Cylinder Head and Barrel
Remove all carbon deposit from the cylinder head, bearing in mind again that the
aluminium is soft and easily damaged if the decarbonising tool is carelessly applied,
and carefully wipe clean to ensure the removal of all loose particles. Most of the
carbon deposit likely to have accumulated in the cylinder will be in the exhaust port,
and cleanliness at this point is most important as explained earlier. Scrape this out
carefully, taking care not to let the tool slip into the bore and damage the surface of
the latter. Examine the transfer and inlet ports for the presence of carbon, although
this is unlikely to be heavy, and finally wipe the ports and the cylinder bore
absolutely clean.
Big end Bearing
While the cylinder is off, it is as well to test the big-end bearing for wear. This is
done by taking hold of the connecting rod stem and pulling it upwards until the crank
is at the top of its stroke. Then, holding it in this position, try gently but firmly to
pull and push the connecting rod in the direction of its travel in order to feel whether
there is any play. If the big-end is in a sound condition there should be no play in this
direction although it may be possible to rock the rod sideways, i.e. at right angles to
the axis of the machine. If vertical play is perceptible in the big-end and you do not
feel qualified to decide whether the amount in evidence is permissible or not, you
should seek expert advice. This point is not likely to give trouble, however, provided
that the machine has been carefully used and adequately lubricated, for the big-end
bearing is of ample dimensions for the work it has to do. But if for any reason the
big-end bearing has deteriorated as the result of neglect or abuse, it should be
replaced and unless you have the necessary experience and facilities for this class of
work it is preferable to have it done by an expert repairer.
Re-assembly
Before attempting to replace the cylinder barrel over the piston, smear the former
generously with engine oil and then place it over the piston, carefully manipulating
the rings into the end of the bore and seeing that they enter freely without the
application of force. As soon as the cylinder barrel is in position, replace the cylinder
head and put the washers and nuts on the four holding down bolts. Tighten the nuts
in diagonal order so as to avoid distortion and, if a torque spanner is being used, it
should be set to 10 lb. ft.
Examine the sparking plug and replace if sound. Check the rubber sealing ring on
the carburetter flange, replacing if damaged. Finally reconnect the exhaust pipe,
carburetter, and petrol pipe.
Sparking Plug
The sparking plug is of great importance to satisfactory engine performance, and it
is necessary that it should be the correct type and in good condition. Champion type
N4 is recommended.
The condition of a sparking plus can be helpful in determining the condition and
tune of an engine, and it is recommended that the plug be removed from time to time
for examination.
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