If you can't find any timing marks for TDC you can rotate the engine clockwise the 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.
If your scooter starts but won't rev or run well, there are basically five most probable areas of trouble:
Problems with fuel flow
Problems with the spark
Problems with the carburetor
Problems with valve clearance
Fuel flow problems can be caused by a clogged fuel filter, a clogged fuel line, a faulty fuel supply valve or a faulty fuel pump (on those scooters that use one). One way to bypass all these components is to rig up a temporary gravity fed fuel flow directly to the carburetor. If you connect a fuel supply directly to the carburetor from a container held above the level of the carburetor and the scooter revs freely, you've identified the problem and you need to check (or replace) each component of the fuel supply to make sure it's working OK.
If the scooter starts, it must be getting some sort of spark, but the spark may be weak or there may be a problem with the ignition timing. The first thing to do is remove the spark plug and take a look at it. If there are heavy deposits on the tip (black or white), or if it looks worn or damaged or if the gap isn't what it should be, it should be replaced. You could clean it and regap it but plugs are cheap ($5 or less), so you should probably replace it anyway and see if the engine runs better. If there's no difference, at least you now have a spare plug if the one in the engine ever does fail!
Today there are very few scooter engines on which you can alter the ignition timing. All timing functions are done via the CDI (Capacitor Discharge Ignition) module. If it's bad, all you can really do is replace it. They're pretty reliable though, so that might be something to try if all else fails to find a problem.
Leaks in the air or vacuum lines can cause problems too. Usually it's leaks in the vacuum lines which control several functions related to the carburetor operation and fuel supply. First check all the vacuum line connection and the lines themselves for splits and cracks. If they look OK you can spray water on them while the engine is running and see if there is any change in engine speed. If there's a leak, the liquid will temporarily seal it and the engine will run better. Some recommend spraying WD40 or carburetor cleaning fluid on the lines, but if you do there can be a risk of fire, so be careful.
Carburetor problems are the most likely source of poor running. First check the idle speed and air/fuel mixture screws (see carburetor adjustment). If the adjustment is OK, or you can't get the engine to idle well, then comes the next step. Over time (especially if the scooter is stored for long periods of time with gas in the carburetor), "gummy" deposits can build up. These are the residues which are left behind when gas evaporates. They can clog internals valves, jets and passages inside the carburetor which can affect fuel flow. The best way to deal with them is to remove the carburetor from the scooter (see carburetor rejetting), take it apart and soak it in carburetor cleaning fluid, which will dissolve the deposits. Make sure the jets are clear too.
Valve clearance ("valve lash", "tappet clearance") can affect the way the engine runs. This is the clearance between the camshaft 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 man have one.
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