5 Signs That Your Dirt Bike Needs A New Top-end – 2 Stroke

If you buy or put a lot of hours on a 2-stroke motocross bike, chances are it will need the top-end replaced. A top-end rebuild is pretty easy to do on a two stroke engine, and the only special tool you’ll need is a torque wrench for most dirt bikes. However, before tearing your engine apart, there’s several signs that can tell you it has a worn top-end. If you have most or all of the symptoms listed below, you can count on replacing at least the piston and ring, and possibly a cylinder and head. Although, if none of the symptoms relate to your bike, there’s a good chance it won’t need a rebuild yet…

It Takes 50 Kicks To Start My Bike…

This is a common symptom for both two and four stroke dirt bikes. Unless you are riding in extreme climate, your bike shouldn’t take more than a few kicks to get started. Certain bikes require more muscle and/or technique to get started, but a solid running engine will only take 1-3 kicks to start, hot or cold. On the flip side, just because your bike takes forever to start doesn’t always indicate a worn top-end. For more info on a ‘no-start’, read this article on Diagnosing A No-Start

My Bike Is Gutless

Does your dirt bike feel like it lost its power-band(sarcasm)? A worn piston/ring will make the engine feel weak and low on power even when WOT (wide open throttle). This often happens after a top-end has so many hours on it, and will get worse/less powerful over time. A 2-stroke piston can last over a hundred hours if the bike was casually ridden and properly maintained, but an aggressive motocross racer can wear out a top-end in less than 20 hours of ride time.

This Thing Eats Spark Plugs

Shattered CR80 Piston
Shattered CR80 Piston

How often are you replacing the spark plug? If you have to put a new one in every ride, stop and inspect before riding again. If the jetting is right, a fouled spark plug can also be caused by a bad top-end or blow-by from the worn piston/ring/cylinder. This definitely indicates a new top-end is required, as well as a new spark plug. I have personally seen a CR80 that fouled plugs because of a bad top-end. It turned out that the piston was cracked and it eventually just lost compression. The cylinder was used again because it was surprisingly still in good shape! Here’s some more causes of a fouled plug.

Look For Scratches

A scored piston or cylinder can be caused by multiple things. A lack of lubrication from not enough 2-stroke oil in the gas or lean jetting can damage the piston and cylinder, as well as the crank assembly and bearings. Scoring is caused by metal to metal contact, which is common on 2-strokes when they aren’t warmed up properly. It makes me cringe when people try to warm up their dirt bike too quickly, and I see it quite often. A lot of top-end damage is done if you start your bike and immediately ride it hard or rev it without giving it enough time to warm up.

A piston has a certain amount of clearance between it and the cylinder wall. This clearance is for “growth”, because it will get bigger as it heats up. The size difference isn’t much, but if the piston heats up too quickly, it will expand faster and get too big to properly fit in the cylinder bore, thus creating scoring, and possibly a “cold seizure”. Read the article in the link above to learn how to warm up your dirt bike without damaging anything. You should also note that forged pistons, as opposed to stock cast pistons, are slightly smaller in diameter and require more warm-up time to expand; so take extra precaution.

You may have to take the cylinder head off to get a good look at the cylinder walls to check for scoring, but there’s a quick and dirty way to give you an idea of its condition without touching the engine itself. Just unbolt and remove the pipe/expansion chamber so you can look in the exhaust port of the engine. Shine a light in there and see if there’s any noticeable scratches. You can look at the piston as well, but any scratches/scoring that looks like something you can feel with your finger nail is damage and will most likely need repair/replacement. If you can’t see the cross-hatching on the cylinder walls then it is worn and probably out of spec.

Top-end Rebuild
Top-end Rebuild

My Bike Is Super Easy To Kick Over

If you don’t know already, this isn’t a good thing. While most 2-strokes are easier to kick over than a four-stroke dirt bike, you shouldn’t be able to push the kick-start lever down with one finger. If it kicks over with next to no resistance, the top-end is probably down on compression. Low compression will usually go hand-in-hand with taking a lot of kicks to start, as well as a weak engine.

A new piston ring may be the only thing that needs replacing if the piston and cylinder are still in good shape. Rings only last so long and eventually go out of spec (refer to your manual). If you have the cylinder off, check the ring gap with gauges and compare to what the manual says. If the gap is too big, replace the ring with a new one. You can usually re-ring a piston once before needing a new piston as long as everything else checks out.

These tips are only guidelines. Nothing is set in stone, as every bike is different. Just because your bike is hard to start doesn’t mean it will always need a new top-end. It could be something completely different, such as a dirty carburetor because the bike hasn’t been started for months. I put this list together because these are the most common symptoms of a bad top-end when all of the simple problems have already been checked.

Have fun, and ride safe!

