2 Stroke Dirt Bike Engine and Maintenance

2 Stroke Dirt Bike Engine and Maintenance

2 Stroke Dirt Bike Engine and Maintenance

Modern 2 stroke dirt bike engines are akin to works of art, their magic lying in their simplicity. Their ‘power centers’ consist of only three main moving parts, the crankshaft, connecting rod and piston. Unlike their 4 stroke counterparts, 2 stroke engines do not contain valves, giving them a lighter weight.

2 stroke engines fire, producing power, once for each revolution of the crankshaft; 4 stroke engines fire once per every alternate revolution. The combustion chamber, each time the piston goes up, is filled with an intricately balanced combination of oil, petrol and air. This intricately balanced combination is compressed, then the spark plug ignites it, resulting in an explosion, forcing the piston to go back down. The process is repeated. There are two strokes of the piston for each single ignition, a compression stroke and a combustion stroke, giving the 2 stroke engine twice the amount of explosive power of a 4 stroke engine of equal size, the 4 stroke engine needing 4 strokes of the piston for one ignition.

Diagram of a 2 stroke dirt bike engine:

2 stroke dirt bike engine

2 stroke dirt bike engine

Engine maintenance

Engine maintenance is one of the most important aspects of enjoying your dirt bike. There are three main factors to keep in mind when it comes to proper engine maintenance: air filters, engine oil and coolant.

Air filters

This is one of the most important parts of dirt bike engine maintenance. If your air filter is dirty, your bike cannot function properly, its airflow being compromised. It needs air and fuel to function properly. Dirty air filters, not properly oiled, can damage the engine, allowing dirt particles to get inside the combustion chamber, scratching it from within, damaging the piston and other parts. A dirty air filter will wear out an engine pretty quickly. You probably want to clean your air filter after every single ride.

In order to clean your air filter, you will need oil, cleaner and rim grease. Put about 3-4 caps of cleaner into a tub of warm water. Remove the dirty air filter from your bike and submerge it in the tub. Be careful not to twist and turn it roughly, or you will end up damaging it. Instead, squeeze it gently in the tub until it is clean. Rinse the cleaner off of it under clear tap water and then squeeze all the water out, leaving it to dry. Once it is dry, you can apply oil on it. Make sure you apply the oil evenly, not concentrating it on one part of the filter. Lastly, go back to your bike and clean the rim with a paper towel, checking the air box for dirt. Then apply rim grease on your rim and reattach the air filter, and you are good to go.

Engine oil

Dirt bike engines are not the same as car engines; dirt bike engines, being small and working hard with high compression ratios, need their oil changed more often. Your 2 stroke engine should have its oil changed after about every 10 hours of riding, or somewhere around that frequency. You can use the oil of your choosing. It doesn’t matter what oil you use as long as you change it frequently.

Coolant

Your engine has to remain cool at all times; you have to make sure that you have enough coolant in your radiator. Even if you have a clean air filter and you’ve changed your oil, having insufficient coolant will damage your engine. So, open the cap of your radiator before each ride, and check to make sure that you have enough coolant. If you can see the level of the fluid, it generally means that you have enough coolant. If not, fill it up with your coolant of choice.

Radiators usually depend on airflow to keep them cool. If you ride your bike in an environment that is a bit tricky, requiring you to go slow, you may not be able to create the airflow your radiator needs. In this case, we recommend buying a fan and connecting it to your battery so that it can prevent your radiator from heating up.

Engine mods

Sometimes the factory settings on your dirt bike may not be up to your specifications. In such cases, you can make mods to the engine yourself, mods usually designed to make the bike faster and more powerful. But before you do this, you need to make sure that you are fairly good at riding your bike in the first place. Otherwise, the mods won’t really help you and may actually end up doing some harm. Here are a few mods you can make to your 2 stroke dirt bike engine, making it faster and more powerful.

