How Do You Fix Low Compression on A Dirt Bike?

Low compression on your dirt bike is one of those things that is relatively inevitable. Whether your engine is a two-stroke or a four-stroke, its compression will eventually run low and will require your attention. 

In a two-stroke dirt bike engine, the likeliest cause for low compression is worn pistons or rings. A four-stroke engine also has piston rings that need to be replaced regularly. However, the latter can also experience worn intake or exhaust valves, leading to low engine compression.

As is the case with most dirt bike questions, there is a lot of detail that goes into these common problems, and the more you understand about the machines, the more capable you will be at solving issues before they happen.

How to Tell If My Dirt Bike Has Low Compression

Low engine compression is the number one reason for dirt bikes working perfectly one day and struggling the next. Fortunately, there are many symptoms that indicate that your engine is in need of attention. In the case of a two-stroke dirt bike, you may notice any of the following signs that indicate your engine compression is low.

  • It becomes difficult to start
  • The kick-starter is too easy to kick over
  • Your dirt bike feels lower power than usual
  • Your spark plug is fouling
  • Your dirt bike won’t idle well

If you have a four-stroke dirt bike, many of the symptoms will be similar to the two-stroke variety, but with a few notable differences. You can expect to see any of the above, but with the following added to the list.

  • Dirt bike backfires or pops when decelerating
  • Complete loss of power
  • Runs very roughly

The only benefit of low engine compression is that it is easy to identify, thanks to these extremely obvious symptoms. Once you find your dirt bike showing any of the above signs of low compression, it’s time to get it tested.

How to Test Your Engine Compression?

Completing a compression test is reasonably straightforward, but there are a few variables to be aware of before starting. There are actually two ways to test an engine, warm or cold. The concept behind a compression test on a warm engine is that it will give you more accurate results because the machine will be closer to its average operating temperature. 

While this has the benefit of accuracy due to the thermal expansion of the piston, cylinder, and rings being more representative of your engine when it’s running, it can be challenging to replicate this when testing. There are a few steps that need to be completed to run the test, and your engine will inevitably cool down as you get prepared, so in the pursuit of accuracy, you may end up with more variable results.

Choosing to perform a cold compression test is much simpler. However, it is important to be aware that the resulting compression values that you get from your test on a cold engine will be lower than a warm one. While mechanics will go back and forth over which choice is better, repeatability is the key to successful testing, and a cold compression test is infinitely easier to duplicate.

Step-By-Step Two Stroke Compression Test

  1. The first step is the simplest one, and that is to remove the seat from your bike. Note that some bikes will give you sufficient access to the engine that you won’t need to remove the seat, but consult your owner’s manual to determine if yours falls into that category.
  2. Remove both the fuel tank and the radiator shrouds from your bike. If your bike has a petcock equipped, turn it off before removing the fuel line. For safety and cleanliness, catch any fuel draining from the line with a rag.
  3. Next, you will need to remove the spark plug cap. However, before you remove the plug itself, use compressed air to blow dust and debris out of the plug cavity so that it can’t get inside your engine. After doing so, remove your spark plug.
  4. At this point, you should install your compression tester into the spark plug hole and ensure that all of your fittings are tight to reduce the possibility of getting false readings.
  5. You will then hold the throttle wide open and kick the bike over five times as quickly and as hard as you can. Then, confirm the number recorded on the compression tester.
  6. Once you’ve recorded the value somewhere, reset the testing gauge, and repeat your test three to five times to ensure that your results are accurate.

Testing compression on a two-stroke dirt bike is not a very complicated process. As long as you follow these steps and confirm the details of your exact make and model of bike in your owner’s manual, you should be able to get consistent readings of your dirt bike’s compression. 

It is worth noting that a four-stroke engine will also involve a compression relief system and will have a specific allowable compression range. Consult your owner’s manual to find this information, and if your test indicates low compression in your four-stroke engine, you should complete a leak down test to determine what components need replacing.

What Level of Compression Should My Dirt Bike Have?

Now that you have a recorded compression value, you will need to compare it to your dirt bike’s recommended minimum compression. As mentioned above, your owner’s manual will have a minimum PSI specification, but there are commonly upheld two-stroke PSI values that you can use as a guideline. Be aware that these numbers are not absolute for every dirt bike.

  • 50cc engine – 120 PSI minimum
  • 65cc engine – 120 PSI minimum
  • 85cc engine – 130 PSI minimum
  • 125cc engine – 140 PSI minimum
  • 250cc engine – 170 PSI minimum
  • 500cc engine – 140 PSI minimum

Along with various makes and models possessing individual minimum requirements, where you live may also make a difference. Suppose you run identical tests on a single dirt bike at sea level and again at a higher altitude. In that case, you will receive different values as the difference in air pressure will provide alternate conditions.