-Tom Stark

My Favorite Two Stroke Motocross Blog

I love two-stroke motocross bikes, there’s no doubt about it. And if there’s one thing I don’t want to see, it would be the discontinuation of two-stroke dirt bikes. They are just so much fun to ride, and cheaper/easy on maintenance I might add. So, I try to keep up on the latest news about the industry. Fortunately, Two Stroke Motocross.com is there to fill me in on everything I need to know, and more!

TwoStrokeMotocross.com

I have a million things to do around here, so reading every little article on the net about this topic is just not feasible. Two Stroke Motocross gathers the best, and most important things to know about the two-stroke industry, so I only have read one article to get the latest and greatest information. They post updates about two-stroke dirt bikes, motocross teams, tips and how-tos, and many other articles to fill your 2-cycle hearts’ desire.

I really liked their post, “Two Strokes and Attitude” because it not only gets you to want to persuade people to start riding two-strokes again, but it shows you the best ways to go about it in the motocross community.

If you are a die-hard two-stroke motocross fan like me, you’d better check out TwoStrokeMotocross.com Here now! You won’t regret it; they even have their own TSM Racing Team!! And remember that the best way to bring the two-strokes back is to go out and buy one!

Good luck, and have fun riding! (only possible on a two-stroke….. haha, just kidding!)

-Tom Stark

Why Does My Dirt Bike Keep Fouling Spark Plugs?

Sick of having to replace the spark plug on your dirt bike almost every ride? Chances are, it’s a fairly easy fix. But there can be several variables to the equation. Incorrect jetting is the number one cause of plug fouling, but we’ll get a little deeper into what the most common causes could be and how you can fix them before spending a ton of cash on spark plugs.

No Air = No Combustion

Dirty Air Filter

First thing to do if your spark plug has been fouled is to check the air filter. If it’s really dirty, that makes it difficult for air to get through to the engine, causing a rich condition. Either clean it thoroughly with some Air Filter Cleaner, or buy a new filter. Before you put it back in the air-box, treat with some air filter oil or spray. BE CAREFUL, though, because adding too much oil can also clog up the filter, resulting in another fouled plug.

What Mixture Are You Smoking?

(For 2-Strokes) While the gas/oil ratio doesn’t equate to what the jetting is, you can still foul plugs, or worse, if the mixture is wrong. Depending on your bike, it’s best to look at your owner’s manual for the correct ratio. For most 2-stroke motocross bikes, a ratio is 32:1 or 40:1, gas being 32 or 40, and oil being 1. For those that don’t it, it means that for every 128 ounces (one gallon) of gas, you add 4 ounces of 2-stroke oil for a 32:1 ration, or just over 3 ounces for a 40:1 ratio.

If you have too much oil in the mixture (such as 12:1), then you will probably end up fouling the plug because it is more difficult to burn that much oil. Don’t try to cut corners with the oil though, because it’s very important for your dirt bike’s engine. 2-stokes need the oil for lubrication on the cylinder walls, so if you don’t have enough (or any for that matter) then the cylinder walls will run dry, causing it to overheat and seize the engine very quickly. So it’s very important that you put in the right mixture pre-mix to your 2-stroke gas tank.

Also, 2-strokes have reeds that air and fuel go through into the cylinder, so if they are cracked or broken the bike will not run right. This could cause the plug to foul, so before you go out and buy anything expensive, make sure the reeds are still good.

Too Rich

Turn Up The Heat!

While this usually isn’t the main problem, it could be that your spark plug isn’t burning hot enough. Having a hotter spark will ignite the fuel stronger, leaving less residual gas/oil in the cylinder that could cause the plug to foul. This isn’t the best choice, but I’d say it’s safe to go one plug hotter than stock. Spark plug manufacturers aren’t always the same, so make sure you find out how they rate them.

4-Strokes Foul Plugs Too…

Don’t be fooled into thinking that only 2-strokes foul plugs. 4-Strokes, although it is not as common, do foul plugs. Like I mentioned before, improper jetting is the main reason why dirt bikes foul spark plugs. Almost every motocross bike comes rich from the factory (especially Honda 2-Strokes when they were being made). It’s usually a pretty simple fix, but many riders are too lazy and would rather continue buying plugs instead of spending a few bucks a jets.

Can 4-Strokes Foul Plugs?

If you know for sure it’s a jetting issue that’s causing the fouling (you’ll know this by looking at the plug, which I’ll cover on how to read one in a future article), then the first thing you should do is go down a size or too on the main jet. Your goal is to get a crisp throttle response at any rpm. If you ride in the lower rpm range often, then you’ll want to go down on the pilot jet and lean out the clip position (learn the Basics of Jetting Here). By leaning out your bike, your bike will not foul plugs as easily, allowing you to ride all day long.

