Does your dirt bike have low compression? Whether you have a 2 stroke or 4 stroke dirt bike that feels like the compression might be getting low, you’re in the right place!
In this article, I’ll show you the most common symptoms of low compression for a 2 stroke and 4 stroke engine, what the minimum compression test numbers should be, how to test the compression, as well as what causes this problem and how to fix it.
What is engine compression?
An engine, 2 stroke or 4 stroke, is basically a pump. It takes in air and fuel (gas) and compresses them under extreme pressure to help ignite and combust the mixture that keeps the engine rotating or running.
If the compression is too low then the mixture will not completely burn. For common low compression symptoms continue reading.
What causes low compression in a 2 stroke engine?
Compression is produced in a 2 stroke by the piston and rings with a specific size tolerance. The rings seal to the cylinder wall and keep the pressure inside of the combustion chamber.
If the piston or rings are worn beyond the manufacture tolerance then it will not maintain proper compression and symptoms will follow.
A bad or blown head gasket can also cause a loss of compression. A bad head gasket will allow the compressed air-fuel mixture to escape out of the cylinder or let coolant into it.
This is why it’s important to keep your 2 stroke top end regularly maintained by replacing the rings and/or piston when the compression starts going down, as well as not letting it get overheated.
Why the cause of 4 stroke engine compression is more complicated
While a 4 stroke dirt bike has piston rings that need to be replaced at certain intervals, the cylinder head is more complex. Simply put, there are more parts that can wear out or fail that can cause low or loose compression.
The intake and exhaust valves will eventually wear out or burn away and not seal. Worn valves will result in a loss of engine compression.
2 Stroke Low Compression Symptoms
Before you even check the compression on your dirt bike, there are a number of signs and symptoms that will tell you that the engine is getting worn out and needs attention.
Here are the most common 2 stroke low compression symptoms on a dirt bike:
Hard to start
When the compression gets low, the engine will be harder to start because it can’t efficiently ignite the air fuel mixture. It will usually run rich and be harder to start when the engine is hot.
Pro tip: you can try holding the throttle wide open when hot starting and then let off the throttle once it starts.
Kick-starter is easy to kick over
Since your dirt bike engine is basically a pump, it needs to compress the air that goes into it. This means that as the piston gets closer to the cylinder head, the compression will increase and make the kick starter harder to push down.
When the engine has low compression, less air is compressing in the engine because it’s leaking out, resulting in the kickstart being easier to kick over.
Low on power
Even if you tune the jetting for proper air-fuel mixture, you’re going to have less overall power when the compression is low. This will usually make the most difference to your low-end and midrange torque.
Bike is bogging
When you lose compression, this may cause bogging because the air and fuel isn’t properly burning. Whether it’s too much air or too much fuel, you may feel a “bog” or hesitation when you try to accelerate.
Spark plug fouling
Another common symptom of low compression is constantly fouling spark plugs. They usually show up as wet and black. This is because the fuel isn’t being completely ignited, so it just stays in the cylinder and basically “rains” on the spark plug until it quenches the fire (spark).
Doesn’t idle well
Since the air-fuel mixture usually goes rich when the compression drops, this often results in the idle dropping as well. Eventually it may not idle at all because it’s running so rich from all the fuel not being burned.
4 Stroke Low Compression Symptoms
A 4 stroke dirt bike will have similar signs when the compression is low, but there’s a couple of different symptoms than a 2 stroke shows. With that said, checking the compression on a 4 stroke isn’t as effective testing a 2 stroke compression.
Since a 4 stroke has valves and other parts that a 2 stroke engine doesn’t have, the rings aren’t the only area where the compressed fuel and air can leak out.
For a more accurate test of a 4 stroke engine, you should do a Leak-down Test. It shows you exactly where the engine isn’t perfectly sealed, costing you power and potentially a future repair.
Here are the top symptoms of low compression in a 4 stroke engine:
- Hard to start
- Kick-starter is easy to kick over
- Backfiring/popping on deceleration
- Running rough
- Complete loss in power
Minimum Compression For A 2 Stroke Engine
Engines need a certain amount of compression to be able to start and run, but what is the minimum compression my dirt bike needs? This is harder to determine since different bikes have a different factory compression number.
If you upgrade to a high compression piston or head then the number will be higher. Your OEM manual may have a minimum PSI specification.
How much compression should a 2 stroke dirt bike have?
With that said, here is a minimum 2 stroke compression PSI that you can follow as a general guideline, but are not absolute numbers for every dirt bike:
|2 Stroke Engine size||Minimum Compression PSI #|
A small 50cc and 65cc 2 stroke dirt bike should have a minimum compression of 120 PSI to run properly. Good compression is about 150 PSI.