Jetting

In case your bike is not properly jetted, it will not run in the smooth manner that it should and could lead to a detonated engine. You want to down jet to the edge of detonation, and no further. You have to live on the edge to generate maximum power. You want to be careful, however, to achieve the right balance. If your bike is already perfectly jetted, putting an aftermarket pipe on it will make it run lean, resulting in making is slower and seriously harming your engine because it won’t be receiving enough fuel. If, on the other hand, you make your bike too rich, you will go through your spark plugs really quickly, also causing your bike to sputter, cough and be generally sluggish. Too lean and too rich, both extremes are bad for your bike. Some people wrongly think that too rich is alright, it not being bad for your engine. But it is bad for your engine with the carbon buildup from the unburned gas. When you modify your bike, you want to make sure it is finely tuned, otherwise it will run worse than when you first got it.

Make your stinger ID larger

In most performance system, the exhaust system’s stinger ID is too large. You can increase your bike’s power by adding a restriction to your stinger ID. However, be careful to do this in baby steps, as if you go too far, it will lead to detonation.

Squish clearance

The squish gap is the gap between the piston and the squish band in the head of the engine at the time when the piston is atop the center. A good rule of thumb is that you want the piston to nearly touch the head at peak rpm. You can achieve this by running a tighter squish gap. The way to do this is to tighten the squish gap until the piston touches the head, then go back a tiny bit, so that there is no gap at all at peak rpm. Having no gap prevents the collection of mixture in the squish band, meaning there is nothing to detonate there. You can tighten your squish band by sanding down the head or the cylinder. This will increase your bike’s power and reduce chances of detonation.

Inspect pipe

Check for and eliminate any resistance that might be there in your exhaust system. Make sure that the pipe is clean from within, and the changes in its sections are distinct.

Intake length

Straighten and shorten your intake length. This will give you better power.

Port match

Match the ports in your cylinder to the ports in your case. Some engines can be really bad from factory in this regard. This is not dangerous at all; it is just a good practice.

With electric start

Two stroke dirt bikes have recently been manufactured with an electric start by companies like KTM. It’s a great addition to two-stroke engine dirt bikes. Now, if the bike were to stall in some ugly place or if you were to fall from a hillside, you will be extremely happy to be able to restart everything with the press of a button. The new electric start models are said to weigh less than the old kick starters as well.

Dirt Bike Engine Rebuilt

Rebuilding a 2 stroke dirt bike engine might sound like it should be done by a maintenance professional, with many ordinary riders balking at the idea, but its really not. It may sound intricate and complicated, and, to be sure, it requires some patience and time, you can accomplish the task without facing too much difficulty.

The truth is, it is not rocket science, and more or less boils down to changing the piston, some rings and gaskets.

However, despite the process being relatively straightforward, most riders have a propensity to wait for too long, until the top end of their bikes absolutely cannot be ridden. The symptoms that your top end needs urgent replacing include but are not limited to what is referred to as piston slap, engine noise and a lack in horsepower. Piston slap refers to a situation where the piston is slapping against the wall of the cylinder, either because there is a lack of compression or because the cylinder is worn out. You will know it when you see it or hear it to be more accurate; it is annoying and loud.

To rebuild the top end, you need knowledge of general maintenance, such as filter and oil change. So, the best advice in this regard is to read the service manual, following the recommendations that it contains. Don’t wait too long, or you risk damaging the engine. And even if you don’t, your bike will perform poorly, and you won’t enjoy riding it.

Get the service manual in case you don’t already have one. You’ll need to wear specs and get the specified torque for your dirt bike model. Make sure you read the service manual carefully, not missing out on anything, using it to guide your entire process. We are providing a general guideline here, but the service manual is what you need to get into the nitty-gritty of the specific nuts and bolts.

You will need a ratchet set and torque wrench. Make sure you first give your bike a thorough cleaning, preventing dirt from getting inside your engine’s casing once its broken down. To reach the top end you will have to remove several parts.

