15 Things Beginners Need To Know About A 2 Stroke Dirt Bike
Are you looking to get your first 2 stroke dirt bike? Maybe you’ve been out of the dirt biking realm for a while and would like to know what has changed. Let’s face it, 4 strokes have taken over much of this sport in the last 20 years.
Or are the tides changing and 2 strokes are making a comeback?
I’d like to start with the basics if you don’t know anything about dirt bikes with a 2-cycle stroke engine and work our way towards choosing the right bike for you and how to maintain it.
What Does 2 Stroke Mean On A Dirt Bike?
2 stroke simply means that the engine completes 2 strokes (up and down for one full revolution) for every “power” cycle. This means that every time the piston comes up in the cylinder, the spark plug ignites the air-fuel mixture to produce combustion and “propel” the piston back down.
Why Do 2 Strokes Need Oil Mixed In The Gas?
The rotating assembly of the engine (crankshaft, connecting rod, piston, etc) are separate from the transmission. This means that the engine oil (really transmission oil) you pour in for an oil change is just lubricating the transmission.
The crank and piston need to be lubricated, so the 2 stroke oil that you “pre-mix” in the gas is what lubricates these parts by coming into contact from the fuel mixture.
Not running oil in your gas will destroy your engine in little time. No lubrication will cause the top-end to overheat and seize.
Do They Still Make 2 Stroke Dirt Bikes?
Despite 2 strokes not being raced by the top professional motocross racers, many companies still develop and produce 2 stroke dirt bikes.
The companies still producing 2 strokes are:
- Gas Gas
Honda, Kawasaki and Suzuki stopped selling 2 stroke 125cc and 250cc motocross bikes back in 2007, give or take a year or two.
Why Did They Stop Making 2 Stroke Dirt Bikes?
The 3 big Japanese companies stopped making 2 strokes for a number of reasons. The biggest reasons are because 4 strokes became the dominant bike in motocross, and the profit of OEM part sales is much higher than a 2 stroke.
2 Stroke vs 4 Stroke – What’s Better?
The great debate of how many strokes: 2 versus 4. What’s better and why?
First, you need to step back and ask what you’re looking for in a dirt bike.
Some simple questions to help you decide if a 2 stroke is right for you would be:
- Do you want a more simple engine?
- Are you looking for the lightest feeling bike?
- What kind of riding do you do?
- Is mixing gas going to be a problem?
- Do you know how to change jets in a carb?
2 stroke engines are simpler by design because they have less moving parts. This means engine rebuilds are generally cheaper.
For example, a stock 2 stroke cylinder head is just 1 part, while a 4 stroke head has valves, cam(s), guides, seals, can chain, etc.
All of those parts can rack up quite the repair bill if there’s major engine damage.
Are 2 Strokes Less Reliable?
I believe that 2 strokes can be just about as reliable. With that said, they generally require more top-end maintenance. This means replacing the rings and/piston more often than a 4 stroke.
A good way to know when a 2 stroke top-end will need a rebuild is by checking the compression. If it’s 20% lower than when the top-end was fresh, then it’s going to need to be rebuilt.
Is A 2 Stroke or 4 Stroke Faster?
Comparing the same size 2 and 4 stroke, 2 stroke will almost always be faster in a drag race. However, there are many nuances that make one so-called “faster” than the other.
A 250 4 stroke motocross bike will be easier to ride than a 250 2 stroke mx bike simply because of its power characteristics. A 4 stroke has more low-end torque and a more linear power-curve (there are exceptions). This equates to smoother power and less shifting to keep it in the “meat” of the power.
Why Is A 2 Stroke Faster Than A 4 Stroke?
A 2 stroke naturally has the ability to be faster than the same size 4 stroke because of how the engine works.
As mentioned above, the engine makes a “power stroke” every 2 strokes (hence the name). This means that it’s igniting and combusting the air-fuel mixture twice as many times as a 4 stroke at any given RPM.
