Norton Commando Workshop Manual

## Engine/Primary Drive

# C

8 | On all 1972 models and 1971 models with the locating groove adjacent to the inlet valve guide shouider, ensure that the inlet guide to stem seals are fitted and located correctly.
9 | With the valve spring seat in position, loose assemble the valve (with lightly lubricated stem) inner and outer valve spring and top valve collar.
10 | Using the valve spring compressor, compress the collar and springs and insert the split valve cotters
11 | When the collets are firmly in position in the valve stem groove, remove the compressor. The cylinder head is now ready to refit.

### SECTION C4

### DECARBONISING

After the motorcycle has been in use for some time, carbon deposits accumulate within the combustion chamber and exhaust ports and also the valves cease to seal completely due to wear and a certain amount of burning on the valve seatings. These conditions result in a lack of compression and an increase in the compression ratio, giving symptoms of a gradual loss of power, poor compression and difficult starting. To remedy these conditions it is necessary to remove the carbon deposits and grind the valves to their seatings to obtain a gas-tight seal. To do so, remove the cylinder head (Section C1) and remove the valves (Section C3).

Unless a carbon dispersant vat is available, the carbon must be scraped from the combustion chambers, exhaust ports and piston crowns. Since the cylinder head and pistons are produced from an aluminIum alloy material which is scratched easily, the carbon must only be removed by a soft metal or hard wood scraper which will not damage the surface. A steel scraper or screw driver blade must not be used. DO NOT remove the thin ring of carbon round the top of each cylinder bore and leave also a ring of carbon around the edge of each piston crown. Removal of this carbon will tend to increase the oil consumption.
The valve stems and heads shouid be scraped clear of carbon but take care not to scratch the facings. Examine the facings for deep pitting which wouid render the valves unfit for re-use.
When ail parts are perfectly clean and in good condition grind in the valves and reassemble.

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