Briggs and Stratton Decompression Valve Problems: Symptoms and Fixes

Small engines like those used in lawnmowers are usually started manually using a recoil rope. Although most of them nowadays have an auto start button, the mechanism used to kick-start these engines, whether manually or automatically, basically applies the decompression mechanism to ease the starting procedure. 

The decompression valve is one of the most critical components required for these engines to start. In this article, we shall explain how a decompression valve works, decompression valve problems and symptoms, and how to fix those problems whenever they arise in a Briggs and Stratton (B & S) engine.

Decompression Valve - How it works

A decompression valve opens the combustion system to release some of the compression that builds up in the combustion chamber when the engine is being started. The released compression reduces the power needed to start the engine, making it easier to kick-start the engine using the rope or start button.

The compression and decompression mechanism eases the starting of the B & S engine by allowing the piston (s) to move up and down freely until they gain sufficient momentum to engage the ignition. Normally, the engine builds up a lot of pressure in the chamber to enable power release when the fuel-air mixture is ignited. The spark ignition is what moves the piston (s) up and down to release engine power.

However, when starting the engine, the pressure (compression) build-up is so much that it is practically impossible to move the piston (s) manually up and down the cylinder without engine power. As such, the compression build-up must be released to let the start-up mechanism spin up without working against the engine thrust. This is where the decompression valve comes into play.

When you press the start button or pull the recoil rope of the B & S engine, the decompression valve opens the exhaust system to allow a certain percentage of the compression to escape. This enables the engine to turn easily and complete a cycle that then cranks it into life. The procedure works as follows:

• The piston is positioned at TDC. As you pull the recoil rope or press the start button, the manual force (or starter force) pushes the piston down.
• As the piston moves down, the intake valve opens to allow the air-fuel mixture to be
• sucked into the cylinder. The exhaust valve closes simultaneously.
• When the piston reaches the BTC, it begins to rise. The intake valve closes and the air-fuel mixture begins to be compressed. At this point, the decompression valve opens to release some compression to enable the piston to overcome the compression force and continue rising.
• The piston reaches the TDC. The spark plug releases the electric spark that ignites the
• compressed air-fuel mixture which explodes.
• The explosion force rapidly pushes the piston down to the BDC releasing usable engine power which cranks the engine to life.
• As the piston rises to the TDC, the exhaust valve opens to let out exhaust gases.
• The engine is now running on its own and the normal valve opening/closing and piston movement kicks in under the engine power.

Once the engine starts, the compression valve closes and the normal combustion process controlled by the engine motion continues. 

There are two types of decompression valves, manual and automatic. An automatic decompression valve is activated (opened) automatically when the engine is off. As soon as the engine cranks into life, an electric pulse from the crank closes the valve to allow normal compression to run the engine.

On the other hand, a manual decompression valve is opened manually before starting the engine. Once the engine kicks into life, the valve resets to the default closed position. It should be noted that failure to open the valve prevents the piston from moving.

The decompression valve reduces the compression force to allow the engine to be started on far less power provided by the starter motor or the recoil rope.

Problems

The decompression valve is located at the exhaust valve and allows a smooth movement of the piston to kick-start the engine. If the decompression valve fails, it becomes difficult for the engine to start as the piston is unable to overcome the initial compression force in the cylinder.

• A faulty decompression valve does not release the required amount of compression to enable the engine to start. The major problems associated with the valve include:
• Failure by the decompression valve to release some compression from the cylinder to enable manual piston movement.
• Decompression valve releasing compression when it should not.

When these issues arise, it becomes very difficult for the engine to be started.

Symptoms

• It becomes very difficult to pull the recoil rope to start the engine.
• The rope keeps breaking every time it is pulled to start the engine.
• The pull start keeps on breaking
• (For auto button start) the electric starter keeps burning out.

Causes

• Worn-out or damaged valve

When the decompression valve becomes damaged or worn out, it is unable to seal or unseal the cylinder properly. This means the compression that is required to be released is not released, or, the amount of compression required to remain in the chamber for the engine to start is not maintained. Thus, the required compression balance is not maintained; hence, the engine will not start.

• Incorrect Adjustments

The decompression valve has to be adjusted to the correct position for it to function properly. The valve position determines the clearance space that it moves to for compression to be released. If the adjustment is too small or too big, the valve may not open or seal the opening optimally as required. This, therefore, means the engine will have difficulties cranking into life. 

• Problem with the head gasket

Sometimes, the head gasket could be incorrectly installed or faulty. This means that the head gasket is allowing some compression to escape or leak out. As such, when you open the decompression valve, more compression is released, leaving behind a compression that is not enough to start the engine.

• Oil and carbon residue

If you have been using the decompression valve to stop the engine by reducing compression, there is the possibility that oil and carbon residues have built up in the valve opening. This happens because when you open the decompression valve while the engine is still running, the open decompression valve becomes the exhaust valve where the un-burnt gases are forced. As the engine dies, the exhaust gases stick and solidify in the valve opening. Thus, when you open the valve to start the engine, the valve gets stuck by these residues and remains open even when the engine starts, reducing compression and forcing the engine to die.

• Broken/Faulty Depression-release Spring

When the decompression lever is engaged to open the decompression valve, a spring on the cylinder head holds the decompression valve in position until the engine starts. Once the engine has started, the engine power activates the spring to release the valve so that it shuts the opening to seal compression. However, if the spring is faulty, it remains stuck in the open position, thus preventing the engine from fully kicking into life due to compression loss.

Fixing

One of the easiest and cheapest methods of fixing decompression valve problems is to have routine inspection and maintenance of the B & S engine and parts. This will ensure the entire engine, including the decompression valve is in optimal state for efficient performance.

Fix and set the decompression valve into the correct clearance position to ensure the correct amount of compression is allowed to escape during the start-up procedure.

Replace the decompression valve if it is found to be faulty. The B & S decompression valve is easily accessible and once it is confirmed that it is faulty, remove it and replace it with a functional one.

If the problem is located at the head gasket, remove the head cylinder and replace the gasket. This should seal any leakage that was leading to excess loss of compression.

If the valve or the valve opening is clogged with oil, dirt, or soot, remove the decompression valve and thoroughly wipe/clean the opening to remove all the dirt and grease build-up. Once this is done, screw the valve back into position.

You could also remove the cylinder head dome and clean it thoroughly with WD40 and a ball of clean steel wool. Check the decompression valve area to ensure the valve is opening and closing the vent easily and properly. If it is not working as expected, replace the valve or the head cylinder if it is the one with issues.

Once these issues have been sorted out, the decompression valve should be able to function properly and release the right amount of compression to make it easy to start the Briggs & Stratton engine. The operator must ensure he pulls the decompression valve open before using the auto start button or the recoil rope to start the engine. 

Remember that maintenance of the decompression system is critical as the decompression system is the basic function that enables the engine to be cranked into life.