The Importance of Valve Adjustment

Most four-stroke motorcycle engines need occasional valve adjustment, or at least a valve-clearance check. Adjusting the valves means setting the appropriate clearance between the base circle of the camshaft, otherwise known as the camshaft heel, and the tip of the valve or rocker arm, depending on the valve train setup. This clearance is important because it compensates for the expansion of the various engine components as the engine heats up, thus allowing optimum engine performance and preventing engine damage.

If the clearance is too tight, for example, it may completely disappear when the engine reaches operating temperature, in turn preventing the valves from closing fully. A valve that doesn’t close fully will allow combustion gasses to escape, which reduces engine performance. Worse yet, though, is that if the valve still closes completely but the clearance is too tight, the valve won’t remain seated long enough to transfer its heat into the cylinder head, which could lead to a burnt valve, particularly the hot-running exhaust valve.

Valve clearances also affect valve timing; if you’ve ever purchased an aftermarket camshaft and looked at the spec sheet, you’ll have noticed that cam timing specs are often given either at zero clearance or a specified clearance, the latter being more realistic, since it takes into account the degrees the crankshaft rotates as it takes up said clearance on the leading and trailing sides of the cam lobe. The valve’s duration (the time the valve is open) will therefore be greater if the clearance is tighter (it opens sooner and closes later), and reduced if it’s looser (opens later and closes sooner). This has a direct effect on engine performance.

Reliability is also affected by valve clearance. If it’s too great, it will cause adjoining parts to slam into each other – hence the tapping noise you hear on bikes that haven’t been properly maintained. This pounding will wear out valve train parts prematurely.

When it comes to a brand-new bike, most manufacturers require that valve clearances be checked right after break in, or at about 1,000 km. This might seem premature, but what happens in a new bike is that valve clearances are set to spec at the factory, but tighten up considerably in the first few hundred kilometres, as the valves work their way into their valve seats. This “seating” of the valves sets them a bit deeper into the combustion chamber, and consequently, the valve stems protrude farther at the other end, taking up precious valve clearance. From what I mentioned earlier about what happens if the valve clearance is too tight, you can understand why a warranty may be voided if that all-important first inspection isn’t performed– it’s not about the oil change. Modern materials have helped extend valve service intervals, but it’s important to have a look, especially if you haven’t done so in a very long time but hear no valve noise – they might be too tight.

Valve clearance is verified by inserting a feeler gauge of the correct thickness between the valve stem or rocker arm, and the cam heel. There are three types of valve adjusters: shim over bucket, shim under bucket, and locknut and screw. The last one is the easiest to adjust, since it involves loosening a locknut and turning the adjuster screw – which is in direct contact with the tip of the valve stem – to set the proper clearance; you then tighten the locknut to secure your adjustment. Access to this type of adjuster is also typically easy, since there is usually a valve-adjustment access cover incorporated into the valve cover.

Shims over buckets are a bit more involved, since you have to remove the entire valve cover for access, and adjusting the clearance requires that you insert shims of different thicknesses. With the proper tools, it’s a relatively easy system to work with; after measuring the clearance and determining which shim is required to set it to spec, you use a special tool to compress the valve enough to remove the old shim and insert the new one, and then move on to the next valve.

The most difficult valve system to work with is the shim under bucket. Measuring the clearance is identical to the shim-over-bucket system, but replacing the shims requires removing the camshafts so you can lift off the buckets and gain access to the shims. When adjusting shim-under-bucket valves, you should record all of your measurements first, get the correct shims to set the clearances, then proceed to remove the cams. While the other systems let you measure and adjust each valve individually, removing the cams makes this highly impractical.

Finally, there are also hydraulic valve adjusters, found mostly on Harleys. If you have this system, it took you longer to read this than to adjust your valves, since you don’t have to.

Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.

BY KRIS AND NATHAN FANT

Adjusting motorcycle valves is both an art and a science. The valves in your motorcycle engine serve two purposes: to bring fuel and air into the engine, and to allow burnt gasses out of your engine.

The intake valves allow a mixture of fuel and air into the cylinder as the piston goes down. Those valves then close, and the piston comes up and compresses the air. The spark plug fires, forcing the piston down, and as the piston comes back up, the exhaust valves open to allow the burnt fuel to leave your engine. This amazingly simple process creates enough power to move your motorcycle.