How to Fix Low Compression?

With all the information you’ve gleaned from your dirt bike’s compression test, you will be ready to fix the actual problem. In most two-stroke engines, the primary reason for low compression will be a worn top-end, which may require replacing the piston or perhaps just the piston rings. However, if your cylinder has worn out of specifications or has physical scratches on it, you may need to rehone or replate it.

If you have a four-stroke engine suffering from low compression, you may need to replace the valves, seals, and potentially a new timing chain to ensure its reliability. Whether this is a task for you or your local mechanic depends on your mechanical aptitude and familiarity with top-end rebuilds.

Don’t Let Compression Get You Down

While the signs of low engine compression can be dramatic and alarming if you are not familiar with it, taking the time to understand your dirt bike’s engine will pay off in the long run. Even if you aren’t mechanically inclined, identifying problems with your machine before they get serious can save you time and money by preventing them from getting worse. 

At the end of the day, knowing more about your bike will make maintaining it more straightforward and ensure that you spend less time worrying about what might be wrong and more time back out on the track!

Are Dirt Bikes Hard to Maintain?

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Dirt bikes are all about getting down and dirty and adrenaline-soaked adventures across formidable terrains. Dirt bikers will swear that no other sport can come close to motocross or trail riding. But how challenging is it to keep those dirt bikes running?

Dirt bikes require more frequent maintenance because they endure impacts and stresses more than common road bikes. To run at peak performance, they require frequent inspection and servicing. Yet most dirt bikes have more simple working parts and are easier to repair oneself than road bikes.

To understand a dirt bike’s maintenance requirements, we have created a list of common problems particular to this bike type. You may then have a clearer idea of the challenges you would expect if you decided to own a dirt bike yourself.

What Is a Dirt Bike?

Merriman-Webster defines a dirt bike as “usually lightweight motorcycle designed for operation on unpaved surfaces.” Dirt Bikes are made to be ridden on hilly and uneven terrain in all kinds of weather conditions. They are used in motocross racing popular in North America, Asia, and Europe. 

There are two larger categories within the term ‘dirt bike’ which are motocross racing and trail riding. Although the term motocross can be used for both, generally, motocross refers to the sport of dirt bike riding on a prepared outdoor track. 

Dirt Bike vs. Motocross vs. Enduro

Though every motocross bike is a dirt bike, not every dirt bike is a motocross bike. Motocross bikes are made for speed and agility and need a high degree of maintenance. Dirt bikes are less specialized and easier to maintain. Enduros are larger, heavier bikes with larger gas tanks for longer distance races.

Your average dirt bike has a long-travel suspension and rugged tires with a high seat position. These lightweights, single-cylinder bikes are designed to tackle any terrain such as desert woods or mountainous terrain.

Two-Stroke vs. Four-Stroke 

When it comes to dirt bikes, riders choose between a two-stroke (2T) and a four-stroke (4T) dirt bike motor. The 2T has one revolution of the crankshaft within one power stroke while the 4T has two. The 2T does not have a dedicated system delivering lubrication to the crankcase while the 4T does.

Common Maintenance Requirements in Dirt Bikes

Although two-stroke and four-stroke dirt bikes are easier to repair because of fewer moving parts, the maintenance necessary to maintain your bike properly is high compared to road bikes to prevent your dirt bike from malfunction or failure. 

Preventative Maintenance Requirements of a Dirt Bike

Many of the maintenance routines for a dirt bike are not excessively difficult or strenuous in themselves. These preventative maintenance requirements become a routine in a dirt bikers’ life and are part of the off-road experience.

Preventative maintenance saves dirt bike owners on costly repairs on their bikes and even prevents them from injury in extreme cases. Responsible riders follow a maintenance routine to get a longer lifespan out of their dirt bikes and overall better performance in the short term. The following is a list of what a dirt bike owner would be expected to do to maintain an optimally running dirt bike.

Wash Your Bike After Every Ride

Wash your dirt bike after each ride, whenever possible. Not only do you keep your bike’s parts free of debris and dust, but you also get a chance to look your bike over and check for any problems. A gentle wash is recommended with water and brushes or cloths. If you use a pressure hose, be extra careful to direct the water away from the bike and not into the engine or electrics. 

A great step to add to your dirt bike routine is to use your washing time to inspect your bike for problems. Preventative measures taken early can save you a lot of money in the long run with costly repairs and replacements.

