It's time again to pull out your chainsaw, backpack blower, & trimmer to get your yard looking its best. Except, when you try to start your motor you find no matter how hard you pull the starter the only thing firing up is your frustration.
Before you start pulling your hair out trying to diagnose your engine, take a look at our simple guide as to what could have caused your engine to fail.
Lean Running Conditions:
In the combustion chamber, lean is lean; it does not matter if air entered the
engine somewhere other than the carb, leaning out the mixture, or if the fuel
flow was restricted, leaning out the mixture. If the engine does not have a
rev-limiter, lean running will cause over-revving. A saw with a lot of run time
will get a polished, buffed appearance on the bottom of the intake skirt, but
over-revving will advance this buffing effect considerably, as the picture on
the right indicates.
Fuel provides piston cooling, so a lean mixture allows the piston to overheat.
However, damage is usually worse on the exhaust side.
Also be aware that a new engine with little buildup of residual oil in the
porosity of the piston and cylinder wall will score differently than an engine
with a lot of run time.
This is sometimes referred to as "4 point scoring", implying that the piston swells at
the four corners where the support webs for the skirt are located.
On a saw, the intake side of the piston nearest the flywheel may not be scored,
because it is getting the cooling air first.
Typically, the exhaust side of the piston will be scored all the way across, with the
intake side showing damage mainly at the skirt webs.
The wrist pin will usually be blue, and the underside of the piston may be black with
burned oil residue.
Lack of Lubrication/Oil
Typical damage from lack of lubrication will be up to 360° scoring around the
piston. It may be due to no oil, the wrong type of oil, or not enough oil in the
fuel. Old fuel may also contribute to this.
Winter fuels or fuel with high levels of alcohol may indicate failures similar to
those observed from lack of lubrication.
If a Stihl engine check is done on the engine and no lean faults or
overheat indicators are found, then the piston is removed and has scoring on
both sides or even all the way around, it is most likely a lack of lubrication
related failure. The only way to be sure is to see what faults are present, and
if none are found then by process of elimination, it is most likely lack of
No Oil or Very Low Oil in Fuel
This engine had no residual oil soaked into the metal surfaces, so it did not last very
This is a stratified scavenge engine design. Notice that the lack of lubrication
caused scoring on the intake and both sides of the piston first and led to a loss of
compression when the rings were pinched by the material wiping across the lands.
Regardless of the engine design, no oil leads to piston contact with the cylinder wall
and scoring and metal transfer.
The crankshaft bearings are fine, and all it would take to repair the unit is a new
piston and cylinder assembly.
Notice the sandblasted, dull look of the intake side of the piston on the left,
compared to the one on the right. In the close-up, you can see the scratch
marks and tell that the machine marks are worn off. An engine with long term
dirt ingestion may continue to run until it has so little power that the operator
will complain about the performance. The dirt may cause the lower rod
bearing to fail since that is the first place it hits as it comes into the
There is usually plenty of evidence to confirm an engine that has ingested
dirt. The air filter may be missing or damaged. There may be evidence of dirt
or grit on the clean side of the filter or on the choke plate of the carb.
A worn piston eventually leads to lean running, as the intake side of the
piston acts as the valve that closes the intake port. This is a close tolerance,
and if the skirt is worn down from abrasive ingestion it no longer seals well.
The engine will have excessive spit-back through the carb. The air filter may
get saturated with fuel in some situations.