Adjusting valves on a motorcycle, ATV, or similar 4 stroke small engine is a fairly straight forward process that will give you peace of mind and save you money by not having to take it to a mechanic. Valves are one of those parts that eventually get out of spec and need to be adjusted. Some machines require it more often than others. If you think you might have issues with your valves (low compression, valve noise, starts or runs poorly, hard to start, bad idle, etc) it is always a good idea to at least check the valve lash to make sure it’s within the correct limits.
Note: The two main types of valve adjustment setups are via a screw and locknut or with shims. This article discusses the screw and locknut style valve adjustment. The shim type valves are usually found on higher compression/performance engines such as motocross bikes.
What you will need to know ahead of time:
- Valve clearance specifications for exhaust and intake valves (usually measured in millimeters such as .08mm intake and .12 exhaust)
- How to get your bike to TDC (top dead center) by turning the crank. (usually a nut that is behind a crankcase cover or you can elevate the back tire and put it in 5/6th gear and turn the motor over that way) You will also need to know where the timing marks are so you know when the piston has reached TDC.
- How to use basic hand tools. Basically if you have never used a wrench before, you shouldn’t be attempting this.
- Feeler gauges – Make sure you have a set of gauges that will work for the valve clearance specs on your machine. (you may need to stack a couple in some cases)
- Spark plug wrench/socket – The spark plug will need to be pulled to release compression when turning the engine over.
- Tools to take off the valve cover – Sockets or allen keys are standard for most covers.
- Socket for cranknut (a.k.a. crankshaft nut)– A socket that is big enough to fit on the cranknut so that the shaft can be turned to get the piston at TDC.
- Tool for turning the adjuster screw – some have a slotted head for a screw driver and some have a square slot that will need a special tool (usually a pair of pliers works fine too)
- Valve cover gasket – Not always required, but might be good to have on hand in case the old gasket gets damaged with removal of valve cover. Always go with OEM gaskets.
How to adjust the valve clearance:
- Remove the spark plug(s).
Remove the valve cover (I usually start with the intake, but it doesn’t matter) by removing the bolts that hold it in place. Be careful as sometimes they can strip easy. (Especially Phillips head bolts!)
- The valve cover and gasket may be stuck on and need a gentle thump of a rubber mallet to get loose. If you’re careful you may be able to reuse the gasket.
Rotate the engine so that it is at top dead center
- You can cover the spark plug hole with your finger and feel when there is pressure (compression stroke). When the timing marks line up with the marks on the flywheel, you should be in business. (see first photo at right)
- In most cases you will be able to wiggle the rocker arm (the curved metal piece that sits on top of the valve stem/spring) slightly. Sometimes the valve may have gotten tight from wear and the rocker arm will be tight in all instances and have no free play so make sure you are at the TDC of the compression stroke.
Measure valve clearance by inserting feeler gauge inside the gap between therocker arm and the valves stem (see photo below)
- If the clearance is correct, there will be a slight drag felt on the feeler gauge. If the correct gauge doesn’t fit, loosen the locknut and then back off the adjusting screw one quarter turn at a time until it fits. When in doubt, it’s always better to have it slightly loose than too tight. When it’s the right clearance, check to see if the next size up fits. (hint: it shouldn’t fit)
Tighten lock nut
- When you get it to the right clearance, hold the screw in place while you tighten the locknut. It will almost always get tighter, no matter how well you hold the screw in place, so I usually get it to the right clearance and then back off the adjuster screw ~1/8 of a turn or so. Tighten the locknut (not too tight that you strip it!) and then recheck the clearance. It may take a couple tries to get it right.
- Repeat and check valves on same cylinder for the other intake and exhaust valves. (Most 1 or two cylinder bikes and ATVs allow you to check intake and exhaust valves with the engine at TDC but some vary, so know your machine’s specs)
Once you are done with all the valves, replace the valve covers
- Make sure the gaskets and mating surfaces are all clean and free of debris.
- Tighten valve cover bolts in a crisscross pattern and be extra careful not to over tighten and strip the threads. (It’s also a good idea to put some anti-seize compound on the threads before tightening.
- It’s also a great time to inspect the spark plug gap and clean or replace if necessary
Reinstall spark plug and any timing covers you may have removed.
- This is also a great time to inspect the spark plug and gap and clean or replace it if necessary.
- Be very careful not to over tighten the spark plug as it can strip easily. (I always screw it in until it seats and is hand tight and then tighten with a socket ¼ turn further and no more than ½ a turn)
- Cross your fingers (if you feel you need to) and fire that engine up.
- Smile with satisfaction at how easily it started and the purr of a newly tuned valve train.
- Now drive it like you stole it.