Dry Your Bike Before Inspecting It

Many dirt bike enthusiasts recommend drying your bike after a wash down by taking it for a short spin. Whether you use the sun or a leaf blower to get the job done, make sure your bike is dry before conducting your maintenance inspection. Once your bike is clean and dry, you can easily spot oil drips or coolant and brake fluid leaks.

Check for Oil Leaks

Oil leaks may be a result of poor maintenance or just general wear and tear on your seals. The first culprits are usually the crankcase or a worn gasket, and a typical leak is from behind the front sprocket, which is called the countershaft seal. Wipe the oil from under your bike, visually inspect for leaks, and keep an eye out for any coolant or brake fluid.

Check your transmission oil levels and reference your user manual to determine your oil quantity and viscosity.

Inspect and Clean Your Drive Chain

If your chain is covered in mud, wait for it to dry and brush it off with a nylon brush. Once your chain is clean, lubricate it well with a high-quality chain lubricator. The lube fills in the cracks and notches where dirt can collect. 

Motocross and dirt bike riders tend to favor non-O-ring chains because they are lighter. This means they need to clean and lubricate their chains quite often, depending on the amount of riding time. It is better to lubricate your chain after your ride so that it has time to soak in and fully penetrate your chain.

Check Your Chain Tension

Your chain should always have some slack to compensate for the movement of the suspension. However, if you can remove your chain from the rear sprocket, it is too loose. The ideal amount of chain play is ½ inch (13mm) when the swingarm is parallel to the floor.

Inspect Your Bolts

Your bolts can sometimes be shaken loose under extreme vibrations and intensive riding. You should always conduct a check that your bolts are firmly in place to prevent anything shaking loose on a ride.

Check Your Bikes Controls and Control Cables

If your throttle and clutch cables show any signs of wear or fraying, you should replace them. You should also check your throttle control for the correct amount of free play. An excellent way to test this play is to:

  1. Place your bike on a work stand, start it, and let it idle.
  2. Rotate your handlebars to their full range of motion and listen carefully for any increases in your RPMs.
  3. If you hear an increase, your throttle needs more free play.

Also, check that your throttle is responsive, making sure it snaps back when twisted.

Check Your Air Filter

Your air filter plays an essential job in keeping sand, dirt, and debris from getting into your bike’s engine. A dirty air filter will affect your bike’s performance and cause damage to the piston and barrel. If you have a washable filter, you can clean your filter in warm water and a household cleaner, such as Simple Green. 

If you need to replace your filter, make sure you buy a high-quality one with a dual-stage bonded foam filter and thick sealing rings.

Check Your Tires

Your dirt bike wheel bears up quite a load, and your spokes could loosen, and your wheels may fall out of alignment. This leads to uneven weight on your hub and rim. A torque wrench is an excellent investment to ensure your spokes are the right tightness. 

Ensure your rims are round and have no dents or cracks and then check your tire pressure. It’s essential to ensure that you maintain the correct psi in your tires. You can use the TireTek Tire Pressure Gauge and ensure that your psi is correct. The average psi for all-round dirt biking is 12psi, but it can fluctuate between 8-18psi depending on the terrain.

Do an Oil Change

If you spend a lot of time in mud or your bike endures extreme duty, you should consider changing your oil more often. The frequency of oil change can depend on the particular bike, your riding conditions, and your dirt bike’s age. Your oil lubricates all the working components in your bike’s engine. 

When your oil levels fall too far, these metal components rub against each other, creating intensely high heat. This could cause your engine to seize and be permanently damaged.

Some dirt bikers advise an oil change each ride while others suggest 8-10 riding hours. Check your owner’s manual for your manufacturer’s recommendations for oil changes.

Check Your Fluid Levels

If you are racing, you should bleed your brakes every 20 hours, and recreational dirt bikers should do it every 40 riding hours. Brake fluid can go bad when it absorbs water and becomes contaminated. Off-road bikes generally use a DOT-3, 4, or 5 brake fluid, but you should check your manufacturer’s specifications in your user manual.

Grease Your Bike

Inspect your:

  • Air filter seal
  • Swingarm
  • Hardware
  • Wheel bearings and seals
  • Shock seals and forks
  • Steering head bearings

If necessary, grease these components to keep out moisture and debris and to provide lubrication. 

Check Your Brake Pads

Your brake pads may wear down over time, and if it wears down too far, it may even destroy the brake rotor. Worse yet, your brakes may fail. You need to routinely check your dirt bike brake pads and replace them at signs of thinning or visible wear. A rule of thumb is to replace the pad when 1.0mm (0.04 inches) of the brake pad remains.

