Adjusting Valve Clearance - Scooter Focus
Valve clearance ("valve lash", "tappet clearance") can affect the way the engine runs. It's the clearance between the rocker arm and the valve stem. If it is too large, the valve won't open all the way (and you'll get a loud "clattering" noise) and if it is too small (or non-existent), the valve may not properly close. Either way is a problem. You either won't get fuel/air into the cylinder properly or you won't get good compression and/or exhaust and the valve may overheat and suffer damage.
To adjust the valves the engine must be COLD. If it's not cold, the setting will be wrong. Then remove the valve cover. On single cylinder engines there are two valves (one exhaust, one inlet). Then follow the instructions in your manual if you're lucky enough to have one that gives them. If not then you can try this. Put the scooter on the center stand and/or a jack so that the back wheel is off the floor. Put a wrench on the nut holding the center of the fan and rotate the engine until it's at what is known as "Top Dead Center" (TDC), which is when the piston is at it's highest on the compression stroke. This may be marked with a "T" on the flywheel (see image below) At this point both valves should be fully closed and the gap between the rocker arm and valve stem (clearance) should be at its maximum. Both rocker arms will be at minimum height positions on the camshaft lobes. There may be marks on the flywheel to indicate TDC, and/or there may be marks on the outside of the cam sprocket. Check your user manual if you have one.
If you can't find any timing marks for TDC you can rotate the engine clockwise the cam lobe is about 180 degrees away from the rocker arm. This means that the clearance between the rocker arm and the valve should be at it's maximum and you should be able to wriggle the rocker arm back and forth a bit (that's the clearance). You can also remove the spark plug, put a straw into the cylinder and rotate the engine until you feel the piston is at its maximum height both valves are fully closed (note that there are two position in a 4-stroke engine at which the piston is at it's maximum height in the cylinder. Be sure you get the right one). Once you're at TDC, loosen the locking nut and adjust the clearance using a set of feeler gauges. When you have it set correctly, tighten down on the lock nut, and re-check it. Now do the same thing again for the other valve. For most 150cc GY6 engines the clearance should be around .004", but check your manual. For 50cc engines I've seen .002" recommended.