You Can Always Just Ride Harder

If you want to go the easy route (well not really!), just ride the bike harder. Spark plugs usually foul at low RPM’s, so if you’re wringing your bike out you shouldn’t have as much of a problem!

IF NOTHING ELSE WORKS!!!

If you’ve tried all of the above, then there’s a chance your dirt bike will need an engine rebuild. A worn out piston can cause a rich condition, which results in the plug fouling. So if the top-end hasn’t been rebuilt for a while, you’ll want to do that. Not only can this solve your plug/jetting issues, but it may prevent a major engine failure in the near future because of worn out parts.

Good luck, and keep that bike running strong!

-Tom Stark

Wikipedia: Revolutions per minute is a measure of the frequency of a rotation.

250cc Motocross Two or Four Stroke?

250cc 2-stroke vs. 250cc 4-stroke

If you’re looking to get into racing motocross at AMA sanctioned tracks and are wondering which bike to buy for the 250cc class, then pay attention. Although the four-strokes have pretty much taken over the motocross market lately, it seems that there are a lot of people trying to bring back the two-strokes, and it seems to be working. AMA made it legal so that a 250cc two-stroke can run in the same class as a 250cc four-stroke. Now, the first reaction I had when I heard of this was without doubt to pick the two-stroke. I looked into it a little bit more and came up with this….

250cc Four-Stroke

YZ250F 4-Stroke

Now most people would think that a four stroke with the same displacement as a two-stroke would not have a chance, and that is why the AMA originally allowed the four strokes to run a bigger engine in 1998. This isn’t the case, due to the fact that companies have put so much more money and effort into making the modern four-strokes extremely high-performance. Some riders that do not like the two-strokes snappy power will often stick to the four-stroke 250 just because they like their broad and easy-to-ride power-band. A 250 two-stroke may have more peak power, but when the day gets long and the track starts to get really rough, that’s when the four-stroke starts to shine. Due to the tractability in rough and slick conditions, the four-stroke will be easier to ride faster late in the day. That doesn’t mean it will be faster than a two-stroke though…

250cc Two-Stroke

Two-strokes are fun to ride because they have a bigger top-end “hit” compared to the four-stroke. If you keep the two-stroke on the pipe then it will definitely be faster than the 250F, as long as the conditions aren’t too rough. For those of you that want more, snappy power, and a lower center of gravity, then go with the two-stroke. It may not always have the traction that a four-stroke has in rough or slick track conditions, but the horsepower makes up for it.

YZ250 2-Stroke

So Which Bike??

After reading the info on each bike you would probably guess that the two-stroke is the better bike for AMA racing, and my answer for that would be, “Yes.” Clearly, it’s a faster bike if the track is in good shape, it handles better because it has a lower center of gravity and is a 2 stroke, it has a lighter front-end allowing you to skim over whoops and bumps, and it smells good. But I will say that if you are a four-stroke guy that doesn’t like the snappy-feeling power-band on the two-stroke, then you will be fine on a 250F. The 250F is by no means slow, and any good rider should be able to win on it. My choice would be to ride the 250 two-stroke though because it’s faster, and it’s a new rule that I would take advantage of. Good luck, and no matter what stroke you ride, have fun!!

-Tom Stark

 

 

What Dirt Bike To Buy For My Kid?

Are you thinking about buying a dirt bike for your kid but don’t know which bike to choose or look for? I will give you some background on name brand bikes for kids to help you make a decision. Choosing the right bike for your young rider is very important, especially if they’ve never ridden before. Kids start riding as young as three years old, so once they are able to ride a bicycle without training wheels they should be capable of riding a dirt bike. And please don’t put training wheels on a little dirt bike, that just defeats the purpose.

There are a few factors that come in to play when finding the right dirt bike for a kid. Age, experience, size, and where you are going to ride all come in to play when looking for a bike. There are 50, 70, 80, 90, 100, 110, 125, and 150cc four-stroke dirt bikes that are mostly meant for off-road and trail riding. For the smaller two-stroke dirt bikes, there are a couple different 50s, 65s, and 85cc bikes. These are used for motocross racing.

Off-Road Bikes

50cc

There are a few different 50cc dirt bikes, but the most common is the Honda CRF50F/XR50R. This is where dirt biking starts for every little kid that has never ridden before. These things are almost bullet proof if you maintain it properly. If you are looking for a first bike for a kid that is under 7 then this is probably the best choice, unless they are bigger/taller than the norm. There is also the Suzuki JR50, Kawasaki KDX50 and Yamaha PW50; all of which are oil-injected 2-strokes that are very light and perfect for the smallest of riders, and the Yamaha TTR-50 which is about the same as the CRF, so it just depends what color you like best.

