13 Signs Your 4 Stroke Dirt Bike Engine Needs To Be Rebuilt

Wondering how to tell if your 4 stroke dirt bike needs an engine rebuild? Has it made any weird noises or something feels different? I highly recommend you stop riding until you figure out what has changed.

Riding a dirt bike that has an engine that’s about to let go is not only dangerous, but it will get even more expensive to rebuild. There’s many symptoms that can give you a heads up. 

Pay attention to the symptoms so this doesn’t happen!

The top signs that a 4 stroke dirt bike needs to be rebuilt are:

  • Hard to start
  • Low power
  • Noisy engine
  • Fouling plugs
  • Running more rich
  • Smoking
  • Milky oil
  • Metal in oil/filter
  • Burning oil
  • Low compression
  • Engine vibration
  • Coolant coming out the overflow
  • Water pump leaking

Does One Sign Mean I Need To Rebuild It?

Does your bike have any of these symptoms? If it does, don’t get upset just yet. Before you tear apart your engine, read through each section that has the symptom that you’re trying to diagnose. 

Just because your bike has one or more symptoms doesn’t mean the engine needs to be rebuilt. It could be another issue causing the same symptom. 

Remember to always start with the easiest things first when troubleshooting a problem on your dirt bike. 

Hard To Start

A 4 stroke that is difficult to start may be a sign that it needs to be rebuilt. This could be caused by valves that are worn or out of adjustment. It could also be a worn top-end that is low on compression. 

Kick Starting An Old Honda XR200

However, just because it’s hard to start doesn’t mean it needs to be rebuilt. A valve adjustment is merely maintenance if they are just out of tolerance. A dirty carb or fuel injector is also a common cause of hard starting. 

Low Power

Is your 4 stroke low on power? Make sure the clutch is properly functioning? A slipping clutch is a common cause of low power feeling. 

If the carburetor is clean and everything is tuned properly then a top-end rebuild may fix your bike lacking power. You can do a compression test to check for low compression, but a leak down test will more accurately tell you what’s worn or leaking on a 4 stroke. It could be the piston and rings, a blown head gasket, or worn valves/seals.

Noisy Engine

Some 4 stroke engines are straight up clanky and noisy. With that said, if your bike is making a louder noise than it has in the past it’s time to investigate. 

Clanking, tapping, and rattling are almost always a result from “loose clearances”. Either the factory tolerance allows for a lot of clearance, or the part(s) are worn out of spec. 

A worn cam chain or tensioner will cause a ticking or rattling noise. Simply replacing the worn parts will make it quiet again and potentially prevent a catastrophic failure. 

A worn piston/cylinder will make a knocking noise because the piston is too small for the cylinder and is rocking as it moves up and down. The sound may get quieter as the piston warms up and expands. 

Any time you hear a new noise, it’s best to shut the engine off and diagnose the problem before it gets worse. The oil can give you an idea of what’s going on inside. More on this later.

Smoking Out The Exhaust

A 4 stroke should not have smoke coming out the exhaust. The only time it’s okay is when there’s just some residual moisture in the system that burns off within seconds. 

White Smoke

If you see white smoke and it has a sweet smell, your 4 stroke is more than likely burning coolant. This is often the cause of a blown head gasket. 

XR600 Smoking
4 stroke dirt bikes aren’t supposed to smoke this much…

Blue Smoke

Blue tinted smoke generally means you’re burning oil. There’s a number or reasons that can cause this. Worn piston and rings or leaking valve seals are a couple of the most common reasons. 

Leaking Valve Seal

A worn or cracked valve seal will allow residual oil to sleep into the combustion chamber. This will result in bluish smoke on startup until the oil is burned up. 

A leaking valve seal isn’t a major problem, but it may indicate potential problems in the near future that will require an engine rebuild. 

Worn Piston/Rings/Cylinder

A piston, rings, or the cylinder that is worn out of tolerance will cause excessive blow-by due to lack of sealing and compression. Blow-by is when oil leaks by the piston rings and gets burned in the combustion chamber. This will also come out the exhaust as smoke.

Blow-by is evident when there’s black burn marks on the piston’s outer circumference at and above the piston rings. 

Running Richer

While air temp and humidity can alter jetting mixture, an unusually rich mixture may mean the engine is getting tired.

A worn out engine will not be able to burn the fuel mixture, so more gas is going out unburnt. This is usually a sign of worn rings or piston. 

Fouling Plugs

In addition to running rich, the spark plug may eventually foul. 4 strokes resist plug fouling more than a 2 stroke. If your 4 stroke dirt bike is fouling spark plugs, it’s more than likely a jetting or engine problem. 

Rich jetting causes spark plug to foul
Black spark plug = running rich

Oil Is Milky

Have you removed the oil drain bolt and had thick, milky looking oil drain out? This is due to water or coolant getting into the oil. 

