Can You Run Mower Without Blade?

Many people ask me about starting their mower engine without the blade, so much so, I wrote a post explaining all.

Many small engines are designed specifically for mowers and as such require the extra mass of a moving blade to help rotate the crankshaft on the compression stroke past top dead center (TDC). Starting such an engine without the blade may cause personal injury and damage the engine.

In this post you’ll learn why most mower engines require a blade to run correctly.

Honda engine

Honda Engine

Small Engine Types?

Many regular small engines are designed to suit many applications from driving a power washer, water pump, turning a cement mixer or driving a tiller.

These type engines are what’s known as horizontal crankshaft engines. They are not designed specifically for any one single purpose and as such will run just as well with or without an attachment on the crankshaft.

water pump

Horizontal engine

Mower engines on the other hand are a little different. They are known as vertical shaft engines. Vertical shaft engines by their layout are perfectly adapted to sitting on top of a mower deck with a blade fastened underneath.

Most of these type engines are designed to have a blade fitted, meaning, the mass of the rotating blade is factored into the overall design of the motor.

I say most mowers because some employ a feature like Toro’s Spin Stop. They use a separate control lever to engage the blade. The feature allows the engine to run without the blade spinning, useful for propelling the mower across graveled areas.

Higher end Honda mowers also sport this feature. If your mower has this feature then your engine will comfortably run without a blade attached.

Likewise tractor mower engines will run happily without blades attached. 

Small engine

Blade spins at 200 mph

Why Does Mower Need Blade To Start

Before explaining why a mower requires a blade, it’s helpful to know a little about the four strokes of an engine cycle. Once understood, the purpose of sufficient mass to rotate the engine becomes clear.

Mower cylinder head

1 Induction

Stroke 1 Induction

As the piston travels down the cylinder the intake valve opens. Gas and air mixture enter the cylinder. As the piston nears the bottom of the cylinder the intake valve closes.

Mower cylinder head

2 Compression

Stroke 2 Compression

The piston meets resistance as it travels back up the cylinder, all valves are closed. Gas and air mixture is forced towards the combustion chamber at the top of the cylinder by the piston. (It’s momentum, helped by the mass of rotating crankshaft, flywheel and blade that powers this stroke).

Mower cylinder head

3 Power

Stroke 3 Power

As the piston passes the very top of the cylinder travel (TDC), the plug fires and the explosion drives the piston downwards towards the bottom of the cylinder. This of course also drives the blade which cuts the grass.

Mower cylinder head

4 Exhaust

Stroke 4 Exhaust

This is the last of the four strokes of a complete cycle. The piston begins to travel back up the cylinder and the exhaust valve opens to allow spent gases exit through the muffler. As the piston reaches the top of the cylinder, the exhaust valve closes and the whole cycle begins again.

Mower flywheel

Flywheel Adds Mass

Mower Engine Needs A Rotating Mass

All small engines require a rotating mass to drive the piston when it’s not under power. This is especially true on the compression stroke, here the piston meets lots of resistance as it squashes the fuel mix into the combustion chamber.

To help the engine achieve sufficient rotational momentum, it employs some weight in the form of a crankshaft, flywheel and of course the blade. If a moving object has sufficient mass (weight), it will carry momentum.

Without the required mass rotating at a sufficient speed (mower blade spins at over 200 mph at the tip) the resistance the piston meets on the compression stroke would simply overcome the rotational force on the crankshaft and the engine loses momentum.

Firing the plug now easily causes the engine to reverse track, aka kickback.

Mower blades

Spinning Blade Adds Mass

Running Mower Without Blade Is Dangerous

Attempting to start a mower without a blade is dangerous, the lack of crankshaft momentum causes the engine to kickback.

Kickback is when you pull start the engine and the pull cord is snapped from your hand sharply.

The cord often whips you as it snaps back and sometimes breaks the pull handle.

This can be a painful experience and is associated with a broken shear-key.

Mower without blade fitted

Bladeless mower

Running Mower Without Blade Risks Engine Damage

Running the mower engine without the blade risks damaging the crankshaft or shear key.

Blades aren’t universal, the correct blade is designed not only to be the right length and width but also the correct weight.

The problem is, as the engine approaches top dead center, it isn’t carrying sufficient momentum. As the plug fires it causes the crankshaft to rotate in the wrong direction snapping the cord from your hand as it does so.

If you need video help, check out the video library, it covers the complete process from diagnoses to replacing or check out a post I wrote “Replacing a shear-key”.

Shear-key

Broken Shear-key

Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.