You’ve likely heard or seen people’s advice to maintain your four-stroke dirt bike regularly since these bikes have more complex parts.
But the real question is, how often should you rebuild a 4-stroke dirt bike?
A four-stroke dirt bike needs a rebuild once it reaches 100 total riding hours to keep it in pristine condition. For four-stroke racing dirt bikes, the rider may need to rebuild their engine on a lower riding hour range, ideally when it hits the 25-30 hour total hours due to high engine stress.
When should you rebuild your four-stroke dirt bike?
If you’re a casual rider and don’t exert much strain on your dirt bike, you can rebuild your four-stroke engine once it reaches 100 hours of riding time.
Several riders rebuild their four-stroke dirt bikes after 80 hours to be safe.
For racers, you’ll want to rebuild your dirt bike engine once it hits the 25-30 hour mark.
You can even start rebuilding for lower total riding hours, especially if you’ve been on strenuous rides lately.
You can refer to your dirt bike’s manual for any information about rebuilding since every dirt bike model varies in its maintenance needs.
6 signs of a four-stroke engine rebuild
When you encounter these six signs on your dirt bike, it’s time to pull out your manual and your tools because it’s rebuilding time:
1. Difficulty starting the dirt bike
Does your dirt bike’s engine struggle to start when you kick the starter? Does it require more effort to get started?
If yes, it could be a sign that there’s something amiss with your engine.
Some common causes of starting issues are:
- Worn-out valves
- Carburetion issues
- Worn top-end
- Air leaks due to cracks in the intake boot or bolts/gaskets between the airbox and engine
2. Low power due to low compression
If you notice your four-stroke giving you lower power than before, there could be an issue with your dirt bike’s clutch, valves, or piston rings.
You can tell this by doing a compression test or if you feel less resistance to the kick starter.
If cleaning your carburetor or re-adjusting the valves and piston rings with a shim kit doesn’t work, you might need a top-end engine rebuild.
3. Smoke in the exhaust
There are two types of smoke you should watch out for on your dirt bike: white smoke and blue smoke.
White smoke usually has a sweet smell that results from burning coolant in the combustion chamber due to a blown head gasket (if your bike uses coolant, that is).
Blue smoke, on the other hand, means you’re burning oil. This happens when oil flows past the valve stem or piston rings and enters the combustion chamber due to:
- Worn valves or piston rings
- Leaking valve seals
- Worn cylinder due to high engine hours
4. Too much noise
There are certain noises you should watch out for on your dirt bike, such as:
- A loud metallic noise due to the cam chain stretching or lack of tension/resistance with the bike’s chain
- Consistent knocking noises from worn pistons or valves
5. Higher oil consumption
Sometimes, a clogged/blocked crankcase breather tube is the culprit behind it since the blockage prevents crankcase air from escaping the engine.
Or it could be something bigger, like worn piston rings, valves, or seals.
6. Engine oil looks different
Your dirt bike’s engine oil is a simple way to tell if something’s wrong with your engine.
For instance, milky-looking oil means water mixes into the oil system, which could be due to a broken water pump seal.
Black-colored oil could mean the dirt bike needs proper maintenance or more combustion byproducts are entering the oil stream due to worn piston rings.
What to do if you notice any of these signs?
You can perform a leak-down test to diagnose your dirt bike’s engine health and if your cylinders are sealing properly.
Leak-down tests involve mounting a pressure gauge on your engine.
It’s usually performed on the compression stroke when the piston is at TDC and all valves are closed.
The first pressure gauge measures the air trapped inside the cylinder, while the second gauge measures the amount of air escaping the combustion chamber.
The difference between the percentages of the two gauges determines if your dirt bike is in proper condition.
A pressure loss of 0% – 5% for racing dirt bikes and 0% – 15% for standard dirt bikes is common.
Once it leaks to 16% and above, the engine might be experiencing issues and may need to be repaired or rebuilt.
Here’s a handy guide on how to do a leak-down test on a four-stroke dirt bike:
How much does rebuilding a four-stroke dirt bike cost
You can expect the cost to rack up around $400 if you’re only replacing some parts on the top end, like the valve train and timing chain.
You can expect the cost to be around $1200 – $1500 for a DIY total engine rebuild and around $2500 – $3000 for an overhaul with a dealer, including parts and labor costs.
Your choice of parts for the rebuild can also affect your expenses for your rebuilding project.
If you don’t mind paying more, you can opt for OEM parts.
Otherwise, you can save a lot with non-OEM or aftermarket parts.
You can expect the cost to be cheaper if you’re doing the top-end or the bottom-end.
That’s why proper maintenance is more critical than ever!
Tip: Use a dirt bike hour meter to track riding hours and maintenance.
Is it hard to rebuild a four-stroke engine on a dirt bike?
A four-stroke is made up of more complex parts and pieces because its process of burning air-fuel mixture to power up a dirt bike is more thorough than a two-stroke.
This could mean more time to learn how to disassemble things, check for damaged or worn-out parts, and then reassemble everything.
But don’t let that fool you into thinking that rebuilding a four-stroke dirt bike is impossible.
Whether you have mechanic skills or not, you’ll still be fine, as long as you have your dirt bike’s manual with you.
Here are some helpful video guides on rebuilding a four-stroke:
How To Rebuild a 4-Stroke Top End in your Dirt Bike or ATV
Detailed Four Stroke Engine Rebuild Course – YouTube
How To: 4 Stroke Top End Rebuild | Motorcycle Superstore
Full Engine Rebuild – Yamaha YZ 250F
$900 dirt bike KTM 250SX complete rebuild – time lapse
Wrapping up
Rebuilding your four-stroke dirt bike’s engine is essential to prolonging your dirt bike’s lifespan.
Most riders rebuild their engines every time they hit the 100-riding-hour mark, although some choose to play safe and start rebuilding at the 80-hour mark.
For heavy-duty dirt bikes, like racing dirt bikes, you’ll need to rebuild the engine every 25-30 hours.
It can even be lower depending on the level of physical exertion you put on your dirt bike.
If you encounter these common signs with your dirt bike, you can do a leak-down test to check your engine’s health:
- Difficulty starting the engine
- Low power
- Sustained smoke in the exhaust
- Excessive noise
- Higher oil consumption
- The different appearance of engine oil
For the DIY approach, rebuilding can cost around $400 or slightly more for rebuilding some parts and $1200 – $1500 for a total rebuild.
Bringing your dirt bike to a mechanic for an overhaul can cost as much as $2500 – $3000 due to labor costs.
