There are many reasons why your lawn mower won’t turn over unless the spark plug is out, but there is one that stands out above the rest. If you are pulling the starter cord or using the electric starter and the engine won’t turn over, then it means there’s a mechanical fault.
So, if removing the spark plug from your mower frees up the engine, then the problem has been identified. Now all you need to do is work out what the diagnosis means. Let’s take a look at what you have discovered and how to fix it.
Why Won’t Your Mower Turn Over Unless the Spark Plug is Out? (The Short Answer)
When you crank an engine, the intake and exhaust valves open and close in relation to the position of the engine’s stroke. When the valves do not open and close correctly, they form a vacuum and lock up the engine. When you remove the spark plug, the vacuum is released, and the engine is able to turn over.
What are Lawn Mower Engine Valves?
To better understand what valves do and how they can malfunction, you first need to know how they work. The intake valve allows fuel and air into the cylinder head, which is then compressed for ignition. The exhaust valve allows the exhaust gases out. As your lawn mower engine turns over, these valves open and close in a specific sequence known as the compression sequence. Here are the different stages of a compression sequence of a four-stroke engine.
- Intake – The intake valve opens to pull air and fuel into the cylinder head (combustion chamber). The exhaust valve remains closed.
- Compression – The intake valve closes to seal the cylinder head along with the closed exhaust valve. As the piston rises in the cylinder, the air/fuel inside the cylinder is compressed.
- Power – With both valves closed and the piston at the top dead center, the spark plug ignites the compressed air/fuel mixture and forces the piston down. TDC – piston at its highest position.
- Exhaust – As the piston travels down the cylinder, the exhaust valve opens to allow the exhaust gases to discharge through the exhaust pipe.
5 Possible Reasons Your Engine is Hard to Turn Over with the Spark Plug In
As you can see from the four stages of the compression sequence, there is a lot of movement of the intake and exhaust valves, and plenty of things can go wrong. So, if your mower won’t turn over unless the spark plug is out, these are the areas of the engine you’ll need to investigate.
Crankshaft Damage
Inside the engine, you’ll find the crankshaft along with two protrusions. These protrusions force the two push rods to move in and out, opening and closing the valves. If these protrusions get worn down for any reason, the push rods will not move as needed.
Even though this is a possible reason why your mower won’t turn over unless the spark plug is out, it is pretty unlikely since the crankshaft is extremely tough. It’s more likely that the problem stems from one of the weaker parts of the engine.
That being said, if you have a very old engine with plenty of use, a worn-out crankshaft might be the cause. So, the best course of action is to check the rest of the valve system first and eliminate other potential problems. If everything else works just fine, your crankshaft could be damaged.
Push Rod Damage
Push rods are thin metal rods that transfer the movement of the crankshaft to the rocker arms. Unfortunately, even though the push rods are made of metal, they can bend under too much stress, say from something like a jammed rocker arm. The quickest way to tell your push rods are damaged is to remove them and check if they are straight.
Rocker Arm Adjustment
The rocker arms sit between the push rod and the valves and are small metal plates that rock back and forth as the crankshaft moves the push rods in and out.
First, the rocker arms need to be able to move freely, and second, they need to move a specific amount. This is where the rocker arm adjustment comes in. With the engine set to TDC, the gap between the rocker arm and the valve must be set to a specific gap, around 0.080 inches. If the rocker’s arm is adjusted incorrectly, both valves can potentially remain closed, trapping the air inside the cylinder head and making the engine hard to turn over with the spark plug in place.
To check the adjustment, you need to research the gap distance for your specific engine and measure the gap with a feeler gauge when the engine is at TDC.
Stuck Valves
So how does a valve actually work? Well, a valve opens by being pushed into the cylinder head by the movement of the rocker arm, push rod, and crankshaft. The valve then closes by means of a spring, returning the valve to the closed position.
If the opening that the valves move through becomes dirty and clogged, the valve can become stuck. This results in the valve either staying stuck open or closed. If your mower won’t turn over unless the spark plug is out, it could mean the valves are not opening correctly.