Why Valves Need Adjustment

In order for this process to work, the valves need to create an airtight seal on the cylinder head. When the engine is cool, there is a very small amount of space (10ths of a millimeter) between the adjuster and the valve stem. This tiny gap is called valve lash. Over time, this gap gets smaller and smaller due to wear. Eventually, the valves are unable to close fully making a seal inside the combustion chamber impossible.This can lead to poor gas mileage, rough running, difficulty starting, and, if left for too long, can cause the engine to not run at all.

When to Adjust Valves

Most service manuals have intervals for valve adjustment as part of basic bike maintenance, usually every oil change or every other oil change. Valves are more likely to need adjustment when the bike is fairly new, or very high mileage. You may also find your valves need adjustment when your bike starts randomly dying at stop lights, like mine did recently.

How to Adjust Valves

(or better titled, How I Adjusted the Valves on my DR650)

This is the amateur mechanic’s description of the valve adjustment process on a Suzuki DR650, with the humorous outtakes thrown in at the end. Always check your service manual and find knowledgeable assistance if this is new territory for you. Shim style adjustments differ from the rocker arms and screw type adjusters shown here.

With a cold engine for proper valve clearance measurement, give your bike a wash or sponge bath. Letting the bike cool off overnight is recommended as the valve lash needs to be measured with a cold engine. On my bike, I needed to remove the side panels, seat, and gas tank. This gives access to the valve covers.

Remove the valve covers, spark plug covers, timing inspection plug and the crankshaft access plug. Loosen one of the spark plugs a bit to let the compression out of the engine. To properly adjust the valves, you must get your engine to Top Dead Center on the compression stroke. Most motorcycles have a method to measure where this top dead center is, such as a timing mark.

Turn the bolt under the crankshaft access plug counterclockwise until you see the line with the “T” next to it through the timing inspection hole.

 When you find this, make sure this is happening during the compression cycle; you’ll know it’s the compression cycle because you can wiggle the rocker arms slightly up and down and they should both be in their most upward position.

You are now ready to inspect your valves. In my opinion, this is more art than science. Valve feeler gauges come in sizes that vary from .03mm to 1mm.

In your service manual, you’ll find a range of specs for your bike. The current model DR650’s specs are Intake: .003mm – .005mm Exhaust: .007mm – .009mm. Since you have gone to the work of taking your bike apart, and over time valves are expected to tighten, it is typical to use the largest gauge that is in spec. You’ll slide your feeler gauge between the adjuster and valve stem, and again, this is an art and a feel.

What you want is the feeler gauge to slide in smoothly, but with a slight amount of drag, as if your motorcycle wanted to keep your feeler gauge, but acquiesces in giving it up. If your bike holds on firmly, your valve is too tight, and if your bike gives it up freely, your valve is too loose.

As predicted, my valves were tight. My valves use a valve clearance adjustment driver as the tool for adjustment. I’m pretty sure three hands would have been helpful for this process, but adjusting valves is a job that can be done by one person in about thirty minutes.

This tool has two pieces that fit inside of each other.

The first is a socket (10mm for this bike) that has a hole all the way through.

The second is a knob with a square inset to fit the top of the square heads on the valve.

You first use the socket to loosen the valve nut, and then use the knob to loosen the valve.

Use the feeler gauge in one hand and turn the knob with the other until you feel just the right amount of drag.

Know that when you tighten the nut, the valve will tighten slightly.

Adjust all four valves, and then recheck them as if you were not the one who just adjusted them.

Once all four valves are in spec, it is simply a matter of putting the bike back together. Since you loosened your spark plug, makes sense to pull that out and take a peek. Put your bike back together and start it up! Listen for clicking clacking, which would indicate you left your valves too loose.

What could possibly go wrong?

The timing inspection plugs are made of a very soft metal. You may accidentally strip it, have to chisel it out, and steal your husband’s plug to test your bike.

You may have your mentor come check your valves, and because of the subtle art of properly adjusted valves, he may tell you to loosen and tighten multiple times.

You may forget about needing TDC to occur during the compression cycle. If so, you will get your bike completely back together and hear clack clack clack clack. Then you get to do the valve adjustment all over again for the second time.

I’m not saying any of these things happened to me, but if they did, they were an excellent learning experience. Evidence: my first valve adjustment took 3 hours and my second valve adjustment took 30 minutes.

Valves are your engine’s best friend, and a valve adjustment is your valve’s best friend. Valves play a critical role in the function of the engine. Most motorcycles, besides some dirt bikes use a four stroke process for their engine. The valves play a critical role in this process by opening and closing the entrance and exit to the combustion cylinders.

