What started out as a motorcycle that was getting harder and harder to start progressed into a loud tap-tap-tapping sound when the bike finally turned over. The motorcycle was down on power while getting disproportionately bad miles per gallon of gas. The tapping sound is a clue that the valves are getting tight and are in need of adjustment. Overtime valves pound into their valve seat fractions of a millimeter but this means that the valve is not opening up fully; allowing spent air-fuel mixture to stay in the combustion chamber, limiting how much fresh air-fuel mixture can enter the chamber. The result is a motorcycle that is hard starting and low on power but the remedy is spending a couple hours and a couple dollars getting the valves back in check.
This task requires a basic supply of tools with the addition of a feeler gauge set ($5) and a torque wrench (varies). First, you will have to remove your seat and gas tank to give you access to the engine. If you have an old Japanese bike you are in luck as all you have to do is remove small covers over each of the valve. Modern bikes will require you to remove the valve cover, which means replacing the valve cover gasket, and possibly draining the bike's coolant. Next you will have to remove a plug on the alternator to set a cylinder at Top Dead Center and remove a plug on the crankshaft so you can spin the engine. It is imperative that the cam lobes do not depress the valve spring when you are checking valve lash, as this will jeopardize any measurement you take.
With the lobes facing away from the valve, insert your feeler between the top of the valve spring and the cam lob. Keep inserting larger feeler gauges until you can no longer fit a larger feeler gauge in the crevice. The largest feeler gauge you can fit in the crevice is your valve lash. Write this number down and move on to the next cylinder. Most modern motorcycles have two intake and two exhaust valves per cylinder, meaning a four-cylinder motorcycle has 16 valves that need to be checked. There are two basic types of valve adjusters: shim under bucket and screw type adjusters. Shim under bucket utilizes shims that fit under a bucket beneath the camshaft to open the valve more or less. If you find a tight valve you must remove the camshaft, bucket and check what size shim is currently installed.
Then you cross-reference the shim size with the adjustment you want the bike to have to find the shim needed. Armed with that information, you go down to a local motorcycle shop where they will exchange your old shim for the correct size shim at no charge. Screw type adjusters require loosening a locknut and threading a screw in or out until you achieve the correct valve lash. Once that is done you torque the locknut down so the valve maintains this valve lash. Install everything in the reverse order from which you removed it and go for a test ride. While this procedure could take a couple of hours, you can expect a motorcycle that starts quicker, pulls stronger and runs farther on a gallon of gas.
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