Riding Lawn Mower Engine Cranks But Won’t Start

Riding Lawn Mower Engine Cranks But Won’t Start

October 12, 2019

Hi, Wayne here from Sears PartsDirect. Today we’re going to troubleshoot a riding
lawn mower engine that spins but won’t start when the starter motor runs. Once it’s spinning, the engine needs 3 things
to run—fuel, spark and compression. Here’s a quick test that can often eliminate
2 of those suspects right away. We’ll check to see if the engine has compression
and spark by spraying a short burst of starter fluid into the engine’s cylinder. The highly combustible starter fluid will
start the engine briefly if the cylinder gets spark and the right amount of compression. In a well-ventilated area, pull off the air
filter and spray a short burst of starter fluid into the cylinder through the carburetor’s
air intake. Now try to start the engine. If the engine doesn’t start at all, you
can skip to the next section on fuel system troubleshooting and move on to checking the
ignition system. If the engine starts briefly and then dies,
you know the spark and compression are okay, leaving you with a fuel supply problem. A dirty carburetor is usually the culprit
when the engine isn’t getting fuel. But, before you replace or rebuild the carburetor,
check these basic fuel supply issues so you don’t waste money on a part that you don’t
need. If the air filter you just pulled off is dirty,
replace it. You need a clean air filter so the right amount
of air can mix with the fuel to start the engine. If you haven’t used the mower for several
months, drain and refill the fuel tank with fresh gas. Over time, gas absorbs water and loses combustibility. In the future, if you don’t use up all the
gas before storing your riding mower for several months, add fuel stabilizer to your gas tank. A clogged fuel filter won’t allow gas to flow
to the carburetor. Replace the fuel filter if you haven’t changed
it within the last year. Here’s a video that shows you how. The fuel solenoid valve shuts off fuel to
the carburetor to prevent backfire when you kill the engine. If the wire’s disconnected, the solenoid won’t
open to allow fuel to flow to the carburetor. Check the wire harness connection on the fuel
solenoid valve. Reconnect the valve wire harness if you find
it disconnected. Check the fuel line from the fuel tank to
the carburetor for clogs or damage. Clear any clogs and replace the fuel line
if damaged. If your engine has a fuel pump, replace the
pump if it doesn’t move gas through the fuel line to the carburetor. If you found no problems when checking these
fuel system issues, then rebuild or replace the carburetor to restore the fuel supply
to your engine. If your engine didn’t start with the help
of starter fluid, check the ignition system. First, make sure you have the spark plug wire
firmly connected to the plug. Replace the ignition coil if you find damage
to the spark plug wire because the wire is part of the coil. Here’s a video that shows how. If you found no problems with the spark plug
wire, pull the wire off and remove the spark plug. Check the spark plug tip for carbon or oil
deposits that could prevent the plug from sparking. Also, check for cracks in the plug’s insulator. Replace the spark plug if the plug’s too
fouled to spark or the insulator is cracked. If the spark plug looks okay, use a spark
plug tester to see if the plug it getting current from the ignition coil. To test it, install the spark plug and connect
the boot of the tester to the spark plug. Connect the engine’s spark plug wire to the
other end of the tester. Crank the engine and see if the tester sparks. If the tester doesn’t spark, it’s likely
you need to replace the ignition coil. If the plug sparks, that means the ignition
coil is good, but the timing for the spark might be off. Here’s how the timing works on a riding mower
engine. The flywheel magnet and ignition coil control
spark timing. When the flywheel magnet passes the ignition
coil, the coil generates and sends current to the spark plug, causing the spark. The flywheel key keeps the flywheel aligned
on the engine crankshaft so the magnet passes the ignition coil at the right time. The flywheel key is a small metal rectangle
that keeps the crankshaft and flywheel aligned when you tighten the flywheel bolt. To protect expensive engine components from
damage, the flywheel key shears if a mower blade hits an object with enough force to
make the flywheel slip out of alignment with the crankshaft. If you hit a rock or stump and the engine
suddenly stops, you may have knocked off the timing by damaging or breaking the flywheel
key. A damaged flywheel key means the magnet won’t
pass the ignition coil at the right time for the spark plug to ignite the fuel to start
and run the engine. If this scenario sounds familiar because you
did hit an object while mowing and the engine stopped, remove the flywheel and check that
key. Replace the flywheel key if you find damage. Here’s a video that shows you how. If you find no problems with the ignition
system, then a compression problem could be preventing the engine from starting. Check the oil level to see if the engine is
overfilled. Drain some oil to drop it below the maximum
fill level. If the oil level is okay, pull out the spark
plug and check cylinder compression using a compression gauge. This test shows whether the piston is compressing
the air and fuel mixture in the cylinder. Connect the compression gauge to the cylinder
through the spark plug hole. Zero the compression gauge. Briefly turn the key to the start position
so the starter motor spins the engine. Check the compression reading. Most engines should measure over 40 psi of
compression. Many engines produce more than 80 psi. If cylinder compression is less than 40 psi,
the piston isn’t compressing air inside the cylinder. Keep in mind that a low compression reading
shows you that a compression problem exists, but doesn’t reveal the cause. You might need to adjust the valves or replace
the piston rings. A damaged cylinder wall can also prevent the
piston from compressing air inside the cylinder. To accurately analyze a compression problem,
have a service technician do a leak-down test on the engine. The technician will use an air compressor
and special tools to find where air leaks. Once the technician finds the cause of the
compression problem and fixes it, you can get back to mowing. I hope this video helps you out today. You can find links to the products and parts
we talked about in the video description below. Check out our other videos here on the Sears
PartsDirect YouTube channel. Subscribe and we’ll let you know when we
post new ones.

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