A blown head gasket or a leaking water pump are common causes of water/coolant mixing into the oil. 

Metal In Oil

Always take note of what your bike’s oil looks like when draining it. You can usually tell when something in the engine changes based on the oil. Just paying attention here can save you hundreds of dollars. 

Small metal flakes can show up during engine break-in time, which is why it’s important to change the oil after an hour or less of break-in.

Chips in oil filter is an early sign that parts are wearing down…

New flakes or chunks means there’s something else going on. It could be the clutch or the piston breaking down. Either way, the oil will show you almost right away. 

Pulling apart the oil filter is another good way to check the health of your engine. Cut it open to check for more debris that it caught. 

Burning Oil

Your 4 stroke dirt bike may need an engine rebuild if it’s using more oil than usual. Worn rings or valves/seals are a common cause of a high oil consumption. 

All engines use some at least a little bit of oil over time. The tolerances of the internal parts would be too tight if it didn’t use any. With that said, if you check the dipstick or eight glass and it goes from “full” to “low” every few hours of riding then it’s using more oil than it should. 

Pro Tip: check for clogged/pinched or blocked crankcase breather tubes. If the breather tube is clogged, the excess crankcase air can’t escape and may cause the engine to consume and burn more oil.

Low Compression

Is your dirt bike getting really easy to kick over? This usually means that the compression is low and the top end will need to be freshened up.

Low compression is most commonly caused by worn piston, rings, cylinder, or valves.

Engine Vibration

Every dirt bike makes some kind of vibration. If it didn’t, how can you tell if it’s running or not (electric bikes just took offense to that)? The bigger the engine the more vibration you’ll feel, as a general rule. There’s always exceptions, though, as some engines are better balanced than others. 

If you’re used to the feel of your 4 stroke and it starts vibrating more and more than usual, it’s time to investigate. 

What To Inspect

Unfortunately, it’s hard to tell what’s going on without digging very deep. 

Without tearing the engine apart yet, remove the clutch cover and inspect the clutch basket, plates, and the bolts/nut holding it. 

You may have to remove some parts to inspect everything, but it will be worth it if you find the problem in there before it gets worse. 

You can also pop the stator cover off to inspect the flywheel/stator end. If something came loose, you’ll see it. 

Coolant Coming Out The Overflow

Coolant naturally comes out the radiator overflow hose when it gets too hot. If it happens repeatedly, you might have an engine problem that requires a rebuild. 

A blown head gasket is a popular issue that pushes coolant out due to extreme heat and pressure getting into the cooling system. 

Water Pump Leaking

Many liquid-cooled 4 stroke dirt bikes have a “weep hole” on the water pump cover. If coolant starts dripping out of it, that means the water pump seal is likely bad. 

Changing this seal doesn’t require a full engine rebuild, but you’ll have to remove right side case cover to access it on most 4 stroke bikes. 

F.A.Q. On 4 Stroke Dirt Bike Engines

Below are the most commonly asked questions concerning engine rebuilds and maintenance on 4 stroke dirt bikes. 

These answers are geared towards motocross and enduro bikes with a single cylinder liquid-cooled 4 stroke engine. 

How Do You Know When To Rebuild The Top-End?

You can make assumptions, but until you start inspecting actual parts, you won’t know for sure what needs to be replaced. 

Starting with the easiest things first is smart if you’re not sure what happened. Each symptom, though, can give you an indication of where to check. The list of signs and symptoms above explain what the most probable cause would be. 

Rebuilding a YZ250F top end.

How Much Does It Cost To Rebuild A 4 Stroke Dirt Bike Engine?

Depending on what parts need to be replaced and whether or not you’re doing the work yourself, a 4 stroke engine rebuild can cost anywhere from $50-3000+ for parts and labor. 

A typical 4 stroke dirt bike top-end rebuild with a fresh piston, valves and timing chain will generally cost about $300-700 in parts. Labor is another $400-600. That is for a basic top end rebuild. 

If there’s more that needs to be replaced, or your engine blows up and parts implode, the cost goes way up. A new cylinder head and cams can cost another $1000 if they are scrapped. 

This is why it’s so important to properly maintain your dirt bike. 

Catastrophic engine failures tend to happen because of lack of maintenance, incorrect installation, or excessive amount of abuse and hours on an engine. 

How Often Should You Rebuild The Top-End?

What kind of riding do you do? How often do you change your oil and clean the filter? What bike do you ride? These factors are the most important part to determining how long your 4 stroke top-end will last. 

A 450 dirt bike can last for several hundred hours before needing a rebuild if regular maintenance was done and it wasn’t ridden hard. 

On the flip side, a 250F motocross bike may need to have new rings or a piston put in every 20 hours if it’s raced hard on a moto track. 

