Camshaft

The camshaft is a short shaft with egg-shaped bumps (lobes) that opens the valves. On an overhead-cam (OHC) engine it sits in the cylinder head and is turned by the cam chain. On a pushrod engine like the K-Light it sits low in the crankcase, turned by a gear, and pushes the valves through pushrods and rocker arms.

What it does

The camshaft turns at half crank speed. Each lobe lifts a valve open at the right moment and lets the spring close it. Some cams carry a small decompressor that eases kickstarting. Lobes and bearings wear when the oil is dirty or low, and a worn lobe opens the valve less, so the engine loses power.

Signs it is worn or failing

• Ticking from the top of the engine that returns soon after setting valve clearance.
• Low power and poor hill climbing.
• Kickstart kicks back if the decompressor fails.
• Pitted or flat lobes, or scored bearing journals, seen during an inspection.

When to replace it

When lobe height is below the limit, the lobes are pitted or flat, or the journal or bush clearance is past the limit. Replace followers or rockers that ran on a worn lobe at the same time. Follow your bike's manual.

What to match when buying

• Match the engine code exactly. Cams with the same shape can have different lobe timing.
• For OHC engines, check the cam sprocket bolt pattern and timing marks. For pushrod engines, check the gear and decompressor.
• Fast-road or "racing" cams change idle and fuel use and can hurt reliability. Fit the standard profile for a commuter.

Fitting notes

• Workshop job. Valve timing marks must line up, or the valves can hit the piston.
• Oil the lobes and journals on assembly. Set valve clearance afterwards.
• See Assess the top end or, for a crankcase cam, Assess bottom end.
• Tighten the bearing caps evenly and turn the engine over by hand very gently to confirm the cam turns freely before starting.

Reference (K-Light 125)