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Two-Stroke Engine Has No Compression: A Troubleshooting Guide


Causes of no compression in two-stroke engines.

A common issue of a two-stroke engine is losing compression, and there are a few different reasons why this occurs.

The most likely reason why a two-stroke engine has lost compression is an issue with the piston or piston rings. The piston is the part of the engine that creates the compression needed to run. If there is a crack in the piston, air can escape and cause a lack of compression.

This article will discuss the steps necessary to figure out why your two-stroke engine has no compression. We will also discuss the essential functions of a two-stroke engine to understand why compression is needed.

How 2 Stroke Engines Work

Two-stroke engines are popular in smaller machines because they provide a high amount of power with fewer parts than a four-stroke engine. This fewer amount of parts means that two-stroke engines are lighter, though they are also louder.

Two-stroke engines are a type of internal combustion engine that is actually very similar to four-stroke engines. In a two-stroke engine, however, the steps are combined. This allows the combustion process to happen in two steps (strokes) instead of four (source).

The main functions of internal combustion engines are:

  • Intake. Air and fuel mixture is drawn into the cylinder.
  • Compression. The piston compresses and the air/fuel mixture is put under pressure.
  • Ignition. The spark plugs fire and ignites the compressed mixture.
  • Exhaust. The excess fuel and debris is expelled from the engine.


If any of these functions are not working properly, your engine won’t start. Let’s discuss the lack of compression in a two-stroke engine.

Check Your Piston Rings

The piston rings sit on the outside of the piston. They are usually made of cast iron or steel and are treated with a solution to make them more durable. The piston rings’ main purpose is to seal the area between the piston and the cylinder to reduce air loss and increase compression in the cylinder below the piston (source).

Since the piston rings’ main job is to create compression in a two-stroke engine, they are usually the cause of a lack of compression. If your piston rings are cracked or worn, they might not be sealing properly as the piston moves. 

A cracked piston ring will allow air to escape from the cylinder and won’t let your engine create the pressure required to run. If your engine will start, but won’t hold compression or sputters and dies when you accelerate, there’s probably an issue with your piston rings.

To check your piston rings, you’ll need to disassemble the cylinder head. This includes removing the piston from the cylinder to check the piston rings. When you do this, be sure to cover the open cylinder to stop any debris from falling into the chamber.

This YouTube video may be helpful in understanding this process:

How to Install a 2 Stroke Piston Ring Quick Tip

Check the Piston Itself

The piston is the main moving component of your two-stroke engine. It is the part of the engine that moves up and down and turns the crank. The movement of the piston creates and releases pressure, also called compression. 

If the piston is cracked or broken, it may not be turning the crankshaft fully. It could also not be extending or withdrawing into the cylinder properly. Both of these issues could cause a loss of compression, and prevent the engine from running properly.

Again, the way to check the piston is to remove the cylinder head and piston from the cylinder. Be sure to cover the cylinder’s opening here as well.

Check Your Head Gasket

The head gasket lives between the engine block and the cylinder head. Head gaskets are usually very durable, and it takes a long time for them to wear and crack. It is possible, though, and if your two-stroke engine’s parts have been severely damaged, you may encounter a “blown head gasket” as it is commonly referred to by mechanics. 

A blown head gasket is a severely damaged or cracked gasket that is no longer doing its job. The head gasket’s main purpose is to create a seal between the engine block and the cylinder head. This seal contributes to the compression within the engine. 

As with the piston rings mentioned above, a blown head gasket will allow air, oil, and fuel to escape the main engine chamber. This will cause a loss of compression. 

Removing the cylinder head is the way to check the head gasket. If your head gasket is “blown,” you will notice right away. The metal piece will be broken or cracked and may appear rusty. If the gasket isn’t blown, it is still possible for it to be leaking.

Since the head gasket faces such a large amount of pressure from the engine, it’s possible for it to spring a leak. This is harder to determine than a blown gasket, and you may need to replace the head gasket just to rule this out as a possibility.

Check Your Reed Valve

Next, take a look at your reed valve. The reed valve is positioned in the air/fuel/oil intake pipe that leads from the carburetor into the crankcase, where the compression happens. The reed valve’s main function is only to allow the air and liquid coming from the carburetor to travel in one direction (source).

When the piston moves up, past the intake, the reed valve opens and allows the air/fuel/oil mixture to enter the chamber. When the piston moves down, the reed valve shuts so the same mixture cannot travel back into the intake.

If your reed valve is broken or cracked, it could allow the air/fuel/oil mixture to escape back through the intake. The piston’s movement would then not create the pressure it was supposed to, and you’d be left with a lack of compression.

Remove your reed valve and check the quality of it. If it seems to be in good condition, replace it, and ensure it is functioning properly. If it is cracked, replace it with a new one and try rerunning your two-stroke engine.

Call a Professional

After you’ve gone through the steps above, you will most likely have found the culprit of the two-stroke engine’s lack of compression. However, if the steps above did not solve your issue, it is probably time to call a professional.

There are other reasons why your two-stroke engine may not have compression, like other broken parts, clogged valves, or loose seals. Many of these things may be hard to diagnose with the naked eye. A professional mechanic will have all of the tools necessary to run the tests needed to find out the problems and how to solve them.

How To Test Compression

If you are wondering how to actually test compression on a two-stroke engine, watch this helpful YouTube video:

How to Compression Test 2 Stroke Engines

Summary

There are a few different reasons why your two-stroke engine has no compression. The most common cause for lack of compression comes from some sort of leak in the parts of the engine that seal in the pressure.

The main seals that create the pressure needed for the engine to run are the piston rings, the piston itself, the head gasket, and the reed valve. It is within these parts that you are most likely going to find the cause of your problem.

Teddy Henderson

Teddy is always fiddling with small engines, picking up thrown-out string trimmers or tearing apart dirt bikes. He shares what he learns along the way. Hopefully, you'll have less headaches than he has had by learning from his mistakes.

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