The blade in a Rotary Push Mower doesn’t need to be sharp as a carving knife or scalpel but serious dents and nicks will result in a less than perfect lawn (to say the least!). In addition, a seriously unbalanced blade can result in excessive vibration and eventually, possible internal damage as well.
First, drain the gas or remove the gas tank. If you will be filing steel, you get sparks. Sparks are not the greatest thing to have around gasoline vapor. Enough said. Disconnect the spark plug wire and tie it safely away from the spark plug or remove the spark plug entirely. Turn the mower on its side, I like to raise the carburetor side to keep oil from filling it up.

CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere. If there are any, you will need to work on the mower just propped up by 45 degrees or so. Or, use this as a good excuse to perform an oil change and drain the oil (even if the engine is cold, most of the oil will drain out but it will take a little longer). Just don’t forget to refill the crankcase with fresh oil once you have completed work on the blade!

Check that the blade isn’t bent. Locate a reference point on one side and note the height of the blade tip at that location. Rotate the blade 180 degrees and check the height of the opposite blade tip. There should be no significant difference – say no more than 1/8″ or so. If it is greater, the blade is bent or the crankshaft is bent. Either will require further investigation as running the mower under such conditions will probably result in excessive vibration and can be dangerous.

Assuming this is fine, remove the blade:

For just some minor touch up, there is no real need to remove the blade. For major grinding and balancing, removal will be needed. Removal will also be required to inspect for a damaged or sheared blade lock key and to replace the blade or blade related parts if necessary.
In either case: disconnect the spark plug wire and tie it safely away from the spark plug terminal (several inches minimum) or remove the spark plug entirely to prevent accidental starting.

If the nut holding the blade on is very tight, use a block of wood to prevent the blade from turning. Use a good quality socket wrench or box-end wrench of the correct size. An adjustable or even open-end wrench may not be enough and will damage the nut and tend to slip off, possibly quite violently. The nut usually unscrews counter-clockwise. However, check this out first! A careful inspection of the threads on the end of the crankshaft will reveal the direction. Or, determine the direction of rotation which will be designed to tighten, not loosen the blade. Most, if not all, single blade mowers rotate the blades clockwise as viewed from above which will therefore use a normal right-hand thread nut.

CAUTION: Make sure that if the wrench slips, your flesh will not contact the blade or other sharp sheet metal – liberal use of rags or newspapers is a good idea. Arrange your position and the mower so you are *pulling* towards you – this is a more stable controllable arrangement.

Pay close attention to the arrangement of the blade and associated hardware so you can replace everything properly.

Use some penetrating oil (e.g., liquid wrench or WD40) on the nut and threads if there are signs of rust or corrosion. Allow it to soak in for a few minutes before attempting to remove the nut.

A hammer or other more violent approaches should not be needed.

Once the nut is loose, unscrew it the rest of the way by hand and remove any washers or mounting plate and note their exact position and orientation. The blade and adapter should come off easily. Some penetrating oil (e.g., WD40) may help if it is difficult to remove.

If your adapter/blade doesn’t pop off after removing the nut or bolt, it may be mounted using a taper like the flywheel. This is somewhat unusual on a walk-behind lawn mower but might be present on a larger machine like a lawn tractor. A wheel puller is best for dealing with this situation but first see if it isn’t just gummed up or rusted in position – try the WD40.

Inspect the key or locking tab for damage. You may have:

A rectangular blade adapter with a welded-on soft metal ring with a tab protruding into a slot in the crankshaft.

A blade adapter that locks to the crankshaft with a rectangular or D-shaped (Woodruff) soft metal key.
If the adapter’s tab is broken off or the key is sheared or damaged, then replacement of the entire blade adapter or just the key (depending on your mower’s design) will be needed upon reassembly. For now, if you will be sharpening the blade, replace all the hardware in the correct positions (except the blade) and finger tighten the nut so you won’t lose anything.

WARNING: Do not install a hard steel key in place of the recommended blade lock key as you will lose the protection that the soft metal provides and the next incident may be the last… See the section:Why soft metal keys must be used.