  • The drain coolant
  • You will have to remove the seat and then the fuel tank. You don’t have to remove the radiator.
  • You will have to remove the top, the motor mount
  • You will have to remove the head and the cylinder

In order to rebuild the top end, you will have to:

  • Remove the old piston
  • Carefully inspect the cylinder, making sure that it is not scarred
  • Measure the cylinder
  • Put the rings on to the piston
  • Use the circlips, needle bearing and pin, connecting the new piston with the rod, coating the piston with premix or assembly lube.
  • Slide the cylinder over your new piston with the use of new gaskets. Torque the bolts of the cylinder
  • Place the head onto the cylinder, as well as the new gasket. Torque the bolts of the head.
  • Replace the radiator, the fuel tank as well as the seat
  • Ass the fresh coolant
  • Now start your bike

To detect cylinder scarring you must know what it looks like. In case you have never before seen it, it appears as vertical scratches within the cylinder. If you find cylinder scarring, don’t more ahead with rebuilding the top end. First, purchase a new cylinder (this option is on the more expensive side, but gets your job done quicker). If you don’t want to purchase a new cylinder, you can re-plate the existing one (this option is more time consuming but less expensive). You will not be able to use your bike for a week or a couple of weeks depending on which option you choose to fix your scarred cylinder.

The cylinder also needs to be measured in order to gauge the size of the piston that will fit inside. You cannot simply install a new piston, putting everything together again, even in case your cylinder is not scarred and looks like it is ready for riding. You have to measure it for the right size first.

The reason for this is that the bore of the cylinder undergoes expansion after being subjected to the piston’s up and down movement for long periods of time, a movement that transforms the shape of the cylinder oblong. In such situations, the cylinder will look alright on the surface, but you will need a caliper of gauge to measure its new size. In case you don’t have either of these, consult a mechanic or make a purchase.

Once you have sized you cylinder, you can breathe easy, now knowing what sized piston you need, enabling you to use the one you have or order a bigger sized one. If you cylinder is way too large for the pistons you have available, you will have to send it to be worked on by machines taking the form of re-sleeving or plating. This is a big and intricate process, not something you can do at home without the proper equipment.

You shouldn’t forego the process of cylinder sizing, installing a piston that doesn’t fit correctly. It will result in reduced power and low compression. You can additionally expect the engine to fail sooner or later and also expect internal parts to get broken.

Once you have the right sized piston, replace the rings, the seals and the needle bearing. Then put everything together back the way it was again. Make sure to add the new coolant; you don’t want your bike overheating. Start the engine, allowing it to warm up; then shut it down again, allowing it to cool down. Restart and ride at an easy pace for about 15 minutes. After this, shut it off and allow it to cool down once more. Then you’re ready to go full throttle.

Dual exhaust

We recommend not going for a dual exhaust on your dirt bike. Since it has only one cylinder, it needs just the single exhaust. Besides which it would not look good. It would also make your exhausts twice as heavy. It would be twice as bad in case of a crash. And it will make your bike run worse than it does currently.

Oil in exhaust and exhaust leak

If your two stroke exhaust is leaking oil, you need to clean it and learn how to prevent it from making a mess. It’s nothing to worry about; it’s pretty normal, especially if your bike’s running rich. Oil leaks most often where the pipe meets the cylinder, though it can also happen where the silencer meets the pipe. Oil leakage is caused by the wearing down of O-rings that sit between the pipe and the cylinder. These rings get worn out by the vibration of the engine and the pipe. To stop the oil from leaking, simply replace the O-rings periodically. Check your pipe mounts and pipe springs to stop the pipe from vibrating excessively and wearing down the O-rings quicker.

Dirt Bike Engine Facts

In four stroke engines, there are four important steps in the power producing cycle: compression power, exhaust and intake, each step possessing its own stroke of the piston. Two stroke engines carry out the same four processes but in only two strokes of the piston. In simple two strokes, the moving piston’s underside and crankcase are used to freshly charge the pump. These are officially referred to as “crankcase-scavenged two strokes.”

