How To Test Compression On A Dirt Bike – Fix Low Compression

Does your dirt bike have low compression? What does it mean when the engine compression is low, and how do I check it? If you’re asking these questions, then you found the right article.

We’ll look at the when and why you should test engine compression, what happens when you don’t, and how to check it.

testing low compression on dirt bike 120 psi
Is your dirt bike compression low? What does it mean?

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?

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. 

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.

4 Stroke Compression Complications

While a 4 stroke dirt bike has piston rings that need to be replaced at certain intervals, the cylinder head is more complex. 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.

4 Stroke Dirt Bike Compression Test
Checking compression on a 4 stroke is a little more complicated.

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 symptoms that a 2 stroke dirt bike has when the engine compression is low

  • Hard to start
  • Kick-starter is easy to kick over
  • Low on power
  • Bike is bogging
  • Spark plug fouling
  • Doesn’t idle well

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. 

Minimum 2 Stroke Compression Chart

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 StrokeEngine sizeMinimum Compression PSI #
50cc 120 PSI
65cc120 PSI
85cc130 PSI
125cc140 PSI
250cc170 PSI
500cc140 PSI
Dirt Bike Minimum Compression Chart

A 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:

  1. 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)
  2. Remove the spark plug from the cylinder head.
  3. Install the compression tester with the correct size/thread attachment.
  4. Hold the throttle wide open (twist the throttle all the way back).
  5. 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. Otherwise, Motion Pro makes a high quality tester if you plan on using it for many years to come.

If you want to read more then learn how much you should spend in this list of top 5 compression testers for dirt bikes (article coming soon!).

How To Fix Low Compression On A 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.

Kelley Fager

Kelley started riding a Honda 50 at the age of 6 years old. The passion for dirt bikes started there and grown into a lifelong pastime of riding and learning how they work. Motocross Hideout is the result of sharing his past, present, and future knowledge and experience.

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