34
Chapter 1 Engine, clutch and gearbox
24 Examination and decarbonisation: cylinder head
1 Remove all traces of carbon from the cylinder head and
valve ports, using a soft scraper. Extreme care should be taken
to ensure the combustion chamber and valve seats are not
marked in any way, otherwise hot spots and leakages may
occur. Finish by polishing the combustion chamber so that
carbon does not adhere so easily in the future. Use metal polish
and NOT emery cloth.
2 Check to make sure the valve guide bores are free from
carbon or any other foreign matter that may cause the valves to
stick.
3 Make sure the cylinder head fins are not clogged with oil or
road dirt, otherwise the engine will overheat. If necessary, use a
wire brush.
4 Reassemble the valves, using a valve spring compressor to
compress the springs. Make sure the valve stems have a
coating of oil before they are replaced in the valve guides. Also
check that the split collets are located positively before the
spring compressor is released. A misplaced collet can cause a
valve to drop in whilst the engine is running and cause serious
damage. To ensure that the collets are firmly located, tap the
top of each valve stem sharply with a hammer, taking care to
strike squarely on the stem and not on the seat. Note that the
outer springs must be positioned with the closely wound coils
nearest the head.
25 Examination and renovation: rocker arms, adjusters and
1 It is unlikely that excessive wear will occur on the valve
gear components unless the engine has been run without
changing the oil or the machine has covered a very large
mileage.
2 Examine the condition of the rocker assembly, which should
operate smoothly, but without discernible radial play. If worn, it
will be necessary to renew the complete rocker assembly, as
the unit cannot be dismantled.
3 Check the condition of the adjuster and lock nut threads,
and renew these if stretched or damaged. The ball-ends of the
pushrods should also be examined, and the pushrod renewed if
they are worn or cracked.
4 Check the pushrods for straightness by rolling them on a
flat surface. If they are bent, this is often a sign that the engine
has been over-revved on some previous occasion. It is better to
fit replacements than attempt to straighten the originals.
26 Examination and renovation: cam gear and lobe, and
cam followers
1 Examine the condition of the cam gear teeth, looking for
signs of chipping and wear. At the same time, examine the
corresponding teeth of the driving gear on the crankshaft. The
camshaft gear can be separated from the cam lobe, if desired,
by removing the circlip which retains it.
2 The cam lobe itself is unlikely to exhibit any great degree of
wear due to its wide profile, but after extended mileage a flat
may begin to appear at the top of the lobe. This can be checked
by measuring the lobe across its widest point. The nominal size
is 32.768 - 32.928 mm (1.2901
1.2964 in). The cam will
require renewal if worn to less than 32.628 mm (1.2846 in).
3 The clearance between the cam gear and the shaft on
which it is carried should not exceed the specified amount. The
condition of the cam followers should also be checked,
especially if the cam lobe is worn. The clearance between the
cam follower bores and the pivot shaft must not exceed 0.1 mm
(0.004 in). The rubbing face of each cam follower should be
examined for signs of scuffing or scores, indicating wear due to
poor lubrication.
27 Examination and renovation: trochoidal oil pump
The trochoidal oil pump is removed as a sub assembly as
described earlier in this Chapter. Dismantling and renovation
are covered in Chapter 2. The condition of the pump should be
checked as a matter of course if the engine is being overhauled,
especially if signs of scuffing are evident on the various shafts
and bushes.
28 Examination and renovation: clutch and primary drive
1 After a considerable mileage has been covered, the bonded
linings of the clutch friction plates will wear down to or beyond
the specified wear limit, allowing the clutch to slip.
2 The degree of wear is measured across the faces of the fric-
tion material, the nominal or new size being
- 3 0 0 mm
in). If the plates have worn to
mm
in) they should be renewed, even if slipping is not yet apparent.
3 The plain plates should be free from scoring and signs of
overheating, which will be apparent in the form of blueing. The
plates should also be flat. If more than 0.20 mm (0.0079 in) out
of true, judder or snatch may result.
24.4 Do not omit inner spring seat
26.3 Check fit of followers on pivot shaft — note thrust washer