Another way to remove a stubborn blade is to take it to your friendly garage or tire changer and have them put an impact wrench on it. Saves knuckles, tempers, etc. Just don’t put the blade back on without a touch of oil or anti-seize compound and of course, don’t use an impact wrench!

Lets Sharpen the blade:

Slight nicks and dents can be cleaned up with a file while the blade is still installed on the mower. Unless you have run into a curb, this is probably all that is needed on an occasional basis. Removing this small amount of metal will also not unbalance the blade enough to worry about.

If the damage is severe, consider replacing the blade entirely – they are not that expensive (usually under $10). Otherwise, you can use a file, a bench grinding wheel, or a grinding wheel mounted in an electric drill (there are special attachments for this specific application).

Since the rotating blade also contributes to the proper air flow, you do not want to upset the shape. Grind in such a way that the original blade angle is preserved. It doesn’t need (or want) to be razor sharp. A 1/64″ edge is fine. Anything finer will quickly be dulled by little bits of stone and dirt in any case. Safety is not the main concern here – if any part of your anatomy contacts the whirling blade, you **will** be in trouble no matter how dull or sharp the blade might be!

Attempt to remove approximately equal amounts of metal from both ends and in roughly similar areas if possible. If there are a few large nicks, it isn’t necessary to remove them completely – your lawn (and neighbors) will never know the difference.

Check the balance by positioning the blade at the center hole location on a pencil or other rod – you don’t need a fancy blade balancer but can use one if you like. If it tips one way or the other, remove more material from the heavy side a little at a time.

Replace the blade along with all its mounting hardware. Make sure you get all parts in the same relationship as they had originally. The blade must have its sharpened edges pointing downward. Don’t forget to install the key if it is separate and DO NOT substitute a hard steel key for the soft metal one that should be used, tighten the nut to the proper torque.

(The blade replacement procedures for riding mowers and lawn tractors generally differs significantly from that of walk-behind mowers. At the very least, it’s probably not possible (or at least easy or desirable) to turn them on their side. Some may require removing the deck or it may be possible to work from underneath (with appropriate precautions and accident insurance). Refer to your owner’s manual.)

The following description applies to most small rotary lawn mowers with direct driven blades. The vast majority of these use either Tecumseh (as found a variety of Sears/Craftsman equipment) or Briggs & Stratton engines. However, similar comments apply to others as well including Lawnboy two stroke engines and the more modern Honda and other overhead valve type of engines.
The assumption is that the engine started and ran normally prior to the incident. Now, no matter how many times you yank the starter rope or run the electric starter, it will not start at all, bucks, kicks back, backfires, or fails to develop enough power to keep going on its own.

If the blade struck a solid boulder while the engine was set on ‘high’, more severe damage is possible as even with soft metal keys locking the blade and flywheel to the crankshaft, the inertia of the rotating blade is acting sideways against the crankshaft in addition to suddenly stopping its rotation. This can result in a bent crankshaft. The end of the crankshaft with the blade adapter could be bent without affecting the bearings or internal parts. This would need to be tested for as well. Not that such an occurrence is that much better – the crankshaft would still have to be replaced but at least the bearings in the crankcase will not be damaged.

If the starter will not turn the crankshaft (assuming you remembered in your haste to engage the safety bar) – it is seized or will only rotate part of a revolution before hitting against something solid inside – then you probably have serious internal damage that will require a complete strip down and replacement of some (expensive) parts. If it turns but much more tightly than you recall (assuming you do have the safety bar engaged!) then the crankshaft may be bent – again very expensive. Repair may not be worth it.

However, in most cases, what has happened is that either or both of the blade lock key and/or flywheel key have sheared to protect the crankshaft from serious (and terminal) damage.

If the blade lock key broke, the blade will no longer turn rigidly with the crankshaft and provide the inertia required by many small engines with undersized flywheels. In this case, the engine may try to start but die out with a few “putt-putts” or even kick back on the starter cord. (As a side note, attempting to use a lawn mower engine as a replacement on a piece of equipment that doesn’t have something to substitute for the blade’s inertia may not work for this reason.)