All engines will have at least two valves per cylinder, one intake for taking in a mixture of fuel and air, and one exhaust for removing air and any leftover fuel after the combustion stroke. Equally important to your engine is the valve’s seal when it is not open. The valve needs the proper clearance so that when it is not open, it is able to seal off the engine’s cylinder.At redline, a motorcycle’s valves could open and close 130 times a second!

Your valves see a lot of abuse due to high engine speeds. Its not uncommon for motorcycles, especially ones with performance engines, to spend a majority of its life above 4,000 rpm. At a relatively low engine speed of 4,000 rpm, the intake and exhaust valves will each open and close 2000 times per minute. This around 33 times per second! On top of this, valves are often exposed to high temperatures which will cause thermal expansion as they are heated and contraction as they cool off. These heat cycles will further stress the valve and wear it out. Also the hotter the engine gets the less clearance the valve will have.

What is Valve Clearance

So what actually is valve clearance? Valve clearance is defined as the distance from the top of the valve and the rocker arm. The as the camshaft rotates, it will exert a force on the rocker arm. The rocker arm will pivot downwards and push the valve open. The valve clearance is the gap between the top of the valve and the rocker arm, when the cam is not pushing the rocker arm down.

In other words you should only be checking valve clearance on a closed valve, because when the valve is opened there should be no clearance. Valve clearance should only be measured when the engine is cold, taking these measurements on a hot or recently ran engine will not be accurate. Now that we know a little bit about valve clearances, lets talk about the signs your motorcycle will give you when its time to adjust the valves.

1. Pre-ignition

Pre-ignition can be caused by an engine that needs a valve adjustment. This condition happens when fuel in the combustion chamber is ignited before the spark plug fires. This can happen because as the engine heats up, its valve clearance is reduced. In a poorly adjusted valve, the reduced clearances causes the valve to spend less time seated. This will prohibit the exhaust valves from dissipating its heat. This excess heat will eventually become great enough to ignite the air-fuel mixture in the cylinder before the spark plug even fires.

Pre-ignition can cause engine knocking and an altogether unpleasant ride. If your bike as pre-ignition issues and you don’t remember the last time you had your valves looked at, it might be time for a valve adjustment.

2. Rattling Noise (Loose Valves)

Ever wonder what that loud rattling noise near the heads of your bike is? Well, it could be that you have loose valves. The valve train makes a lot of noise. Sometimes even a well calibrated engine will make rattling noise, but if over time your engine starts to develop a rattling or clattering noise, don’t ignore it. A sudden change in valve train noise is sign that your bike should get an adjustment.

Loose valves will cause accelerated wear in all your valve train’s components. In addition to increased wear, loose valves will have increased clearance which can cause compression issues. An engine that looses compression will have significantly reduced performance. If your motorcycle’s acceleration seems slower than it used to be and you have clattering valves, you should get your valves adjusted.

3. Overheating (Tight Valves)

Overheating is another possible sign of an engine in need of a valve adjustment. This can be caused by valves with too little clearance. This lack of clearance causes the valves to close slower than they should. In other words the valves are open too long. The exhaust valves dissipate a lot of heat when they are seated, but they aren’t able to dissipate heat when they are opened. The increased time the exhaust valve is open could overheat the engine.

In addition to overheating, valves with too little clearance can cause a loss of compression in the cylinder. This will cause severe performance issues. Overheating and loss of compression are serious issues. An overheated engine could cause the valve to deform are even break. A deformed valve won’t be able to seal properly when closed causing combustion issues. A broken valve could contaminate your entire engine causing a costly rebuild. Either way if you have a loss of compression and an overheating motorcycle checking your valves is a good place to start.

4. Increased Fuel Consumption

If your motorcycle used to be as efficient as prius and now its reminding you have your truck at the pump, you might have an issue with your valves. Your valves could have a combination of loose and tight valves on the same cylinder. This could cause the exhaust valve to be opened while the intake valve is letting fuel and air into the cylinder. This fuel and air mixture could go right through the cylinder and out the exhaust.

5. Valve Train Failure

Poorly adjusted valves will cause wear and tear on the entire valve train. Valves with too much clearance can cause damage and failure of the camshaft lobes, rocker arms, or even the valves themselves. If you have experienced failure of your valve train components its a good idea to keep a close eye on your valve clearance.

Conclusion

Valve adjustments are crucial to your engines health and should not be overlooked. They’re not fun to do or pay for, but they are completely necessary for long term engine health. Consult your owners manual for your bike’s valve adjustment interval.