When compressed, the two-stroke piston rises, its underside pulling a partial vacuum within the crankcase. Some kind of intake port opens to allow air within the crankcase using a carburetor. When the piston is near the top of the head, a spark ignites the compressed mixture. The mixture is burned, transforming its chemical energy into heat energy, raising its pressure, the pressure forcing the piston down, the crankshaft rotating. The piston, as it goes down, exposes an exhaust port within the cylinder wall, where the gas that has been spent goes out. Simultaneously, the piston compresses the mixture of air and fuel underneath the crankcase.

This is another thing that separates 2 strokes from 4 strokes: their exhaust and intake are both integrated within the same compression movement, meaning 2 strokes don’t need valves because the compression chamber’s wall contains an exhaust port as well as an inlet.

Gas/mixture ratio

It can be challenging to mix the correct ratios for your 2 stroke dirt bikes. But it is essential to perform this task correctly, or you risk causing great damage to your engine, if not destroying it. Many riders appear to be confused over what ratio to mix the oil and fuel at. Your best bet, the easiest and safest bet, is to follow the ratio that your manufacturer recommends, since the manufacturer has the greatest knowledge of the engine, even more than the professional mechanic. The right ratios help keep the rings and piston adequately lubricated.

Upgrades

Checking for upgrades is common amongst dirt bike riders. It is possible to upgrade pretty much all aspects of your dirt bike but many of these are unnecessary. At the same time there are some upgrades that are worth making, improving your bike’s handling and power and its performance on the track. You may want extra power, or to replace worn out old parts or a simple replacement upgrade or perhaps just something to make your ride smoother. In all these situations selecting the right aftermarket upgrades can pay off immensely. But be careful not to waste money on unneeded upgrades. Here are some upgrades we recommend.

Dirt biking skills

Upgrading your riding skills doesn’t require any money unless you decide to take classes. But it will pay off better than any other upgrade you will make, so practice, practice, practice!

Suspension

This will be an expensive upgrade, one that is difficult to install. But you will definitely notice a difference in your ride afterward. You may consider hiring a professional mechanic to do the job for you.

Pipe

This can also make a big difference to your riding experience. But the kind of upgrade you make will depend upon your riding style and will take a bit more research accordingly.

Reeds

Upgrading reeds results in better airflow for your cylinder and better response of the throttle. Carbon fiber is used to manufacture aftermarket reeds, a material that helps eliminating the flutter at high RPMs.

Gearing

And upgrade to your sprocket and chain will result in improvements nearly as great as when you upgrade your suspension. Your goals with dirt bike riding should guide the changes you make to gearing.

Steering Damper

These help in stabilizing the handlebars. They also reportedly reduce arm pump. There are a few steering dampers the permit adjustments to your steering during your ride. They are particularly useful upgrades for off-road riders, absorbing the impact from the rough terrain, protecting your arms and hands.

Miscellaneous

Upgrades to your seat, levers, foot pegs and grips can help you feel more comfortable on your dirt bike. They might not improve your bike’s performance, but comfort is also a factor. Plus, such upgrades can increase your bike’s life-span.

2 stroke dirt bike wheelies

The key to performing a wheelie on a two stroke is keeping it at the low end of the power band to stop you from looping out. Keep the throttle between one-fourth and half to start the wheelie, then reduce the throttle to less than one-fourth to maintain it. Keeping the throttle minimal and smooth is crucial. Keep the engine revs low and go slow.

2 stroke dirt bike weight

2 stroke dirt bikes’ weight varies according to their type and make but they are generally lighter than 4 stroke bikes.

Gear box oil

To change your gear box oil, you will need rubber gloves, a torque wrench to undo the bolts, your choice of oil, a funnel and a measuring cup. Turn on your bike and let it warm up for a couple of minutes. Take out the plug from your gear box using your torque wrench to undo the bolt and let the oil drain. Clean your plug before putting it back in. Make sure all the oil drains from the bike; you may want to shake your bike a little to accomplish this; it may take approximately ten minutes. Your bike will probably need 700ml-800ml of oil but check the engine to be sure. Then insert your funnel into the gear box and use it to fill it up. There will be an oil level check screw to help you make sure that it is properly filled up. Once you have checked this you are good to go. Change gear box oil after about every 20 hours of riding.