It’s all about how you take care of your bike and paying attention for any signs that it’s giving that might lead to a rebuild. 

How Long Does A 4 Stroke Top-End Last?

A 4 stroke top-end can last 500+ hours of casual riding if it’s properly maintained. You can also wear out a top-end in less than 20 hours if you’re riding it hard and constantly hitting the rev limiter. 

How Long Does A Bottom-End Last

The crankshaft is generally referred to as the bottom end of a dirt bike engine. The bottom end will generally last much longer than the top-end if it’s getting proper lubrication. 

A bottom end can last several hundred hours if ridden conservative or moderately. 

Engine vibration is an indicator that the bottom end needs to be rebuilt. 

Checking for excess “play” in the crank with the top-end off is a sure way to tell. If there’s up and down play or a lot of side to side play, the crank bearings are more than likely worn out at a minimum. 

How Do You Rebuild A 4 Stroke Top-End?

You buy an OEM manual and follow it step by step… but seriously, OEM service manuals are really pretty good these days. 

A routine piston replacement isn’t much harder on a 4 stroke dirt bike than it is on a 2 stroke. 

To rebuild a 4 stroke top-end, you must:

  • Remove valve cover
  • Loosen & remove cams or rockers
  • Removing cam chain tensioner
  • Hold the cam chain up so it doesn’t fall into the engine case (an old wire coat hanger works well)
  • Unbolt cylinder head studs and remove head.
  • Remove cylinder
  • Remove cir-clip from piston
  • Take out the wrist pin and the cylinder comes off
  • Check for play in the crankshaft by wiggling it. Up and down should have no free play. A little bit of side-to-side is okay
  • Scrape off any old gasket material
  • Check new ring gap with feeler gauge by placing ring in cylinder (file to fit if necessary)
  • Install new piston, rings, pin, clips
  • Place new base gasket on crankcase
  • Install cylinder, then new head gasket

How Much Compression Should A 4 Stroke Have?

What bike are we talking about here? A KTM 450SXF or a Honda CRF230F? The 450 is a high performance motocross bike with high compression compared to the 230 that basically has a lawn mower engine for technology. 

How Many Hours Is High For A 4 Stroke Engine?

The amount of hours you will get out of your 4 stroke dirt bike depends on a few variables. 

200 hours would be a lot for a 250F motocross bike, but normal for an air cooled trail bike. 

It’s pretty common to see a 450 4 stroke with 300-400+ hours on the original engine if it was just casually trail ridden. 

How Long Does A 4 Stroke Engine Last?

How you ride your 4 stroke dirt bike and how well you maintain it are the key factors to how long it will last. 

Riding your bike hard in the upper RPM constantly on an MX track, it may only last 10-20 hours before needing a rebuild. 

On the flip side, conservative trail riding with proper maintenance (oil and filter changes), your engine could last 100-300+ hours. 

How Often Should You Rebuild The Top-End 4 Stroke?

Your rebuild intervals should be dependent on your needs and what your bike needs. With that said, if your 4 stroke dirt bike is showing multiple symptoms from the above list, now is a good time to rebuild it or at least inspect the top-end before it potentially grenades. 

What Is The Most Reliable 4 Stroke Dirt Bike?

That’s a loaded question if I ever heard one. 

Let’s say you take an air cooled Honda CRF250F and a liquid-cooled Honda CRF250R. Both are 4 stroke, single cylinder 250cc dirt bikes, but the CRF250R is much higher performance. 

If you maintain them the way the manual says and ride them the same way, the air cooled CRF250F engine will generally last longer and be the most reliable. 

Do You Need To Break In A New Engine?

Most engines still need some break-in time. Newer dirt bikes with nikasil plated cylinders don’t take as long to break in as an older style cast-iron cylinder engine.

Breaking in an engine properly will seal the piston rings to the cylinder. If they don’t seal well then the engine will be down on power and use more oil than an engine that was properly broken in.

If you break an engine in on a dyno, you can actually see the numbers increase the more runs you do. This is because the rings are sealing and seating to the cylinder wall to increase the compression and power it puts out.

How Do You Break-In a 4 stroke engine?

There are so many ways to “Break In” an engine. What is the best? I have found that properly warming it up on first start up, as another other cold start, is paramount. Once it’s warm, I moderately ride it around for a few more minutes to make sure the engine and transmission are fully warmed up to operating temp.

Once it’s warm, I ride it hard, shifting up and down the gears. I do that for about a half and hour to an hour, which is the most important hour in breaking an engine in. After that, there’s not much you can do.

I then proceed to park the bike and let it cool while I change the oil. Any newly “rebuilt” parts that “broke in” and got collected in the oil or filter will come out.

If everything looks good then I add fresh oil and it’s ready to go!

Kelley Fager

I help new riders learn how to safely ride and understand how to tune and fix their dirt bike in their garage.

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