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.
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.
