Signs A 4 Stroke Dirt Bike Needs Valves Adjusted
Wondering if the valves need to be adjusted on your 4 stroke dirt bike? Letting them go too long can turn into major engine problems. Are there actually signs that tell me when the valves are needing an adjustment?
In this article we’ll look at the top signs and symptoms of a 4 stroke dirt bike that needs the valves adjusted. You’ll also learn what causes it, how to help prevent it, and how often you should be doing this routine maintenance.
The most common signs of a 4 stroke dirt bike that needs valves adjusted are:
- Hard to start when cold
- Popping in deceleration
- Loss of power
- Hanging idle
- Lower compression
What Happens When You Don’t Do A Valve Adjustment?
Aside from the symptoms above, not adjusting your valves can and will bring bigger consequences.
If you’re fortunate, the valve will just burn enough away to lose compression and the engine will stop running.
In a serious event, the valve could hit the piston, causing the engine to seize or pick-up in a split second.
What Causes Valves To Go Out Of Adjustment?
Depending how what type of valve train your 4 stroke dirt bike has, certain parts simply wear out. Each manufacturer has set limits or tolerances of how much clearance each valve should have. Once the clearance is above or below that tolerance, you will generally start noticing the symptoms listed above.
A titanium valve has a special coating that is made to increase the durability. Once that coating goes away the valve will quickly go out of adjustment. Normal wear and heat will burn the coating away over time.
A stainless steel valve generally doesn’t have a coating, but it still wears out for the same reason; wear and heat.
How Do I Know If I Need To Adjust My Dirt Bike Valves?
The only way to know for sure if your dirt bike needs a valve adjustment is to check the valve clearance.
If you have most or all of the symptoms above then you can deduce that a valve adjustment is probably needed, but you really needed to check to see if the clearances are in or out of factory given specifications.
How Much Does A Valve Adjustment Cost?
A typical shop will charge about $150 to adjust the valves on your 4 stroke dirt bike. This is because it’s quoted as a 2 hour job at an average $75/hour shop rate.
Learning how to do it yourself can save a lot of that money, depending on the tools you already have. Valve shims are expensive if you buy a kit, but they should last for years and multiple bikes.
How Do You Adjust Dirt Bike Valves?
Adjusting valves is not as complicated as it sounds on most dirt bikes. The most time consuming part is often just removing other parts to get access to the top of the cylinder head.
A 4 stroke dirt bike that has a rocker arm/tappet valve train is pretty simple to adjust. It’s older technology, mostly in air-cooled bikes, but it still works well.
There will be a locking nut that holds the tappet adjuster. Break that loose and then you can tighten or loosen the adjuster. Use a feeler gauge to check the clearance between the rocker arm tappet and the valve. Adjust to the specified clearance given by the factory. This must be checked when everything is properly torqued to spec and the engine is cold.
Shim & Bucket Valve Adjustment
Most motocross and enduro 4 stroke dirt bikes have a shim and bucket valve train. They are similar on how to check the clearance. You use a feeler gauge to check the valve clearance. To adjust it, you must have a shim kit and adjust the shims to get the right height under the bucket.
How Often Should Valves Be Adjusted?
According to my Yamaha manual, an older YZ250F should have the valve clearance checked every.
Is this really necessary? If you are an expert level motocross racer, then I’d say yes. However, most riders do not need to check them as often, depending on how you ride.
Checking them on a routine schedule can be a good idea to prevent a possible failure.
Valves only need to be adjusted when they are out of spec.
If you’re regularly hitting the reg limiter on a 250F then you’re going to need to check and/or adjust them a lot more often then if you’re casually trail riding at lower RPMs.
How Long Do Dirt Bike Valves Last?
Now that is a loaded question. The valves in your 4 stroke will last as long as they stay within spec and properly seal against the valve seat. If you’re looking for a number of hours, there’s no set number that you can count on needing to replace them.
You, as the owner and rider of your dirt bike need to know what your bike is doing and how it sounds when you ride it. There’s ways to tell when something needs attention. The symptoms listed above will let you know when the valves are most likely needing adjustment.
If you need to adjust the valves multiple times in a short amount of time, then it’s time to replace the valves.
Does A Valve Adjustment Improve Performance?
Adjusting the valves to proper specification can make a slight improvement in power. The biggest improvement will be the negation of symptoms. If your dirt bike was hard to start, then it will be easier to start after adjusting the valves. Misfiring should go away if that was a symptom.
