Maintenance - Minor Service

5.1 Checking the valve clearances

The engine must be cold before the valve clearances can be checked accurately. First remove the spark plugs and the engine left-hand outer (cylinder head) cover. See Chapter 1 - Engine. Select top gear and rotate the crankshaft to the desired position by turning the rear wheel.

The valve clearances must be measured at the base circle of the cam lobe i.e. with the lobe pointing directly away from the valve stem. This position is approximately just before Top Dead Center (TDC) on the compression stroke for the exhaust cam and just after it for the intake cam. To find TDC on the compression stroke, rotate the crankshaft until the (upper) intake cam lobe for any particular cylinder has opened and closed its valve, then shine a torch down the spark plug aperture and slowly turn the rear wheel until that piston comes to the top of its stroke. It is easiest to work methodically, starting from number 1 (front, or cam chain end) cylinder and to then work backwards i.e. for 100 models from number 1 cylinder at TDC a half turn of the crankshaft brings number 3 cylinder to TDC, a further half turn brings number 4 to TDC, and a final half turn brings number 2 to TDC.

Note that if the ignition trigger assembly cover is removed the crankshaft can also be rotated (anticlockwise, looking at the trigger from the front of the machine) by means of an Allen key applied to the rotor retaining bolt. Whichever method is used, position the cams as described and us a feeler gauges to measure carefully the clearance between each cam lobe and the shim sitting on its respective cam follower recess. The correct thickness feeler gauge blade wilt be a tight sliding fit between the two components. Carefully record all clearances on a sheet of paper.

If the clearance at any valve is outside the specified range, the shim must be replaced by a thicker or thinner one, as appropriate.

This procedure is described in Section 5.2 with service tools and Section 5.3 without service tools.

Cylinder is at TDC on compression stroke when piston is at the top of the bore with both valves closed. Number 1 cylinder, 100 model shown

Valve clearance is measured with cam lobe pointing away from valve as shown