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4-stroke-engine Valve Adjustment Made Easy

8 Easy Steps to Optimum Performance
by John Reid

Four-stroke engines are marvels of miniature mechanical engineering. They’re small enough to fit in the palm of your hand, and they produce up to 2hp. Even though they have twice as many moving parts as 2-stroke engines, 4-strokes provide hours of reliable service with very little maintenance. With so many moving parts, it is easy to see that timing is critical to smooth operation. Over time, a 4-stroke’s moving parts wear, and this increases the gaps between them. Among the more crucial gaps are those between the rocker arms and the intake- and exhaust-valve stems. These gaps control valve timing, which in turn affects an engine’s power output. If a gap is too large or small, it can greatly affect how an engine runs.

When is valve adjustment required?

Engine-valve clearances are usually correctly set at the factory. You may never need to adjust your engine’s valves; it depends on how much you use it. There are certain symptoms, however, that indicate a need to check and adjust the valve clearances. Check the valve clearance if there is a noticeable loss of power and after you’ve disassembled and reassembled you engine. Here’s how to do it.

To adjust the valves on an O.S. 4-stroke engine, you’ll need a thin (0.04mm) and a thick (0.10mm) feeler gauge, a 1.5mm Allen wrench and a 5mm wrench. Engines by other manufacturers may require tools of different sizes.

Begin by removing the valve cover to expose the rocker arms. Check and reset the valve clearances only when the engine is cold. If you check when it’s hot, the metal will have expanded, and the clearances will be smaller than when the engine is cool.

Before you adjust the valves, make sure that both rocker pushrods are at their lowest positions, i.e., the piston is at top dead center (TDC) between the compression and power strokes. There is also a TDC position between the exhaust and intake strokes. To find the TDC of either stroke, remove the glow plug and watch the piston while you turn the prop shaft. When the piston reaches its highest point in the cylinder, it has reached TDC. Insert a toothpick in the glow-plug hole and watch as it rises; stop turning the prop just before the toothpick starts to drop back into the cylinder. On the prop washer/hub, scribe a thin line that lines up with a line on the crankcase. (The O.S. 4-stroke shown already has a TDC mark on its prop hub.)

Next, find the correct valve and pushrod orientation between the compression and power strokes. Install the prop and glow plug, then turn the prop counterclockwise until you feel compression. Turn the prop 1/4 revolution more until the mark on the prop washer/hub is lined up with the one on the crankcase. If you rotate the prop shaft back and forth about 1/8 inch and the rocker arms move, this is the wrong TDC position; the engine is between the exhaust and intake strokes. Rotate the prop one full turn counterclockwise until the marks are again lined up. This is the TDC between the compression and power strokes; rotating the prop shaft back and forth will not make the rocker arms move.

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