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BASIC TROUBLESHOOTING OF ENGINES USED ON LAWNMOWERS

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BASIC TROUBLESHOOTING
OF
ENGINES USED ON LAWNMOWERS

SPECIAL NOTES TO THE READER:


Note 1-
The writer is not implying that all methods and techniques apply to all engines used on all lawn mowers.

Note 2-
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.

SPECIAL NOTE ABOUT TROUBLESHOOTING-It is up to you to not only read all the information about your problem, but it is also your responsibility to make an informed decision. If you have not properly diagnosed a problem and you just "throw parts" at your problem, don't get upset with anyone except yourself if you don't understand what you are doing or understand the complexity of the problem. You still have the choice or alternative of taking your equipment to a trained repair shop. If you buy parts and it did not fix your problem, you either improperly identified the true problem and purchased the wrong repair item or you have more than one problem.

ENGINE WILL START,
BUT CHOKE MUST STAY ON TO KEEP ENGINE RUNNING-

This is normally some sort of fuel restriction.
Applying the choke causes more vacuum in the carburetor throat and causes this vacuum to pull more fuel up from the bowl into the air flow.
If you don't have a clog somewhere in the carburetor, i.e. the main jet is not clogged. Check to see if your fuel solenoid has power going to it. The fuel solenoid is designed to stop fuel flow into the engine after shut-down. However, if it does not work properly or does not have the proper voltage going to it, usually 12 volts DC it will not open up permitting full fuel flow into the carburetor throat. Some fuel will flow as you pull open the choke, but the closed fuel solenoid valve will restrict some if not all of the fuel flow. Check wire connections going to the fuel solenoid. Improper connections or corrosion can cause a problem. Test the fuel solenoid by applying 12 volts directly to the fuel solenoid wire.

ENGINE WILL NOT ROTATE-

Basic principles needed,
BATTERY POWER (VOLTAGE)-
Does your battery have a "sustained" voltage during starting?
Just because your battery is indicating a good voltage reading,
lets say 12.7 volts does not mean your battery is good.
Does it sustain a 12 volt indication during your start attempt?
If your battery drops to a lower voltage during starting, then
your engine will not rotate due to your battery not sustaining 12 volts during start. Our experience with bad batterys has been with very low voltage during start attempts. Voltage usually drops below 9 volts. It is safe to say the lower your voltage "drops", the more likely you need a new battery.

GOOD CONNECTIONS AT THE BATTERY-
Your battery is good, but how are the battery connections?
Is there any corrosion or are the connectors loose?
You can have 12 volts indicated on points in the circuit, but if you have
a bad connection the "full amperage" can not flow through a small connection. Yes, I said amperage, not voltage. Where there is higher amperage, you will have a higher voltage drop. That is why sometimes "voltage disappears". A hand held volt meter draws very little amperage from your check point. But if anything is "turned" on, your voltage check point may lose its voltage indication.
To get a 12 volt reading with a hand held volt meter is possible, but with any small amount of load applied, voltage can "disappear". CHECK YOUR BATTERY CONNECTIONS, BOTH NEGATIVE AND POSSITIVE CONNECTORS!
GOOD CONNECTIONS AT ALL ASSOCIATED CONNECTORS-
Is the voltage indications passing through all connection points?
Splice connectors are good sources for corrosion and loss of voltage potiential. Is there a broken wire somewhere?

FUSES-
While this was not listed first, it may be one of the first and obviously the easiest thing to check.

STARTER RELAY-
Do you have 12 volts going to the starter relay with the key switch being held in the start position?
If yes, then you have a starter related problem, including a bad ground connection with the engine to the battery negative post.
If no, then you have a break in a wire, a switch or relay in the circuit that is preventing power from going to the starter relay "coil" terminal.

STARTER GROUNDED PROPERLY-
Many equipment manufacturers are realizing how important that it is to have a good ground connection between the engine and the machine frame and back to the negative battery post. If any contact point along this path is a "weak" connection, then amperage will not flow through the starter and the starter will not rotate. Some manufacturers are running their main frame grounding point straight to the same place the starter mounts to the engine. This ensures the starter has a direct grounding to the battery negative post. Some manufacturers don't do this and rely on the engine to find grounds back through the frame of the engine and the frame of the machine to the negative post. That is a lot of contact points subject to corrosion. In some cases, we have replaced battery cables with longer cables to make a direct run to the starter mounting bolt to make equipment work properly. You may consider this option if you suspect you have a lot of corrosion to clean up or paint to scrape.

ENGINE WILL ROTATE, BUT 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 plugs. 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 coils 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 cylinders 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.