70 & 80cc

Honda made the CRF/XR70 and 80cc bikes for kids that are just starting, but a little too big for a 50cc. The 70 has a taller seat height with a little more power than the 50, but the 80 has a clutch which is perfect for training the little ones without letting them ride on a fast bike.

100/110/125/150cc

Honda has been known for their XR/CRF100 which reaches to a variety of riders. It has a clutch and is just right for the older kids to learn on if they are too big for the CRF80F. The 100 is a great trail bike because it has just enough power, and it’s also big enough for an adult, so this bike should be easy to find used, and cheap I might add. Kawasaki and Suzuki decided to make the ultimate pit bike for kids and adults. The KLX110/DRZ110 (Parts are interchangeable) has become popular because of how much power it has, or is able to put out, for its size. The 110 is an automatic, so it’s great for smaller riders that want more power than a 70 without going to a clutch yet. Yamaha also has a bigger off-road bike for kids, the TTR-125. The TTR-125 is a popular model for bigger kids just starting out. It has a clutch with enough power to haul riders around, young or old. Honda also has the CRF150F for riders that want more power and a taller ride height than the 100. Although the set-back for the 150 is that it weighs about as much as a full-size motocross bike. These mid-sized off-road bikes are popular for a reason, and this means that they have usually been ridden a lot and probably abused. So if you’re looking at buying a used one then make sure it is clean, it has somewhat low hours, and has been well maintained.

Motocross Bikes

50cc

There are a couple different name-brand 50cc motocross bikes for kids, and they are both KTM two-strokes. KTM has the 50 SX Mini, which is for the smallest beginning racers. There is also the 50 SX which is bigger and suits a little taller beginning racer. These bikes both have a one-speed automatic transmission and are liquid-cooled. KTM 50s are expensive, but they are perfect race bikes for your little kids that are ready to tear up the tracks. There are also Chinese companies with 50cc motocross bikes, which aren’t made with as much quality.

65cc

KTM and Kawasaki are the only name brand companies that still make a 65cc two-stroke motocross bike. KTM has the 65 SX which is more expensive and usually the racers that are serious buy them. Right from the factory KTM puts aftermarket parts on their bikes, such as VForce reeds on the two-strokes, handlebars, and Brembo brakes. The Kawasaki KX65 is what riders that just want to race buy because they are so cheap. That does not mean that they aren’t fast, they just aren’t always as reliable and don’t have a very high re-sale value. Both are great race bikes for intermediate younger riders in around the age 7-10 that have been riding for some time. These bikes are usually not used for trail riding because they are not as easy to ride and lug around as a four-stroke is.

85/105/150cc

85cc motocross bikes are the start for a young teen aged racer. They have more power than most any kid could use, which makes these bikes a blast for adults or adrenaline-junkies as well. KTM, Suzuki, Kawasaki, and Yamaha all make an 85cc two-stroke mx bike. Honda put a stop to their two-stroke inventory after 2007, but people still buy their late model bikes. Instead, Honda wanted to start a revolution for mini-bikes with their all new 150cc four-stroke motocross bike. It was well talked about, but once it came out it died slowly. They are powerful bikes and are easier to ride than an 85, but the weight and cost lost it for them. Usually these CRF150R’s (Big and small wheel) were bought by racers with money because they also needed quite a bit of maintenance. If you have money and your kid loves the easy ride-ability of a four-stroke then this would be a good race bike to buy.

If you don’t have a deep pocket, then an 85 or 100/105cc two-stroke stroke is a great choice. Your kid will love you for getting them one of these bikes; before they flip it at least once, that is. If you find someone selling a used 85 that hasn’t been flipped, then you’re either lucky or they’re lying. 85s are pretty much race-only bikes. I’ve trail-ridden with one, and it was not very fun because of the lack of power down low. The Honda CR85R and Yamaha YZ85 hit harder and are more snappy than the others because they don’t have a power valve. KTM’s 85 SX comes with aftermarket goodies from the factory like all of their other bikes, so it’s probably the fastest out of the lot. Kawasaki’s KX85 and Suzuki’s RM85 are very similar and are used by many riders that are on a budget because of how cheap they can be. Honda and Suzuki both have a larger wheel 85, which helps those taller kids that quite aren’t ready for a 125 or 250f.

Kawasaki and KTM have a big-bore 85 to compete in the supermini class (86-112cc). Kawasaki’s KX100 is a big wheel and has plenty of power for any young racer. KTM has a 105 SX which is one of the best power-to-weight ratio bikes you can buy, and also has larger wheels. When it comes down to choosing an 85,100/105, or a 150cc four-stroke motocross bike for your kid, it depends how big your pocketbook is, what fits your kid, and what color you like best.

If you know what bike you want to get, check out my article “How To Buy A Dirt Bike“. Thanks for viewing, and good luck buying the right bike for your kid.

-Tom Stark