Brands

2 stroke dirt bikes may not be as popular as they once were but there is no substitute for them. Here we are going to take a look at the best models on the market, some models being brand new, others old classics. Some sources erroneously report that 2 stroke dirt bikes are not being produced any longer. They, as we’ve said, are mistaken. Other sources report that it is difficult to maintain 2 stroke dirt bikes. This is also incorrect; if you’re good at maintaining things generally, maintaining a 2 stroke dirt bike won’t be a problem. A third misconception that appears to be prevalent is that you should not purchase old 2 stroke dirt bikes because they will be too ragged to ride. There may be some truth to that, but you should bear in mind that 2 stroke dirt bikes are designed to take a lot of punishment. They can handle it.

Many big brands are still manufacturing new 2 stroke dirt bikes, albeit not to the same extent as they did previously. Ever since the 2 stroke dirt bike went downhill, manufacturers have become hesitant when it comes to developing new 2 stroke engines. However, others are still going strong. Yamaha, Suzuki and Kawasaki all have 2 stroke dirt bikes in their lineups. In Europe, Gas-Gas, Husqvarna and KTM are subsisting on 2 stroke machines. The 2 stroke era may not quite be dead, but it certainly is dying. KTM have come out with a fuel-injected 2 stroke engine, an engine that could revolutionize dirt biking in the future. Honda are reported to be developing something similar to this if rumors are to be believed.

Even though new models are being manufactured, we’ve included a few older ones in this list. After all, everyone can’t afford a new bike; and when you’re buying your first dirt bike, you may choose to go with a second-hand one, investing the money you save into a new piston and other dirt bike parts.

Suzuki RM250

The Suzuki RM250 is arguably the best used dirt bike on the market, one definitely worth bearing in mind for those in the market for 2 stroke dirt bikes. We particularly recommend the models that were manufactured between the years 2003 and 2008. We most recommend the 2008 model, that being the last year that the RM250 was produced, the model being the ‘latest’ in the series. In its time, the RM250 counted amongst the giants of motocross sports and its later models were manufactured to meet extremely high standards. The RM250 receives lavish praise for its fast and powerful 2 stroke engine as well as its springy suspension. If you are aware of a late 2000’s model up for sale, you may want to go for it, it being very hard to go wrong with.

KTM 250SX

The KTM 250SX is a heavy-duty dirt bike, one that ticks off all the right boxes. It has garnered worldwide praise for its excellent 2 stroke engine, reliable, powerful and fast and capable of taking a lot of punishment. It is arguably the fastest 250cc 2 stroke dirt bike on the market, a claim that we are happy to support. It is a little pokey and one of its downsides is it suspension, a problem that has plagued KTM. However, KTM riders will be accustomed to this and won’t mind as much. We recommend the SX motocross version, a version that contains both, a function for started electrically as well as a function for kick starting. KTM bikes in general are a bit on the expensive side, so finding one that’s extremely cheap is highly unlikely. You may not, therefore, want to go for a second-hand KTM, but purchase a new one, should you have the money to spare. These dirt bikes are still being manufactured.

Kawasaki KX500

This model first entered the fray as far back as 1983. It had already gained legendary status back then. It is 499cc, possessing a single cylinder. It is an absolute powerhouse of a 2 stroke dirt bike, with some going so far as to refer to it as being amongst the greatest dirt bikes in history when it comes to power. It won a vast number of racing competitions the world over, before being discontinued in 2004. Incredibly powerful, furious and fast, this 2 stroke dirt bike is for the aficionados of horsepower. Being immensely powerful in their prime, these bikes are readily available today, the older models being as cheap as $500. However, you may want to pay a bit more than that for a dirt bike that’s better in condition and newer, something around $2500. Getting parts for the models manufactured between 2000 and 2004 is easy as well.