More power strokes = More power (as a general rule).
Why Are 2 Strokes Banned?
2 strokes can be raced anywhere as an amateur.
The only place where 2 strokes cannot race against the same size 4 stroke is in professional motocross racing. This means a 250 2 stroke cannot race in the 250 class (full of 250cc 4 strokes).
What Are The Disadvantages of A 2 Stroke Dirt Bike?
If 2 strokes are faster and lighter, then why doesn’t everyone ride a 2 stroke? That’s a good question, but it’s relatively easy to answer…
It all comes down to personal preference.
The most common reasons why people don’t buy a 2 stroke are:
- Less off-idle torque
- Requires premixing gas in most cases
- Tend to vibrate more (newer models are counterbalanced)
- Smoke/smell more
KTM and Husqvarna, as well as Beta have been raising the bar for off-road 2 stroke dirt bikes. So much so, that Yamaha has gotten back in the game to develop their 2 stroke models again.
250 and 300cc 2 strokes are some of the most popular trail and Enduro bikes because they have plenty of torque and horsepower, and they feel considerably lighter than a 450 4 stroke. This means you’ll get worn out less and be able to ride longer before your arms pump up and can’t hang on anymore.
Which 2 Stroke Should I Buy?
Now that you know what 2 strokes are all about, how do you decide which one to get?
The criteria that will determine which bike you should get is: what is your skill level, what kind of riding will you be doing, and what is your height?
The last one isn’t as important unless you’re just learning how to ride a dirt bike. Balance is a key factor to being a good rider, so you shouldn’t need to reach the ground with both feet.
Is A 2 Stroke A Good Beginner Bike?
Certain 2 stroke dirt bikes can make a great starter bike. It all depends on which model you get. A Yamaha YZ250 motocross would be a terrible example of a good beginner bike. It’s very powerful with an abrupt powerband, it’s tall, and the bike is set up specifically for motocross racing.
The Kawasaki KDX200 is another good 2 stroke if you are looking for something more old-school. It doesn’t have electric start or anything high-tech, but it’s easy to ride with plenty of power and has a slightly lower seat height.
Kids 2 Stroke Dirt Bikes
Maybe you’re just looking for a dirt bike for your kid. The Yamaha PW50 is the tried and true “first dirt bike” for kids ages 3-6. It’s oil-injected, so you don’t have to mix gas (just pour the 2 stroke oil in a small reservoir tank).
The Kawasaki KDX50 and Suzuki JR50 are comparable competitors to the PW50 if you don’t want a blue machine. They’re just harder to find.
KTM has their 50 SX 2 stroke, but it is a higher performance race bike. I would not recommend this for a first bike. It’s also a lot more expensive than the other 50cc 2 stroke bikes.
Cobra and Husqvarna also have 50 cc 2 stroke mx bikes that compete with the KTM 50 SX.
65cc 2 Strokes
KTM, Husqvarna, Cobra, and Yamaha all have 65 2 strokes that are competition motocross bikes.
They’re all similar in size and power, which each bike having subtle characteristic differences that come down to personal preference.
85cc 2 strokes
Stepping up to the Mini class in motocross, Yamaha, KTM, Husqvarna, Kawasaki, and TM are the major competitors in the class.
KTM has a 105 SX for the supermini class, while the other brands have to be fitted with a kit (bigger wheels and up to 112cc 2 stroke)
Enduro Vs Motocross Bike
Are you considering buying a cheap 2 stroke motocross bike and converting it into a trail bike? There’s a lot of mods available to make a bike, such as a YZ250 more enduro friendly, but at a cost.
The biggest disadvantages of having a motocross bike for trail riding are:
- Harsh suspension
- Power is abrupt
- Close-ratio transmission
- 19″ rear wheel
- Weak stator (for powering lights)
- No kickstand
Starting out with a proper enduro/trail bike can actually save you a lot of time and possibly money.
