Mower Engine Valve Adjustment
So what is lawn mower engine valve adjustment? Valve lash is a precise specified gap between the valve tip and the rocker. As an engine wears, this valve lash (gap) gets larger and needs to be checked and adjusted.
- Starter cord hard to pull
- Hard starting
- Lack of power
- Rough running
- Oil leaks
- Noisy engine
What Are Valves?
Usually stamped on the valve cover or on the engine cover.
Valve Adjustment & Hard Starting
Excessive cylinder pressure causes the gasket to fail.
Valve Adjustment & No Start
A compression test will confirm if your engine is at the end of its life.
Engine Just Lacks Power
No power, hard on gas and its hard to start.
How To Adjust Lash
Remove the plug and turn off the gas if fuel tap fitted.
Remove the rocker cover, a rag will be needed as some oil will drip out.
The metal rocker cover will have a gasket, if not damaged may be reused, ideally it should be replaced.
Valves & Rockers
Rotate the engine by turning the blade, observe that both valve springs are being compressed in turn.
Note the push rods are in place and undamaged. These won't be fitted if your engine is over head cam (ohc), but all other info applies.
Push rods should be straight, its not uncommon for them to bend causing a no start condition. Replacing is a simple job. I have removed for demonstration purpose.
DO NOT REMOVE PUSH RODS TO ADJUST VALVES.
This is the exhaust valve, seen here adjacent to the muffler (exhaust).
Rotate the engine clockwise (as seen from above the engine) by turning the blade.
Keep turning until the intake valve spring is fully compressed (valve open). This is the first stroke of the 4 stroke sequence.
Insert a blunt object into the plug hole against the piston.
Turn clockwise again and watch as the blunt object is pushed out by the piston.
When fully out, it's known as Top Dead Centre (TDC) and is the 2nd stroke (compression).
Keep turning until the blunt object starts to go into the cylinder by approx. 1/4 inch. Note that both valve springs are unloaded. This is the beginning of the 3rd stroke (Power stroke). This is where we adjust valves.
Slip the gauge blade between the rocker arm and valve tip. Move the gauge forward and back and feel the resistance.
If you feel a nice amount of resistance, meaning, the gauge is snug but does move when you push on it - Your good, no need to adjust this valve. If you don't feel any resistance, start adjusting.
Here I am holding the adjuster nut (inner nut) with a wrench. The lock nut (outer nut) must be released before I can turn the adjuster.
Your engine valve set up may be different but the idea will be the same. In some cases the adjuster grub screw will be small Torx head, or other variation.
With the gauge in place move the adjusting nut in about a 1/8 turn and check gauge. In most cases only small adjustments are needed
Here I am turning adjuster small amounts while checking the gauge. You may have to use a wrench to adjust.
Rotate engine one turn 360° and check again, readjust if necessary.
Some valve setups can be tedious to get right, hang in there, it'll pay off.
Most of the time you can reuse the old gasket, but careful not to over tighten the cam cover, it causes them to leak.
If you need to replace them - gaskets other than cork can be replaced with a silicone gasket maker.
That wasn't so bad!
What does OHV mean on a lawn mower? OHV stands for overhead valve, it's an engine layout where the valves are positioned directly above the combustion chamber. This provides smoother running, more power and fuel efficiency.
Briggs & Stratton stuck exhaust valve? This can happen to mowers when they lay up over winter. Trapped moisture between the valve stem and sleeve turns to corrosion and causes them to stick. Removing the valve cover and taping them lightly with a piece of wood or rubber hammer will free them.