Yamaha YZ250

The Yamaha YZ250 is more recent, possessing a history that is both illustrious and long, dating back to 1974. However, you probably don’t want a 1974 model unless you happen to be a proper vintage aficionado; otherwise you will probably prefer something more recent. Yamaha, fortunately, are still producing the YZ250. But in case your budget doesn’t allow you to purchase something brand new, we highly recommend any model from 2005 and beyond. The two stroke dirt bike possesses a 250cc engine, water-cooled, claiming 32lb-ft of torque and 48.8hp. It’s lightweight at only 212lbs, a machine that is impressive and beautiful. One drawback of earlier models is the suspension, but the 2006 model and beyond came with an upgraded suspension. The dirt bike has won an impressive 14 motocross and supercross titles combined. The starting price of newer models is $7,399.

Honda CR250R

Honda stopped manufacturing 2 stroke dirt bikes in 2007. The CR250R had been in production all the way back since 1972, possessing a history to rival that of the Yamaha above, in fact with even more titles to its name. It is still available second-hand and for extremely good prices, some bikes still in outstanding condition going for around $1000, an excellent deal.

Yamaha YZ125

For some riders, the Yamaha YZ125 is superior to the larger Yamaha YZ250. It is recommended for people who aren’t as proficient at dirt biking as others. It is small, weighing 190lbs, but it packs a punch, its power sufficient to take you over all kinds of dirt trails. It possesses a 124cc, water-cooled 2 stroke engine. This bike is indestructible, yet wouldn’t hurt you if it falls on you, which is inevitable in dirt biking. We recommend models from 2005 and beyond; they can be purchased relatively cheaply at $1500, although a brand-new model of Yamaha MSRP will cost $6,499.

Husqvarna CR125

Husqvarna represents one of the final mainstays of powerful and furious 2 stroke dirt bikes. Unfortunately, however, the dirt bike was discontinued in 2013. You should bear in mind that this beast is not for the faint of heart. Ostensibly, it seems innocuous, a 124cc 2 stroke engine, but it is modeled along established racing technology. Here’s one dirt bike that’s hard to get a hold of in the market. So, if possible, get it when you can, even if the price is a bit high; it will be well worth the money you spend on it.

Gas-Gas EC 300

Gas-Gas is a Spanish brand, not as widely known as large Japanese firms or, for that matter, European ones (like Husqvarna and KTM), but it is not to be discounted. We strongly recommend their 2018 EC300, but their previous models, 300 and 250, are also worth the money you spend on them. Gas-Gas has been bought out by electric bike manufacturers, so the EC300, according to some sources, will be their last gas-powered motorcycle. Purchasing their last 2 stroke dirt bike would be a fitting way to commemorate this.

Honda CR500R

This 2 stroke dirt bike can be seen as the CR250’s big brother, its reliability at par with the best. A second feature is that it is more powerful compared to the CR250, which counts in its favor for the majority of dirt bikers. While this bike wasn’t idolized as much as the Kawasaki KX500, we believe that it is the better bike. Because of its understated reputation, you are likely to purchase this one much cheaper than its competitor. This is Honda’ most powerful motocross bike, with a 491cc, liquid cooled 2 stroke engine. Some riders have complained that the bike is too difficult to ride because of how powerful it is. You may have to spend $4500 and above for this one.

KTM 300 EXC

This bike’s engine has been referred to as greatest 2 stroke engine when it comes to power. The KTM 300 EXC is an extraordinary dirt bike, with KTM proclaiming it to be the best bike for riding off-road and racing, even when competing against 4 stroke bikes. This bike has been available for a while. If you’re unable to purchase it brand new, we recommend you purchase a used model, anything after 2004, preferably after 2008, the 2008 model coming with an engine that is extremely powerful and a bike that is very comfortable, buffeted with an electric start. Prices for these are usually on the high side, but with a little bit of luck you can buy one for about $3500. A brand new one is on the expensive side at $9,099.