If you’re pulling on your lawn mower’s starter cord and getting no resistance, then it’s likely because the engine has no compression. So, what are the different causes of low or no compression in a lawn mower engine, and can it be fixed? Let’s start off by looking at how to diagnose and confirm small engine compression problems.
Why Does My Lawn Mower’s Engine Have No Compression? (The Short Answer)
Low compression on a small lawn mower engine is usually the result of air being able to escape from the cylinder head when it’s not supposed to. Two common problems of no compression are a broken head gasket or either the inlets or exhaust valves not operating correctly.
2 Possible Causes of a Lawn Mower Engine with No Compression
When you pull on your starter cord, you’re actually rotating the crankshaft and moving the piston up and down inside the engine. Once the piston starts to move, it cycles through the four strokes of combustion: induction, compression, power, and exhaust. A key part of this process is the engine holding and releasing air from within the cylinder at specific times.
So, if your engine can’t tightly hold the compressed air, then the four cycles can’t be completed. Why can the air escape in your lawn mower to cause no compression? Well, let’s take a look at the two most common causes of a lawn mower engine with no compression and find out.
The Head Gasket
An engine body consists of two main parts: the block and the cylinder head. Picture it as the engine block is a box, and the cylinder head is the box’s lid. To get to all the parts inside the engine, you lift the box lid, install the parts, then place the lid back onto the box.
Now, the head gasket is the only thing that seals the cylinder head to the block. As a result, if the head gasket breaks, air can escape from the top of the cylinder when it shouldn’t. Here are some common signs of a blown head gasket and what you should look for.
- Lack of Engine Power
- Overheating Engine
- Poor Performance
- Trouble Starting Your Engine
- Trouble with Stalling
- Whooshing Sound
- Oil Leaking from the Head Gasket
- Oil Smoke from the Muffler
- Air & Oil in the Coolant
- Coolant & Oil in the Cylinder Head
Inlet & Exhaust Valve
There are two valves inside the cylinder head. One is the inlet valve which allows air and fuel into the cylinder, and the other is the exhaust valve, which allows exhaust gasses out. So as the crankshaft rotates, these valves open and close in relation to the position of the piston. Well, this is the plan anyway. If you have no compression, then this might not be happening as it should.
If one of the valves is stuck open, there will be nothing to hold the air inside the piston during the various combustion strokes, leading to no compression. So, how do you tell if there is an issue with one of the valves? Well, if you remove the valve cover from the engine and expose the valves, you’ll be able to see how they operate.
To do this, remove the spark plug and the valve cover, then slowly pull the starter cord. Usually, I get somebody to give me a hand pulling the cord so that I can keep an eye on what the valves are doing. The valves should open and close as the rocker arms press against each of the valves.
You will probably find one of the valves sticking in the open position, causing your lawn mower to have no compression.
How to Fix a Lawn Mower Engine With No Compression (2 Solutions)
So, we have looked at what I believe are the two main small engine compression problems and symptoms. Now, how to get compression back in your cylinder? Let’s take a look at two possible solutions.
Replacing a Lawn Mower Head Gasket
Replacing a lawn mower head gasket is a lengthy process and requires you to remove a lot of different engine parts just to get to the cylinder head. Because of this, I have put together a list of steps you can follow that will give you a pretty good idea of what is required. So let’s take a look.
Steps to Replace a Lawn Mower Head Gasket
- Remove the spark plug cable from the spark plug.
- Remove the spark plug from the cylinder head.
- Remove the engine cover.
- Remove the inlet manifold from the cylinder head.
- Remove the exhaust manifold from the cylinder head.
- Remove the valve cover bolts.
- Pry the valve cover off the valve housing using a flathead screwdriver.
- Rotate the flywheel to loosen the rocker arms against the push rods.
- Loosen the locknuts on the rocker arms.
- Rotate the rocker arms clear of the push rods.
- Remove both push rods from the cylinder head.
- Remove all the bolts securing the cylinder head to the engine block.
- Remove the cylinder head.
- Remove the old head gasket.
- Clean off any gasket residue from the cylinder head and the engine block using a razor blade and spray contact cleaner.
- Mount the new gasket on the gasket mounting pins located on the engine block
- Reposition the cylinder head.
- Thread the mounting bolts into the cylinder and loosely tighten them in an alternating pattern.
- Tighten the bolts in the same pattern to the correct torque setting using a torque wrench.
- Insert both push rods.
- Reposition the rocker arms on the push rods and the valve caps.
- Lightly tighten the rocker arm adjustment nut.
- Rotate the flywheel until the piston is approximately a ¼ way past its top position.
- Insert a feeler gauge between the rocker arm and the valve cap (see owner manual for feeler gauge thickness).
- Adjust the rocker arm adjustment nut until the rocker arm to valve cap is set to the specified distance.
- Repeat rocker arm adjustment for both rocker arms.
- Tighten both locknuts on both rocker arms.
- Clean off sealant residue from both the valve housing and the valve cover.
- Apply rubber gasket sealant to the valve cover.
- Position the valve cover back onto the valve housing.
- Thread the valve cover bolts.
- Torque all the valve cover bolts to the required torque setting using a torque wrench.
- Reinstall the exhaust manifold & inlet manifold to the cylinder head.
- Reinstall the spark plug & spark plug cable.
- Reinstall the engine cover.
Tools & Parts Require to Replace a Lawn Mower Head Gasket
- Spark Plug Wrench or Socket
- Nut Driver
- Socket Wrench
- Razor Blade
- Spray Contact Cleaner
- Feeler Gauge
- Rubber Gasket Sealant
Freeing Up Stuck Inlet & Exhaust Valves
If you have already removed the cover from the valve, then you have already done the first part of the fix. Now it’s time to take a look at the stuck valve that is causing you the compression problem.
First, you’ll want to get the rocker arm out of the way. If you have an engine like a Honda engine, you’ll just be able to slide the pin out of the rocker arm without any tools. But if your rocker is held in place with an adjuster and lock nut, then you’ll need to loosen these off before you can get to the valve. A small wrench should work fine. Now it’s time for the stuck valve.
You’ll usually find that the valve is gummed up and stuck either fully open or part-way open. So, if you grab some spray lubricant like WD-40 and spray it into the hole where the valve is located, then try to pull the valve out of the engine, you should be able to free it up. A pair of pliers really come in handy as valves can be small and hard to get a good grip on.
So once the valve is pulled out, give it another spray with WD-40, then push the valve back in. Repeat this process until the spring on the valve starts pulling the valve out for you. Keep spraying WD-40, push the valve in, and let the spring pull the valve out repeatedly until you believe all the gum has worn away. That’s really all there is to it. Well, kind of.
If you find that the valve is totally stuck and you can’t pull it out, then you have to remove the cylinder head and approach the valve from the top. So if you follow the steps for replacing the head gasket, you’ll get to the top of the valves since they sit next to the piston in the cylinder head.
When you remove the head, spray the valves with WD-40, then tap them. Instead of trying to pull the valves, you’ll be trying to push them instead. This can be much easier if the valve is really stuck open. Once you get the valve moving, follow the same approach as before of spraying, pushing, and pulling until the spring takes over and the gum resistance is gone.
Finally, put everything back together, and the compression should be back to normal.
Tools and Parts to Free a Stuck Valve
- Socket Wrench to Remove the Valve Cover & Rocker Adjuster
- Rubber Gasket Sealant to Seal the Valve Cover Back On
- Plastic Mallet to Tap the Stuck Valve
- Feeler Gauge to Set the Rocker to Valve Cap Gap