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. If you need video help, check out “Replacing mower blade video”.
Most mower engines won’t start without blade attached.
Small Engine Types
There are two common engine configurations, the Vertical crankshaft and the Horizontal crankshaft layout. Vertical and Horizontal crankshaft is self explanatory. The orientation of the crank will naturally lend itself more suitable to one task over another.
It’s not possible to turn a Horizontal shaft engine vertically and simply call it a Vertical engine. These engines have been designed to operate in their particular configuration. For example, most small engine oil systems are splash lube, it relies on splash paddles hitting a puddle of oil at the base of the engine. You can see the problem here. But the differences go deeper than that, as we’ll see shortly.
Horizontal small engines are designed to suit many applications, unlike the vertical engine. The Horizontal engine is more common generally. They are best suited to driving power washers, water pumps, cement mixers, tillers, snow blowers etc. 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.
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 to the crankshaft 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 manufacturers employ a feature like Toro’s Spin Stop. They use a separate control lever to engage the blade. This 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, since they aren’t designed to have a blade fitted directly to the crankshaft.
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.
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.
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).
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.
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 Engine Needs A Rotating Mass
Flywheel Adds Mass
All small engines (vertical and horizontal) 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 weight in the form of a crankshaft, flywheel and of course the blade is factored when the engine is designed for a mower.
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 its momentum. Firing the plug now easily causes the engine to reverse track, aka kickback.
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 occurs when you pull starter cord to start the engine and the 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 more commonly associated with a broken shear-key.
You can read more about a shear-key here.
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, a mower engine is designed to run with a specific blade, by specific I mean not only correct length and width but also mass and lift.
As an engine approaches top dead center, and if 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 “Fitting mower shear-key video”, it covers the complete process from diagnosis to replacing or check out a post I wrote here “Replacing a shear-key”.