MotoMojo: Valve Adjustment

Aug 13, 2013 View Comments by

MotoMojo: Valve AdjustmentValve adjustment, which can often be done by DIYers, is necessary routine maintenance on many motorcycle models. Check the owner’s manual for service intervals.

If an engine has insufficient valve clearance (lash), valves have less time to give off heat into their seats, which leads to burned valves. Tight valves also may cause hard starting, low power, or rough running. Valve clearances decrease due to seat recession, which occurs over time as a valve pounds itself into its seat. It’s more common in older or high-mileage bikes but also occurs on newer motorcycles that are ridden hard and revved high. Loose valves are noisy and also cause excess wear.

Types of Adjustment

The easiest valve gear to service is the screw adjuster. Another method uses shims of various thicknesses to adjust valves. Shim adjusters are typically used on high-revving engines, and there are two major types of shim-adjusted valves; shim-over bucket and shim-under bucket. With shim-over bucket, the cam lobe rubs directly on the shim. Generally, these shims can be removed and replaced without removing the camshaft. Shim-under bucket adjustments call for mounting the shims under the valve buckets, which greatly complicates adjustment because the camshaft(s) must be removed to change shims.

Changing shims requires special tools and is more likely to lead to mistakes and problems than adjusting screw adjusters, especially when removing camshafts. Fortunately, shim adjustments tend to seldom need changing until high mileage. Therefore, we suggest you measure the lash, determine if your engine needs shims changed, and have a pro do it. Many times it will not, and you will save the cost. Desmodromic valve actuation, found on Ducatis, requires precise detailed procedures. Therefore, also leave these to the pros.

Some bikes, including many Harley-Davidsons, have hydraulic valve adjusters that require no adjustment. Consult the owner’s manual to determine if your bike has them; if so, you’re off the hook.

Read the service manual, and gather everything you’ll need before starting. Adjusting valves on bikes with screw adjusters usually requires only a few items. These include gaskets, a set of feeler gauges, wrenches and screw drivers, any special tools needed to fit the valve adjusters, a spark plug wrench, and clean rags.

Allow the engine to cool overnight. In many cases, the gas tank and some bodywork must be removed. Clean any dirt from around the cover(s) and spark plug wells, and remove the spark plug(s) to allow the engine to turn freely. Remove the valve cover(s), and place a clean cloth over the opening to prevent small items from dropping into the engine.

Measuring Valve Lash

Valve lash specifications may be found in the owner’s or shop manual or on a tune-up/emission information label on the bike. Valve clearances must be checked and adjusted when the piston of the cylinder you are working on is at top dead center (TDC) on the compression stroke. At TDC, all valves for that cylinder are closed.

To find approximate TDC if cam lobes are visible, rotate the crankshaft until the raised lobes are directly opposite from where the valve lifters, shims, or rocker arms contact the camshaft. Turn the crankshaft in its normal direction while watching the valves you want to check or adjust. If lobes are not visible, you’ll have to follow the procedure in the shop manual.

If your crankshaft has no convenient way of turning it, position the bike on the centerstand or a motorcycle jack with the rear wheel clear of the ground. Place the transmission into its top gear, and slowly rotate the rear wheel by hand in a forward direction to turn the engine.

Normally, measuring and adjustment is started with cylinder number one and follows the firing order on multi-cylinder engines. Slip the blade between the screw adjuster (or shim) and valve stem tip. The blade should have light drag but still pull through without binding.

Adjusting Valve Lash

If a valve with screw-type adjuster needs adjusting, loosen the locknut and turn the adjuster clockwise to decrease lash or counter-clockwise to increase lash. When clearance feels right, hold the adjustment screw in place and tighten the locknut securely. Recheck lash with a feeler gauge; often clearance changes as you tighten the locknut. Mark each adjuster with a crayon to denote that it is done. Repeat the procedure until all valves are done. Once all the valves have been checked and adjusted, reinstall all components that were removed. Double check everything, start the engine, and listen to how it runs.

Text and photography by Ken Freund

 

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