4-stroke-engine Valve Adjustment Made Easy

8 Easy Steps to Optimum Performance
by John Reid

To check the rocker-arm valve clearance, insert the thin (0.04mm) feeler gauge; it should slide easily into the gap. Now try to insert the thick (0.10mm) feeler gauge; it should not fit into the gap. Be careful here; it doesnít take much force to depress the valve and allow the thick gauge to slide into the gap. The valves need to be adjusted if the thick gauge slides easily into place or if the thin feeler gauge doesnít fit.

To adjust the gap, carefully use a wrench to loosen the locknut on the rocker arm about 1/2 turn. Then use the Allen wrench to back the adjustment screw out about one turn (usually counterclockwise).

Insert the thin feeler gauge between the valve stem and the rocker arm, and gently tighten the adjusting screw until the rocker arm and the valve stem lightly touch the gauge. The adjusting screw can make a big difference, so use a light touch when making this adjustment. Grasp the Allen wrench using only your thumb and index finger as shown.

Retighten the locknut while you hold the adjustment screw firmly with the Allen wrench. Remove the wrenches and the feeler gauge, rotate the prop a couple of revolutions, and recheck the gap. If the clearance is correct, repeat these steps for the other rocker arm and valve assembly. After you have properly adjusted and checked everything, replace the valve cover. Thatís it!

When you set the gap for valve adjustments, itís important to remember that it is better to have too much play in the valve assembly than too little. With a wide gap, the valves wonít stay open as long as they should. This doesnít really hurt anything and incurs only a small loss of power, but having a very small gap or no gap at all can burn and erode the valves and valve seats. You can always avoid this by making sure that the thin gauge slides easily between the rocker arm and the valve stem. Periodic valve adjustment will ensure that your engine runs smoothly and will help to ensure hours of reliable 4-stroke engine service.

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