In order for any scooter engine to run properly, both intake and exhaust valves must open on time. The rocker arms are responsible for opening the valves. At the end of a scooter rocker arm is the tappet. With wear and tear, tappets go out of adjustment and should periodically be re-adjusted. The recommended adjustment varies from brand to brand and it is recommended that the service manual for the specific type of scooter be consulted.
Valve lash also know as tappet clearance, will have a direct effect on engine performance. Valve lash is the minute distance between the tappet and the corresponding valve stem. It is critical that this clearance be within specification (specified by the particular scooter brand) in order for the scooter to run. Valves that fall out of adjustment over time can cause a loss of compression, reduced power and can lead to engine, damage. In some cases your scooter may not even start.
With regular usage, you should incorporate valve adjustment as part of the regular maintenance schedule. This should be done in line with any recommendation from the manufacturer, which can usually be found in the documentation that came with your particular brand of scooter.
Before you start, one of the first things that is recommended is that you have the right tools for the job. While you can get away with using some very “basic” tools, it’s always recommended that you definitely source out tools that are job specific. For example, while you certainly could make the tappet adjustments with a 9mm wrench and a pair of needle nose pliers (which I have seen done) you would achieve easier, more accurate results using a tappet adjustment tool. At the time of writing this article, a decent tappet adjustment tool was about thirty dollars online. If this sounds like a lot, just remember, as you incorporate valve adjustment into your regular maintenance schedule, you can see that the cost of the tool broken out over many jobs makes the expense really small.
For this job you should have a set of feeler gauges and a tappet adjustment tool. Likewise, if you do not have the tappet adjustment tool, you can use a 9mm wrench and a pair of needle nose pliers. You absolutely need the feeler gauge set for achieving the proper clearance.
Start with your scooter engine COLD. The reason you want the engine cold is because as an engine heats up, metal expands. This expansion would most assuredly make the clearance larger once the engine cools. This would result in too large of a valve lash which would likely not allow the valve to open properly. Put the scooter on the center stand so that the back wheel is off the ground. Remove the valve cover and place the screws in a small container so they do not get lost. There will be two valves. One is the intake valve and the other is the exhaust valve.
Next rotate the engine fan until it reaches “Top Dead Center”, which is when the piston is at the highest point on the compression stroke. The Top Dead Center marking is stamped on the flywheel and is denoted usually as a “T”. At this point, both valves should be fully closed and the valve lash between the rocker arm and valve stem should be at its max. Both rocker arms will be at minimum height positions on the camshaft lobes. In addition to the marking on the flywheel, and/or there are also markings on the outside of the cam sprocket that can be used as well.
Once you’re at Top Dead Center, loosen the locking nut. To adjust the clearance, use a set of feeler gauges. You should be able to slide the correct sized gauge (per manufacturers specifications) between the tappet and the top of the valve stem with only the slightest resistance. Likewise if you can freely pass the gauge through, the clearance is too big. Once you have the correct clearance set, tighten down the lock nut, and re-check it. Now do the same thing again for the other valve. Again, the recommended adjustment varies from brand to brand and it’s recommended that the service manual for the specific type of scooter be consulted.