Check Your Sprockets

Your sprockets transfer power to the rear wheel via the drive chain, and they endure heavy energy loads. Your countershaft and rear sprockets will wear out over time, and you should do visual checks often. A rule of thumb is to replace your sprockets when you replace your chain or every 15,000 miles. Refer to your owners manual for their particular specifications.

Other Common Dirt Bike Maintenance Requirements

Do a Compression Test

An easy way to check that your dirt bike engine condition is to perform a compression test. You may find motorcycle compression kits online, such as the OTC Motorcycle Compression Kit. The tester comes with multiple plug spark fittings and works by placing the hose and adapter into your spark plug hole. You then open the throttle and kick the bike over 5-10 times and watch for the reading at maximum pressure.

A well-functioning 250cc engine should read at about 175-200-plus psi, and a mini or 125cc should read at about 150-200 psi. If your compression is 20% lower than your baseline, you need to replace your piston and your rings.

Check Your Piston or Rings

Average intensity riders should replace their piston and rings every 20 hours on an 85/125 and every 40 hours on a 250cc. If you don’t have an hour meter on your bike, you can install an aftermarket one as its a handy way to remind yourself that it is time to perform some maintenance. Sign that your piston and rings are worn is excessive smoke and a loss of power. Pistons come in different sizes, and it’s best to use the specs to replace the piston if it has excessive wear. 

Check for Main Seal Leakage

If you notice oil residue around the seal on the bike’s magneto (left side), you can check the crank bearings for looseness to determine the leak. Main seal leaks can cause air to be drawn into the crankcase and make the bike run lean or too much air and not enough fuel.

The main seal on your transmission side will cause the oil to leak into the crankcase. The signs would be excessive smoke, the bike running rich or too much gas, and the transmission oil will smell rank. If you are uncertain, a mechanic can run a crankcase pressure test to determine the leaking seal.

Check for Other Leaks

Head gasket leaks will typically cause your motor to misfire at high rpm or when it is hot and produce white smoke that may smell of coolant. You might find leakage will occur at the top of your head gasket, and air leaks may develop at the air boot and manifold. The head gasket should be checked that it is flat and in spec. Both gasket and intake air leaks may cause erratic acceleration.

Check if Your Exhaust Valves Are Clean

Your owner’s manual should tell you how often you should clean your exhaust valves clearance. If you don’t have access to a manual, the inspection should be 15 hours to 15,000 miles. Carbon can clog your valves, and you won’t get much top end, and you will notice your spark plugs are being burned through often.

Check Your Silencer

Part of your dirt bike maintenance routine includes repacking your bike’s silencer or muffler. As a rule of thumb, you should aim to repack your muffler after every 50 hours of riding. The owner’s manual should specify your particular bike’s requirements.

Repacking is simple if you follow these simple steps:

  1. Remove the slip-on from the rest of the exhaust.
  2. Remove the outlet cap on the end of the pipe. Usually, these are screwed in, but you may need a rivet gun if it has rivets.
  3. Take out the old packing wrapped around the inner core tube.
  4. Place your new packing and wrap it tightly around the inner core tube.
  5. Slide back into the exhaust pipe.
  6. Replace the outlet cap and the slip-on.

Check Your Reeds

Your reed valves control the air and fuel mixture that goes into the cylinder. As the piston rises, it creates a vacuum in the crankcase. The reed ensures that the mix won’t move back into the carburetor and only moves in one direction from the carburetor to the crankcase.

Reeds should ideally be checked every 10 hours of riding and replaced at least once a year. If your reeds are worn or damaged, you may have difficulty starting your bike.

Check Your Fuel

Always use fresh, high-quality fuel with a suitable octane rating for your particular dirt bike. Refer to your owner’s manual to ensure you are using the right fuel. Octane 87 is acceptable for fuel injected 4-Strokes, but in a 2-stroke, the best gas would be an ethanol-free 92 or 93 octane.

A lower octane can cause pinging or knocking noises caused by detonation when your fuel burns too fast. Try your best only to use non-ethanol blends where you can, but never have more than 10% ethanol-based fuel.

Conclusion

Dirt bikes are a hands-on affair with more frequent checks, tweaks, and required changes in other types of motorcycles. One must bear in mind the unique functions of these small but seemingly indestructible machines and the off-road experience.
It’s no surprise that some of the top MotoGP icons, such as Valentino Rossi, have a motocross background. For some, it’s the ultimate riding experience. You just may have to put in a bit more work.