An easy test is to push the valves into the engine yourself manually. If the valves are stuck and won’t move, you’ve discovered the root of the problem. However, if the valves are stuck in the open position, then you have a lawn mower engine with no compression. This problem has a different set of symptoms, like your lawn mower engine won’t fire up.
Push Rod Not Installed Correctly
Finally, if the push rod isn’t seated on the rocker arm correctly, then the push rod will just move in and out without moving the valves. It’s not uncommon for the push rod to slip off the rocker arm if the valves are not adjusted correctly.
To check this, remove the valve cover and see if the push rod is out of place. If you discover this is the problem, then it will pinpoint why your mower won’t turn over unless the spark plug is out.
How to Fix This Issue (3 Ideas)
Now you know all there is to know about your lawn mower’s valves and the problems they can face. So let’s take a look at three fixes that should get your lawn mower up and running again.
Replacing Damaged Push Rods
While all valve systems work the same way, manufacturers like to use slightly different designs. Here are the steps for replacing damaged push rods on a Briggs & Stratton engine which you can adapt for most engines.
- Remove the valve cover
- Remove the push rod lock nut
- Slide out the old push rod
- Insert the new push rod
- Reattach the push rod lock nut
- Clean off the old valve cover sealant
- Apply silicone gasket sealant to the valve cover
- Replace the valve cover
Tools & Materials to Replace a Push Rod
- Screwdriver
- Nutdriver
- Replacement Push Rod
- Silicone Gasket Sealant
Freeing Stuck Valves
Again, this is the method I use for my lawn mower’s Briggs & Straton engine, which you can easily apply to your own lawn mower. Your engine might be slightly different, but these steps will give you an idea of what you need to do.
- Remove the spark plug
- Remove the valve cover
- Remove the push rod lock nut
- Slide out the push rod
- Rotate the rocker arm off the valve
- Spray the valve with a penetrating spray
- Allow the spray to soak in
- Push the valve into the engine
- Pull the valve back out with a pair of pliers
- Repeat pushing, pulling, and spraying until the valve is able to freely return to the closed position under the power of the valve spring
- Reposition the rocker arm
- Insert the push rod
- Reattach the push rod lock nut
- Clean off the old valve cover sealant
- Apply silicone gasket sealant to the valve cover
- Replace the valve cover
Before you complete this repair, inspect the rocker arm and the push rod for any potential damage. Also, it’s a good idea to use a product like Sea Foam to clean the inside of the cylinder head. While you could take the engine apart for this repair, I prefer this method. It does a pretty good job and saves a lot of work. If you’re not too experienced with engines, then this is definitely the best approach.
Tools & Materials to Free a Stuck Valve
- Screwdriver
- Nutdriver
- Replacement Push Rod
- Silicone Gasket Sealant
Valve Adjustment
The first step before you tackle your lawn mower’s valve adjustment is to research the valve gap of your specific engine. Usually, this information will be in either your lawn mower user manual or the service manual. If you don’t have your manuals, then you can look online.
Just search your specific engine model number and look for the valve gap measurement for the intake and exhaust valves. Once you have the information, you are ready to tackle the repair. Here are the steps you’ll need to follow.
- Remove the spark plug
- Remove the valve cover
- Manually turn the engine so that the piston is at the top dead center
- Insert your feeler gauge between the rocker arm and the valve cap
- Measure the gap
- Loosen the rocker arm adjustment lock nut
- Either tighten or loosen the adjustment nut of the rocker’s arm to set the valve gap
- Once the gap is set, tighten the rocker arm adjustment lock nut
- Remove the feeler gauge
- Clean off the old valve cover sealant
- Apply silicone gasket sealant to the valve cover
- Replace the valve cover
- Install the spark plug
Tools & Materials to Adjust the Intake and Exhaust Valves
- Screwdriver
- Nutdriver
- Feeler Gauge
- Silicone Gasket Sealant
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