DIXE CHOPPER USES A MERCURY SWITCH-
Is your mercury switch "tilted" or "upside down"?
Is your mower upside down? (I could not resist)
The wires coming out the mercury switch must be pointing straight up coming out of the mercury switch. If the mercury switch is shorted while the machine is on level ground, the engine spark will be grounded. Case in point-
We found once on a Silver Eagle, the hinges on the seat base plate to be "bent". This caused the base plate to be twisted in its relative position to how it hits the switch found under the base plate. This base plate had holes in the plate near where the switch was "pushed on" and because the base plate hinges were bent, the holes in the base plate permitted the push pin on the switch to fall into one of the holes causing the switch to keep the engine from starting. Not all Silver Eagles have these holes in the base plate.

NO SPARK DUE TO SLOW ROTATION-

Is the engine being turned to proper "minimum" RPM to generate a spark?
A weak battery causing the starter motor to turn slower than necessary could be your cause. Don't be fooled by an engine that rotates, but does not produce spark. This slow rotation could be slow enough to not produce enough Flywheel magnet induction when it passes the ignition coil (armature). Some engine specifications requie 250 RPM for proper "minimum" rotation to produce a spark. Ask, do I have Bad or weak Battery, does the battery need to be charged, is the starter weak?

An electric starter that is turning, even though it is turning the engine over, it can be a weak starter turning the engine slower than required. It only takes about 30 to 40 minutes and a new starter to find out. Don't test the old starter, put on a new one and be done with it. The time it takes to put on a new starter is less than testing one and your bench test might not be able to reveal "Torque" that the starter produces. The true test is to install a new starter.
Try a new fully charged battery first. It is cheaper than trying a new starter. If you can't start it the next day, did your battery go dead, causing the engine to rotate slower than it should?
Keep in mind also,
High compression can cause an engine to be "tight"
and not turn fast enough to produce spark. Normally this is only on new engines that the valves have not "grown". If your engine has 600 hours on it then your valves are probably not the problem, but still could be contributing.

OIL VISCOSITY-
An engine oil with higher "cold" viscosity can cause an engine to turn slower during cold seasons. You may have to warm your engine up or try a thinner oil during the winter months.

NO SPARK DUE TO WEAK FLYWHEEL MAGNETS-

The flywheel magnets can be weak.

SCREWDRIVER TEST

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.
Compare magnets on new and subject flywheels.
If you have a new flywheel to compare to the subject flywheel, try placing the flat blade of a screwdriver onto each magnet. Compare the amount of force required to remove the blade from the magnet. The new flywheel should have a rather strong attraction compared to the subject flywheel. If you believe the removal force is the same, then the flywheel magnet is probably not the cause. Unless the new flywheel magnet is also bad. If you have asignificant difference in the force, then install the new flywheel.

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!

Did your fuel filter "collapse"?

Fuel filter collapse is caused by a fuel pump pulling all the air out of the filter and no fuel going back into the filter. This is usually an indication of a restriction somewhere betwen the fuel filter inlet and the fuel tank outlet or pickup tube. Check the pickup tube for restrictions if it has one. Check the bottom of the tank for a clogg. This is usually a total clog if your filter has collapsed due to vacuum pull from the pump.

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 tank clogged inside at the bottom outlet ?
Plastic shavings from manufacturing the fuel tanks could collect
in the area of the fuel outlet fitting and restrict the flow of fuel.

Is fuel coming out of the fuel tank?
Is the fuel outlet on the tank clogged?
Is the fuel valve open?

Is the fuel filter clogged or full of water?
Is the fuel pump working?
If your engine has a fuel pump, is it pumping fuel?
Is your fuel pump an electrical fuel pump or Vacuum operated fuel pump?
Is fuel coming out of the fuel pump during rotation?
Is fuel coming out of the fuel pump?
Is fuel getting to the fuel pump? An external fuel filter could be blocking fuel flow. Check the filter element to see if it is saturated with trash causing the obstruction. If any doubt, replace the element.

If you do not have a good external fuel filter you may have a clog inside the fuel pump.

Is the fuel solenoid valve opening?
Look on the bottom of the carburetor, your engine might have a magnetic controlled valve that opens by magnetisim. If battery power is not available to the valve coil then the valve will not open. Electricity could be present and coil could be bad or stuck. Is the valve stuck closed?
Is the main jet on the carburetor clogged? The main jet in the carburetor can be clogged. The jet is removable from the carburetor assembly. You may just want to replace it with a new jet.
Is some sort of external fuel shutoff valve closed? A valve between the fuel tank and the engine may exist. Someone may have closed it for some reason. If you find this to be the case, you should find out why they closed it, there might be a problem.
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-


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?

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Last modified: JANUARY 7, 2017

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