BASIC TROUBLESHOOTING
OF STRING TRIMMERS

SPECIAL NOTE TO THE READER:


While the following is somewhat basic, the writer has intentionally left out various things which one would expect the reader to know about before attempting to apply what is written. Basic understanding of electricity is a must and safety concerns must be considered before attempting to troubleshoot any mechanical or electrical system. If you do not feel you have this knowledge or training you should consult a properly trained service technician.

ENGINE WILL NOT START-

First you must determine if the basic elements for starting your engine exist. You must have air flow through the engine, fuel getting to the top of the cyliner, and ignition spark at the spark plug. If any one of these three elements do not exist or are restricted in some fashion then you will not have the required conditions for engine starting.

The following is not necessarily in a set order, read all the possible causes before attempting to determine what the problem is. It is up to you to determine what will be the "easiest" to check for first.

NO SPARK BASICS-


Are the ignition coil producing spark?
Look for cracks in the spark plug "ceramic insulators".
A very fine crack can allow for the spark to jump through the crack to ground.
Is the air gap for the spark plug properly set AND CLEAN?
Is the connection between the spark plug wire and the spark plug a good connection?
Is the ignition coil producing a good spark?
Be careful with testing an ignition coil for spark, you could feel the power of the ignition coil if you are not careful! Also be careful of any fuel spills that may become ignited when working with the spark plug wires and testing for spark.

IGNITION COIL AIR GAP-

Is the ignition coil air gap properly set?
Some engines work better on the high side of the air gap specification.

Is the small wire going to the ignition coils being pinched and grounding the spark?

Are the spark plug electrodes wet with fuel or water?
Either will prevent a spark. Remove the spark plugs and check, turn the engine over to ventalate the cylinder before putting the spark plugs back in place. Be cautious of loose spark plug wires producing stray sparks and fuel pumping out of the cylinders. A fire could be caused by not paying attention to these elements.

SAFETY SWITCHES NOT FUNCTIONING PROPERLY-

Is the spark being produced grounded? A switch or safety device may be bad. A bad switch could be causing the spark to be grounded to the machine frame when it should not be. Are the mechanical parts pushing on the safety switches when they are supposed to? Is the wiring from the ignition armature being grounded somewhere between the armature and the off switch? Case in point-
We have found the small wire going to the stop switch to be pinched by engine covers or heat sources mealting the plastic wire covering to cause a ground to the frame.

NO SPARK DUE TO WEAK FLYWHEEL MAGNETS-

The flywheel magnets can be weak.
The only way to really test this is to put on a new flywheel. Putting a screwdriver blade on the flywheel magnet will show you that you do have magnatisim, but how strong is it? Keep this in mind when all else fails, you may have a weak flywheel magnet. For the same reason of slow rotation, if your magnet is weak, your ignition coils will not produce enough spark to start the engine.

NO FUEL

Are you out of fuel?
Don't be fooled by a small amount of fuel in your tank.
Low fuel levels in tanks can cause an engine to not start.
Some fuel tanks have fuel pickup tubes that don't go all the way to the bottom of the tank.
PUT FUEL IN THE TANK!

WATER IN THE TANK-
What is in the bottom of your tank ?
Water is heavier than the Gasoline
and will settle to the bottom of the tank.
Your fuel pump could be pulling water from the bottom of your tank.

Is the fuel filter clogged or full of water?
Is the fuel primer pump working?
Are you simply out of fuel? After all this, did you check to see that you have fuel in the tank? Good luck.

NO AIR FLOW THROUGH THE ENGINE COMBUSTION CHAMBER-4 CYCLE TRIMMERS


Are the intake valves opening?
Are the exhaust valves opening?
If air does not flow through the cylinders, fuel can not be carried to the top of the pistons for ignition. Check the valves. You might find one stuck closed because of the rocker arm coming apart or a push rod not pushing on the rocker arm to open the valve. Did the push rod fall out of place? Is it bent?

TRIMMER HEAD WILL NOT TURN

IS THE ENGINE CONNECTED TO THE DRIVE SHAFT?
WE HAVE SEEN WHERE THE OUTER SHAFT SLIPS OUT OF THE ENGINE
AND THEN THE INSIDE DRIVE SHAFT BECOMES DISCONNECTED.

ENGINE WILL START BUT RUNS ROUGH AND LOW POWER

IS THE SPARK ARRESTOR PLUGGED WITH CARBON ?
REMOVE, TEST RUN ENGINE TO SEE THE DIFFERENCE WITH
SPARK ARRESTOR SCREEN REMOVED.
CLEAN AND REINSTALL OR INSTALL A NEW ONE.

IS THE MUFFLER COMING APART INTERNALLY?
ON CATALITIC MUFFLERS, THE INTERNAL STRUCTURE OF THE MUFFLER CAN
COME APART CAUSING AIR FLOW DISRUPTION AND RESTRICTIONS.
AIR FLOW IS IMPORTANT AS FUEL FLOW, if the air flow is "choked off"
then the fuel can not flow through the carburetor properly.
Inspect the engine exhaust port with the muffler removed.
REMOVE ANY CARBON BUILD UP, don't let any carbon to enter the combustion
chamber, this could cause piston scratches and wear.
AGAIN, RUN THE ENGINE WITH THE MUFFLER REMOVED TO SEE IF
ANY DIFFERENCE HAS BEEN MADE BY REMOVING THE CARBON BUILD UP
AND ALSO TO DETERMINE IF THE MUFFLER MAY BE OBSTRUCTED INTERNALLY.

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AAAA Generator and Pressure Washer, Inc.
DBA: ARKANSAS OUTDOOR POWER EQUIPMENT
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Last modified: NOVEMBER 12, 2009

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