Decarbonisation. Instead of the usual stipulated mileage interval between
periods of decarbonisation, it is recommended that this is undertaken
only when the need becomes apparent because of loss in power, heavy
petrol consumption or generally reduced performance.
When undertaken, unless it is thought necessary to inspect the piston
and rings, the cylinder barrel is best left undisturbed.
Before starting this work have available a gasket set, and if the machine
has covered considerable mileage, a new set of piston rings also.
Carbon formed on the piston crown and in the sphere of the cylinder
head, can be scraped off with a cheap steel rule, with the sharp corners
removed, or similar tool. Deal with the cylinder head before removing
the valves, and do not use emery cloth or other abrasives for this work.
To remove and replace the valves and guides. The hair pin valve springs
are removed by inserting the index finger through the coil of the spring
and pulling upwards sharply.
A light tap on the valve spring collar will expose the valve split collets
(which should be put in a place of safety), then take out the valve,
Both valve guides are located by an external circlip. The cylinder head
must be gently and uniformly heated before attempting to remove or
replace the guides.
With the head pre-heated tap the guide upwards out of the port
sufficiently to enable the circlip to be prised out of its groove. Reheat
the head and drive out the guides through the cylinder head. When re-
fitting the guides, pre-heat the head and verify that the oil holes are in
alignment with holes in the cylinder head. Use a valve spring compressor
where coil springs are used.
Valve grinding. The grinding is accomplished by smearing a thin layer
of fine grinding paste (obtainable ready for use at any garage) on the
valve face and then, after inserting the valve in the head, partially re-
volve, forwards and backwards, while applying light finger pressure to the
head, raising the valve off its seat and turning to another position after
every few movements. (Never revolve the valve continuously in one
direction).
When the abrasive ceases to bite, remove the valve and examine its
face. The grinding may be considered to be satisfactorily completed when
a continuous matt ring is observed on both valve face and seat.
After grinding, all traces of abrasive must be carefully washed off with
petrol and a piece of rag, moistened in petrol, should be pulled through
the bore of each valve guide to remove any abrasive that may have entered.
A holder for the valve, when grinding id the valve, can be supplied. The
part number is 017482.
Replacing the valves. A valve spring compressor is required to compress
the springs and a special tool which is inexpensive, can be obtained from
dealers. Before fitting the valve springs, position correctly the valve spring
seat. The raised portion on the underside is located with the depression
(5) in cylinder head. (Fig. 4.) The inlet valve is the larger of the two valves
and it is vitally important to locate correctly the two split collets into
the grooves on each valve stem.
Clean the valve guide bores with a piece of clean rag, apply a little
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