Realistically, you can make any dirt bike work for trail riding.
The real question is, what kind of trail riding are you doing and what are the options that you “need”?
- KTM 150XCW
- Beta 200RR
- Yamaha YZ250X
- KTM 250XCW
- Husqvarna TE 300
Carb vs Fuel Injection
Fuel injection is not new technology, but it’s only recently been mass-produced on a few 2 stroke dirt bikes.
Carburetors have been the longtime fueling system on bikes. They still work great, but are they going to go out of style now?
The main advantages of a fuel injection system is the automatic compensation of climate change (temp, elevation, humidity), as well as throttle response, ease of starting, as well as less fuel and oil consumption.
If there’s so many advantages to fuel injection, then why are some riders so opposed to it?
Complexity, reliability, and cost are the main drawbacks. Or so they say.
Bikes cost more when new because of the technology; there’s no getting around that fact.
When it comes to complexity, that’s debatable. There may be more parts that can fail, but the reliability is actually not that bad. Tuning fuel injection systems is starting to become easier than tuning a carburetor (i.e. with your smartphone).
2 Stroke Dirt Bike Maintenance
Every dirt bike requires routine maintenance if you want it to be reliable and last many hours. No matter what kind of bike you ride, 2 or 4 stroke, the harder you ride it, the shorter the service intervals will be.
Piston rings might only last 5 hours on a 125 2 stroke if you’re holding it wide open on a motocross track. But if you trail ride the same bike and are much more conservative on the throttle, you could get up to 50 or 100+ hours on a stock top-end.
Keys To Keeping Your 2 Stroke Reliable:
- Warm the engine up the right way: warming it too quickly will cause premature wear and possibly cold seize the engine.
- Run proper premix oil ratio: higher RPMs require higher oil ratios.
- Proper jetting is for power & reliability: poor jetting can make your bike hard to start, low on power, or shorter engine life.
- Keep the air filter clean: Dirt in the engine = less engine hour life.
- Regularly change the oil: dirty/low oil will eat away your transmission.
- Rebuild the top-end when compression gets low: Worn/loose parts create more wear and reduce the lifespan of parts.
2 Stroke Top End Rebuilds
The top-end is the piston and ring assembly, as well as the cylinder on a 2 stroke. Rebuilding the top-end is a part of regular maintenance.
Low compression is usually the easiest way to tell your rings are worn and need replacing. Be sure to measure the size of the piston and cylinder (top and bottom, as well as roundness) before installing your fresh top end.
2 Stroke Carb
Even with KTM’s new TPI (transfer port injection), carburetors still dominate the used 2 stroke market these days. 2 stroke jetting is extremely critical (more so than a 4 stroke) to get the most power and efficiency.
Jetting affects so much of how the engine runs.
Common 2 Stroke Jetting Problems/Symptoms:
- Plug fouling
- Won’t start
- Hard to start
- Flooding over
- Gas leaking out of carb
- Poor throttle response
- No over rev
- Low MPG
2 Stroke Exhaust
The exhaust is very important on a 2 stroke when it comes to tuning. The size, length, and shape of the expansion chamber determines what RPM it is tuned for.
Stock pipes are usually tuned for the broadest power, while aftermarket pipes usually shift the power more towards peak horsepower or low-end torque.
The silencer doesn’t have as big of a role in affecting power, but it is important to have it correctly sized for the type of power you’re trying to achieve.
Spark arrestors often reduce sound output, as well as a possible decrease in power. The loss of power isn’t very noticeable to the average rider unless the spark arrestor screen is clogged and restricting exhaust flow.
Best 2 Stroke Mods
There’s many mods you can do to make your 2 stroke dirt bike perform better or differently.
The best mods that actually make a difference are:
Tuning the jetting is often going to be the best mod for performance when it comes to 2 stroke bikes. It is also one of the cheapest ways to get more power and better throttle response.