Unless the valves were way out of spec, in which your bike probably wouldn’t run anyway, you’re not going to feel a huge gain in power.
With that said, reliability is the most important reason you need to adjust the valves if they are out of tolerance.
Can Valve Clearance Affect Compression?
Valve clearance can affect compression. If one or more valves are out of adjustment, it may be open during the engine combustion cycle when it’s supposed to be closed. This creates a small leak-by because the valve isn’t fully sealing against the valve seat. This leak will cause a drop in compression.
Do Valves Get Tighter Or Looser?
Generally, valves will get tighter as they wear. They can get looser if something else becomes loose in the cylinder, but that is rare.
Why Do Exhaust Valves Get Tighter
As a valve wears, it will sink into the valve seat and get closer to the cam, thus making it a tighter clearance.
The valve seat can also wear out from the valve hitting so many times. This causes the valve to sink into the cylinder head. Valves seats can be re-cut for better seal and performance, but it’s better to replace the seat if it’s sunk in too far.
How To Make Valves Last Longer
The two biggest factors that affect the longevity of the valves in your dirt bike are how you ride and how well you maintain the bike.
Valves wear out because of heat and wear. There are some easy ways to prevent premature wear. There are other ways that are simply not reasonable. More on this in a minute.
Common Causes That Wear Out Valves Quicker:
- Lean Jetting
- Overheating the engine
- Idling for long periods of time
- Riding at high RPM
- Constantly hitting the rev limiter
- Not keeping the air filter clean
- Higher power output
Jetting For Reliability
Lean jetting can mean more power, especially if the air/fuel ratio is too rich. However, going too lean will create too much heat.
This heat increases the wear on all the internal parts. Valves can lose their costing and/or seal because they burn away over time.
This is why it’s so important to learn how to jet your dirt bike. Using a fuel controller is becoming a standard practice as well with the fuel injected motocross bikes.
Overheating Is A Killer
As mentioned, heat is the enemy of valves. Whether it’s lean jetting, or idling too long, you need to be aware of how hot your 4 stroke is getting.
Overheating is actually a fairly common problem, but if you let it happen on a regular basis, the engine is going to take a toll. Learn more about overheating here and how to prevent it.
How Do You Ride?
Do you ride your dirt bike to its full potential? Most riders don’t, but many like to sound like it…
Riding at a high RPM not only sounds cool, but most bikes produce high horsepower numbers at high RPM. There is a point where power tapers off and goes down. The rev limiter then kicks in shortly after.
Riding in the upper RPM range is often mandatory while racing if you want to go fast. With that said, constantly hitting the rev limiter is not. It’s there for protection so the engine doesn’t over-rev and break parts from spinning to fast.
If you like hitting the rev limiter just to hear it, do your bike a favor and stop. Not only is there less power at redline RPM, but you’re wearing out the valve train faster. The valves are moving faster and hitting the valve seats harder when they come back to deal against the head.
How To Maintain Your 4-Stroke Engine
To make your dirt bike last as long as possible, properly maintaining it is the most important thing to do. Regular oil changes when it gets dirty, and keeping an eye on how the oil looks. Are you seeing a lot of metal flakes in the oil? That’s a sign that something is probably wearing out faster than it should.
Keeping the air filter clean is crucial to making the valves last longer. A dirty or improperly oiled air filter will allow dirt to pass through the engine. Dirt will eat up engine parts. It may not happen overnight, but the engine rebuild intervals can be greatly increased if you keep clean air going through the engine.
Is It Better To Have More Power?
You can never have enough power, right? Well, as much fun as having mega horsepower can be, whether it’s more efficient or not, there is a downside.
More power equals more heat. Yep, that means that the faster your bike is, the more heat it will potentially create. This is why keeping it cool can prevent a lot of problems.
Modifying a dirt bike to go faster doesn’t necessarily make it less reliable. It’s when you actually use the extra horsepower when things start to wear out at a slightly quicker rate.
If your 4 stroke is hard to start, pops/backfires, is low on power, then it might be time to adjust or replace the valves. To make valves last longer, keep your engine running cool and properly maintain it.
These things will make your dirt bike last for many hours or years to come.
4 strokes tend to run at a higher operating temperature than a 2 stroke. This can result in overheating problems.
The radiators not getting enough air to cool it down is a common issue. For more symptoms of an overheating dirt bike and how to fix it to prevent long-term problems click here.