Skip to Content

6 Telltale Signs Your 2 Stroke Dirt Bike Needs A Top-end

Does your 2-stroke dirt bike need a new top-end? In this post I’ll show you the top 6 signs and symptoms that your 2 stroke dirt bike may need a top-end rebuild. I’ll also explain why each symptom can happen and how to fix it.

If you buy a 2-stroke dirt bike with a lot of hours or you ride a lot every month, chances are it will need the top-end replaced sooner rather than later.

Is a 2 stroke top-end hard to rebuild?

A top-end rebuild is fairly easy to do on a two stroke engine if you take your time and follow the manufacture’s service manual, 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 more than likely count on replacing at least the piston and ring, and possibly a cylinder and head if there is excessive wear.

Although, if none of the symptoms relate to your bike, there’s a good chance it won’t need a rebuild yet… And if you want to learn the simple steps to prevent a catastrophic engine failure that could leave you stranded click here.

It takes 50 kicks to start my bike when cold…

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 when properly jetted.

Certain bikes require more muscle and/or technique to get started, but a solid running engine should only take 1-3 kicks to start.

On the flip side, just because your bike takes forever to start doesn’t always indicate a worn top-end. More on that later…

Hard to start when hot

Starting your 2 stroke dirt bike should generally be easier once the engine is warm. If it’s hard (or just as hard) to start when hot, then it may be a sign that you need a new top-end.

A hard-starting engine when hot is generally because it’s running too rich. The rich jetting condition is caused by a worn top-end that can’t properly burn the air and fuel mixture.

My bike is gutless (has no power)

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.

How many hours on a 2 stroke top end?

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.

Do you ride trails?

Let’s take a 250cc 2 stroke trail bike for an example, such as a KTM 250XCW. You are an average trail rider that puts on 50 hours per year and typically ride in the low to mid RPM range, but occasionally like to open it up for that adrenaline rush.

If you do your regular maintenance, your 2 stroke carb jetting is tuned, and you run a proper pre-mix ratio, you could see up to 100+ hours of ride time before the compression starts to drop. I

n fact, it’s not uncommon to see over 200 hours on a 250 or 300cc 2 stroke trail bike that was ridden conservatively.

Struggling to stay in control on the trails? Click here to quickly learn the basic riding techniques to give you more control to prevent big crashes.

Don’t cold-seize your engine by doing this!

Starting up your 2 stroke dirt bike and immediately going for a ride is one of the worst things you can do to it.

If you want the top-end, as well as the bottom-end to last longer and save you money, you need to properly warm up your dirt bike. Click here to learn how.

CR80R Piston 2 6 Telltale Signs Your 2 Stroke Dirt Bike Needs A Top-end
Shattered CR80 Piston

This thing eats spark plugs (fouling)

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!

Look for scoring

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.

Quick Heat Expansion Causes Problems.

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”.

You should also note that forged pistons, as opposed to stock cast pistons, are slightly smaller in diameter and require slightly more warm-up time to expand; so take extra precaution.

Peaking In Through The Exhaust Port

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 tolerance.

Top end rebuild
Scored 2 stroke piston

My 2 stroke dirt 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.

Low compression?

To test your engine, you can get a you can get a compression tester (Amazon) to get an actual PSI number that will tell you what condition your top-end is in. For more info and a tutorial on how to check engine compression click here.

Common 2 stroke low compression symptoms are:

  • Hard to start
  • Spark plug fouling
  • Low power
  • Engine turns (kicks) over very easily

How Many Hours Before Rebuild?

How many hours does a top-end last in a 2-stroke engine? There’s a lot of variables, but once you start losing more than 10% of your compression is the best time to replace the piston and/or rings off you want to keep it at optimum performance. The number of hours it takes to reach that point will differ for each bike and rider.

The engine components only last so many revolutions before they start wearing out. This is why a 125cc 2-stroke that is ridden by an expert level motocross rider at high RPM consistently will wear out in less time than a 250cc bike that is ridden by a novice rider at a lower RPM on average.

This is why it’s difficult to determine the number of hours you should go before rebuilding the top-end. Going by the book, a 125 should be inspected and/or replaced every 5-10 hours of racing.

In the other hand, if you’re just casually trail riding and don’t care if you lose a little bit of power after a while, you could easily go 50-100+ hours before needing to replace engine parts.

What Parts Should Be Rebuilt?

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).

Common symptoms of bad piston rings on a 2 stroke dirt bike:

  • Low power
  • Hard to start
  • Plug fouling
  • Bogging
  • Sputtering

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.

To learn how to rebuild a top-end and how much it will cost read this.

Not all dirt bikes work the exact same

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.

If your bike won’t start, there’s a simple protocol to go through to figure out why. Is your bike getting fuel? Did you turn the gas on? Does it need a new spark plug? Is the jetting off?

How to make your top-end last longer with jetting

Whether it’s starting your 2 stroke hot or cold, getting the best throttle response, or preventing plug fouling, proper jetting is the key. For a step-by-step guide on how to get started jetting in just a few minutes click here.

bmane

Saturday 2nd of April 2022

bought a 1980 dt100 idles great but bogs when you give it throttle at idle. pulled the carb cleaned it about 5 time doesn't change. pulled the reeds out they look perfect and no major scratching on piston. now it will only start with starting fluid. the bike only has 1906 original miles. great spark. ill do a compression test tomorrow. any ideas?

Kelley Fager

Thursday 7th of April 2022

How did you clean it? On a bike that old, it may need some deep cleaning or the pilot jet replaced if it's too plugged up. Check out this article for my favorite way to clean any carb. Thanks for the comment, and let me know how it goes!

byrin

Monday 13th of December 2021

my b\ike is a 2 stroke 2020 crf 150rb with 50 hours on top end

Joe Beals

Friday 25th of June 2021

99 kx 250 just got cylinder back from milenium bc they had to replate it. The bike has lectron carb, vforce 3 reeds. Gnarly pipe power valves polished and my oil is clean. The bike starts second or third kick. But is so easy to kick. When looking through exhaust port i see all the cross hatches. But the brand new piston has some streaks on it vertically. Idk if im paranoid or if everything is fine. The bike rips. It is hesitant in the bwgining of gears but once i pop the clutch the monster comes out.

Kelley Fager

Saturday 26th of June 2021

Hard to say over text. Was the cylinder matched to the piston size? Have you tried tuning the carb at all? Could the head be warped from overheating in the past? Thanks for the comment, Joe.

Brent

Saturday 26th of September 2020

I have a 2001 Suzuki 250 2 stroke the guy I got it from said he drove it on the freeway a few times now it was locking up while driving an now the kick start doesn’t go down it’s stuck

Kelley Fager

Saturday 26th of September 2020

Sounds like it seized up. Two probable causes based on what you're saying would be a lack of 2 stroke oil in the fuel or the jetting was too lean. You'll want to take the top-end off to know for sure, unfortunately. Hope it's not too bad!

Emmanuel

Sunday 25th of August 2019

I have a 2007 yz85 and I was running all gears and I down shifted and was about to stop and it stalled on me I tried to kick it it wouldn’t start and I bumped it also wouldn’t start and I got home and checked the spark plug and it was really messes up and wasant tight on there I can take it off with my hand , does this mean my bike needs a rebuild or just a new spark plug ?

byrin

Monday 13th of December 2021

i gots da same itsuere it hepen ervy tine u ride i replace to new splug park