An 85cc 2 stroke dirt bike should have a minimum compression of 130 PSI to run well. Good compression is 150-160 PSI.
A 125cc 2-stroke dirt bike should have at least 140 PSI and about 180 with a fresh top-end.
A 250cc 2 stroke should have 170 PSI or more and 210 PSI when new.
A 500cc 2 stroke should have at least 140 PSI of compression and 170 PSI for good performance.
How To Test Compression On A 2 Stroke Dirt Bike
Checking the compression on a 2 stroke is actually pretty easy and doesn’t take long. Here are the 5 simple steps to test the compression on your 2 stroke dirt bike:
- Remove the seat and gas tank (some dirt bikes have enough room under the tank and don’t require you to remove it. I would continue the next steps unless there is not enough room to work with)
- Remove the spark plug from the cylinder head.
- Install the compression tester with the correct size/thread attachment.
- Hold the throttle wide open (twist the throttle all the way back).
- Kick the kick-starter over quickly with 5 strong kicks (only test on a cold engine that hasn’t been run in the past 6 hours). Check the compression PSI number of the tester gauge.
It’s really that simple to test the compression on your dirt bike.
Electric start dirt bikes are basically the same process. Instead of kicking the bike over 5 times, just hold the starter button for 5 seconds (make sure the battery is fully charged so it will properly crank the engine to get the most accurate reading).
Choosing The Best Compression Tester
Now that you know how easy it is to keep an eye on the life of your 2 stroke engine is, you need to decide which tool is best for you. If you’re just looking for a good compression tester that will get the job done without spending a lot of money then this is the one for you (Amazon).
Otherwise, if you’re a mechanic then I recommend getting a high quality tester kit (Amazon) if you plan on using it for many years to come.
How To Fix Low Compression On A 2 stroke Dirt Bike
The main reason for low compression in a 2 stroke dirt bike engine is a worn top-end. A general top-end rebuild on a dirt bike is replacing the piston and/or just the piston rings(s).
However, the cylinder may need to be re-honed or re-plated if it is worn out of spec or has physical scratches on it. A 4 stroke top-end rebuild may include new valves and valve seals, as well as a new timing chain for maximum reliability.
Is rebuilding a 2 stroke top end hard? How much does it cost? Learn how to rebuild your 2 stroke here.
How to fix low compression on a 4 stroke dirt bike
Depending on what’s causing the low compression, there may need to be more parts replaced if you have a 4 stroke dirt bike engine.
The most likely ways to fix low compression on a 4 stroke dirt bike are:
- New piston rings
- New piston & rings
- Complete new cylinder and piston
- New valves
How to prevent a major engine failure
If you’ve ever had your bike die on you or not start while riding then you know how much of a pain it is to push it back to the nearest road or campsite. There are a few common mistakes that cause a dirt bike to blow up, but I want to give you a free guide so that you can help prevent that from happening – click here to learn how to prevent a catastrophic failure.
Sunday 23rd of April 2023
I was also wondering if it was possible to have good compression despite a scratched piston. How long can the bike run if it has good compression despite a piston scratch?
Sunday 23rd of April 2023
Hello, I have a 1994 suzuki rm 80cc, it is hard to start but only when cold, when warm it starts fine, my compression is at 138 psi. Is this normal?
Monday 24th of April 2023
That sounds like pretty good compression. If it's hard starting when cold then it might just be the jetting. Check out my article on tuning the air screw here. As for a scratched piston, it really depends on how bad it is and where. If you're properly warming the engine up (not too fast) before riding hard, it might last longer. It's hard to tell for sure over text on the internet.
Monday 10th of April 2023
Great article. Racing RGV 250, mine has 106 psi bogs down when the power valves open.
Tuesday 11th of April 2023
Hey Richard, that's a sweet bike! I'm not too familiar with that specific engine, but 106 sounds a little low. Thanks for reading!
Monday 27th of February 2023
My grandson has a 2012 kx450 When it’s cold out it won’t start kicking it 100 times We heat the jug and it fires right up 2 kicks What can be the problem?
Monday 27th of February 2023
Hey Peter, has it always done this? When you say "cold out", what's the temp?
Thursday 19th of January 2023
My other yz250 2 stroke was 160 psi and but my other yz250 is 150 psi is this the reason its not working my bike? I may have a little scratch since before but it was working now after I open and close again didn’t work.
Thursday 19th of January 2023
150 is within 10% of 160, so I wouldn't think that would be the main cause. Has anything been done to the bike recently, Yussuf?