attempt to form good seats by an excessive amount of grinding. This will cause pocketing and restrict the flow of gases. If a pocket has already been formed this must be removed by cutting with a valve seat cutter larger in diameter than the valve head. 

    Test the valve guides for wear by trying fit of a new valve in them. Both valves should be quite free, but the exhaust valve has more clearance (.002") than inlet valve.
    To remove the valve guides from the head, two special tools are required which can be easily made. The first is a piece of tube with an internal bore of not less than 7/8". The second is a mandrel about 4" long made from 9/16" diameter bar with the end turned down to 1/3" diameter for a length of 1/2". Support the cylinder head on the tube, which fits over the collar of the valve guide. Using the mandrel, force the guide out of the head with a hand press or by using a hammer.

    To fit a new guide, support the head at the correct angle and use a hand press and the same mandrel. If a hand press is not available, the guide can be replaced using a hammer and a mandrel, to prevent damage to the guide. Check the length of the valve springs which are originally 2.020" and 2.095" for the inner and outer springs respectively. If these have reached the specified service limits they should be renewed.


    If the decompressor holds compression and operates freely there is no need to interfere with it except to remove the carbon from the head of the valve. If the valve is leaking, it will be necessary to regrind it on its seat. This can be done without completely dismantling it. Having disconnected the control cable from the handle bar, unscrew the decompressor from the cylinder head. Compress the spring and remove the spring cap. Unscrew the adjusting screw and locknut from the cable block and pull the cable sideways out of the block. Push the spring upwards and pull the cable nipple out of the body. It will now be possible to remove the cable and nipple through the spring, leaving the decompressor body and spring detached from the control cable.

    The spring and the cap should now be replaced. The valve may be ground in by applying a thin coating of grinding paste on the seat of the valve and twisting to and fro by means of the cable block at its upper end and occasionally lifting the valve from its seat. Do not rotate the valve through a complete revolution before lifting, as this will groove the seat.

    After grinding, wash the whole assembly thoroughly in petrol, opening and shutting the valve while doing so. Make sure that all traces of grinding paste have been removed. If the paste should get into the cylinder serious damage would be caused. If the valve shows a tendency to stick-up in the body but otherwise is satisfactory, this can be cured by washing in petrol, though in this case it will not be necessary to disconnect the control cable. If the decompressor valve is badly burnt or bent it must be replaced.

    10. Re-assembly after Decarbonising :

    Before building up the engine, see that all parts are scrupulously clean and place them on a clean tray, work bench or over a clean sheet of paper. While re-assembling it is advisable to fit a new gasket between the cylinder barrel and the crankcase. Smear clean oil over the piston and space the ring gaps. The second ring is a taper ring and is marked TOP on the upper surface.

    WARNING: This mark should be on top when fitted. Reversing the ring will result in pumping of oil into the cylinder and consequent smoking.

    Place the piston over the connecting rod small end ensuring the split skirt is facing the front and insert the gudgeon pin. Secure the gudgeon pin with the circlips. Oil the cylinder bore and gently push barrel over the piston while keeping the rings compressed in their grooves and seat it gently on the barrel gasket. Refit the 1/4" nut above the timing chest. When fitting the head again, apply jointing compound sparingly on both sides of the gasket. Replace the six nuts and tighten them progressively and diagonally from one side to the other to prevent distortion.

    WARNING: Excess compound may block oil-ways.

    Place the push rods with the adjustable parts downward. The shorter pushrod is the Inlet. Ensure valve stem caps are fixed on the valve stems. Position the rockers and bearings, making sure that the oil feed holes are at the bottom and that the caps and bases are in line when tightened down. Adjust the push rods after ensuring piston is in TDC on compression stroke. 
    The silencer could be cleaned of carbon using a hot caustic soda solution, if necessary.

    NOTE:The cylinder head and base nuts should be checked again for tightness, after the engine has been run long enough to get thoroughly warm.

    Tighten clamp nuts on crankcase finally. For torque tightening of cylinder head nuts please refer torque chart on page .100

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