Mower Engine Valve Adjustment

Checking and adjusting valve lash is usually forgotten about, even by the pros. Your mower engine valve lash is important, when the lash is out it puts an unnecessary load on other components.

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. 

Symptoms include:

  • Starter cord hard to pull
  • Hard starting 
  • Lack of power
  • Rough running
  • Oil leaks
  • Noisy engine
In this guide we'll check and adjust the valves on a walk behind mower, but the same principles apply to tractor mower engines, zero turns, snow blowers etc.

Checking and adjusting valve lash isn't difficult, but there is a very particular procedure to follow. If you need video help, check out "Valve lash adjusting video", it walks you through the whole process step by step, from setting the engine to checking and adjusting lash.

What Are Valves?

All four stroke single cylinder engines have valves, at least 2 per cylinder which open and close in a sequenced cycle. An engine breathes it inhales an air/gas mix through the inlet valve and exhales the spent gasses through the exhaust (muffler) valve.

 

Most valves are driven by push-rods and or camshaft, which are all connected and timed to the crankshaft. The push-rods open the valves by pushing on the rockers which in turn push on the valve tip. The clearance between the valve tip and rocker is known as valve lash. 

 

Most but not all engines will have adjustable lash, some engines are set from factory and don't provide for adjustment. (Kawasaki twin cylinder tractor engines).

 

If you have an ohv engine, it's most likely you have valves that need adjusting.

mower carburetor gumming

OHV

Usually stamped on the valve cover or on the engine cover.

Hard To Pull Starter Cord

A hard to pull starter cord is a common symptom associated with excessive valve lash. The reason the starter cord can be hard to pull is because of excessive cylinder compression.

 

Most mowers will have a compression release mechanism, its job is to open the valves a little early, releasing cylinder pressure. This makes pulling the starter cord less physically demanding.

 

When the engine hits 350 rpm, the plug fires and the engine starts, the release mechanism is then deactivated by a centrifugal flyweight.

 

A faulty mechanism will cause a hard to pull starter cord but is a much less likely cause than excessive valve lash.

Other reasons the starter cord is hard to pull:

 

 

Blade obstruction – broken branches, dry grass etc.

Faulty pull assembly

Hydro-locked cylinder – faulty carburetor needle

Faulty flywheel brake

Broken flywheel key

Internal engine damage – bent crankshaft

Valve Adjustment & Hard Starting

Hard starting isn't always caused by excessive valve lash but it should be checked once per season. When the valve clearance is out of spec, the valve train timing is out.

 

The whole valve train is running behind and this can cause other issues such as blown head gaskets and oil leaks.

 

Mower head gasket

Gasket

Excessive cylinder pressure causes the gasket to fail.

Valve Adjustment & No Start

No starting may be caused by poor valve lash, damage or wear to valve train components. A sticking valve is a common complaint on engines that lay up for long periods. The symptoms will be a no start and no compression. 

 

If your engine is lacking compression, suspect a sticking valve. To fix a sticking valve - use a screwdriver to gently lever the spring into the released position. 

 

mower engine test

Test

A compression test will confirm if your engine is at the end of its life.

Engine Just Lacks Power

Excessive lash will cause your engine to under perform and use more gas. Depending on how bad it is, you may also hear a tapping noise. If ignored it will cause the engine to prematurely wear out.

 

The engine isn't breathing correctly, its always off balance and may sound rough. A simple case of adjusting will have it sounding sweet again.

mowers

No Oomph

No power, hard on gas and its hard to start.

How To Adjust Lash

If you have an ohv engine and are experiencing any of the symptoms, then yes it's worth checking. In this guide we will be checking and adjusting the lash (gap) between the valve tip and the rocker arm.

 

Over head valve single cylinder engines have two valves and two rocker arms. The Intake valve opens when the rocker arm pushes on it, this allows fuel and air into the cylinder. After combustion the exhaust valve opens to allow gases out, this operation is sequenced.

 

An inexpensive tool called a Feeler gauge is needed to set the valve lash.

 

Each engine manufacturer will have a different spec for their engines, intake and exhaust measurements will sometimes differ also.

 

Check your valve lash spec with your engine maker.

 

Briggs and Stratton specification

 

Honda specification

 

Kohler specification

 

Kawasaki specification

 

Mower engine cover

Ohv

How do you know if you have an OHV engine? In most cases it will be stamped on the rocker cover. This is the cover at the very front of the engine. It may also be printed on the plastic engine cover.
mower plug

Remove

Remove the plug and turn off the gas if fuel tap fitted.

Mower engine cover Mower engine cover

Remove

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.

mower valves

Valves & Rockers

Have a helper rotate the engine while you watch that both valves are  compressing and releasing in turn.
mower engine

Pushrod

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.

mower pushrod

Pushrod

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.

Mower engine

Intake

Locate the intake valve, it will be in line with the intake manifold and carburetor.
mower valves

Muffler

This is the exhaust valve, seen here adjacent to the muffler (exhaust).

Mower valves

Rotate

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.

Mower engine Mower engine

Set

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.

mower valve adjustment gauge

Gauge

Measured in Metric and Imperial. The exhaust and intake clearances will differ. Exhaust value lash is usually larger as it gets hotter. Valve clearances differ from model to model so check with your engine manufacturer.
mower starter

Check

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.

mower valve adjust

Locknut

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.

Mower valve adjust

Adjust

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.

Mower valve adjust

Tighten

When your happy with the amount of resistance, hold adjusting nut in place with the wrench and tighten lock nut.
mower valve adjust

Rotate

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.

Mower valve adjust

Repeat

Now repeat the process for the intake valve and note the gauge size may be different.
mower valve adjust

Refit

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!

Related Questions

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.

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.