If the flywheel key broke, the ignition timing will likely be totally wrong and the result may be no ignition, backfiring, kickback, or weak or total loss of power.

To diagnose, proceed as follows:

First, pull off the spark plug wire and tie it securely away from the spark plug terminal (several inches minimum) or remove the spark plug entirely so that there is no chance of the engine accidentally starting. Even though it will not start now no matter what you do, the underlying problem could actually be a flooded carburetor or something else which may correct itself while you are working. Never take chances.

Drain the gas or remove the fuel tank. This will prevent gasoline from spilling out the gas cap vent hole or flooding the engine through the carburetor since you will need to tip the mower to get underneath.

Set the mower on its side (carburetor side up).

CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere.

The mower can usually be set on its side for a few minutes without harm but if these occur – you will have to work with it tipped less than 45 degrees or so – propped on wood blocks. Or, use this as a good excuse to perform an oil change and drain the oil (even if the engine is cold, most of the oil will drain out – it will just take a little longer). Just don’t forget to refill the crankcase with fresh oil once you have completed your work!

Using an old rag and/or proper work gloves, grasp the blade and attempt to rotate the blade and crankshaft.

CAUTION: despite your lack of maintenance, the blade may be sharp!

The blade and crankshaft should rotate together. If there is slippage, the key has broken and will require replacement of just the key or the entire blade adapter plate depending on design. If it appears to be intact, then you can assume the flywheel key has broken. The blade key may be broken as well but it is not likely the reason for your failure to start. You should remove the blade to determine this for sure before restoring the mower to service in any case.

You can possibly avoid removing the flywheel for inspection of the key by unscrewing the sparkplug, rotating the crankshaft so the piston is at TDC, and noting the location of the magnet on the flywheel relative to the magneto coil pole pieces. The magnet should be pretty close to the magneto in that position. If this is not the case or just to be sure, the flywheel will have to come off to inspect and possibly replace the key.

To get at the flywheel key itself, some disassembly is required.

Fly Wheel Clutch Tool

Briggs Fly Wheel Puller

Basic Adjustment

The basic carb adjustments are about 1 turn out for the low speed screw and 1 and a 1/2 turns out for the high speed. This should get the engine running.

Most settings for low speed are around 1 to 1 and 1/4 and 1 and 1/2 to 1 and 3/4 turns out for high speed.

Final adjustments

The LOW speed screw (Engine at Idle Speed) should be adjusted (OUT) until the engine puffs out some black smoke, runs a little rough and appears to be getting too much fuel. Then turn the screw (IN) slightly until engines runs smooth.

Next adjust the HIGH speed screw with the engine running fast (Full Throttle). Turn the HIGH speed screw (OUTWARDS) until the engine puffs black smoke or begins to run rich. Then back the screw (IN) about 1/4 turn.

Once the engine is adjust correctly, going from idle to full throttle the engine will not die, bog down, hesitate, etc.

These procedures will also work with most other small enginnes.

As winter approaches, it reminds us that it is time to winterize our air cooled gasoline-powered lawn mowers . Winterizing will help extend the life of our lawn mowers. This post is for air cooled engines only. There is a lot of steps that are the same for liquid cooled engines but there is also a lot of steps that are not covered here. My next post will be for liquid cooled mowers.

The first thing I like to do is put a mixture of Fuel Stabilizer an gas in the fuel tank. Do not fill tank full, I will only put a small amount of this mixture in the tank. (We will come back to the fuel system later in this post).


Next Step: blow(with compressed air) or wipe the mower off, removing sticks, grass clippings an so on. The deck should be washed with soap and water and a stiff bristle brush, then blow the water off with compressed air. Spray the underside of the deck with W-D 40 or some other rust preventer. Clean the outside of the engine making sure to blow the air vanes that keep your air cooled engine cool. Do this with compressed air making sure to blow all debris from the air vanes.
You can wash the whole mower if you want, just be sure to dry it off as much as possible. Where I work at I washer all of our mowers with an agent called Luster. Luster has the washing detergent and polish mixed together.

Next Step: grease all grease fittings(Zerk Fittings) and a little 3 in-one oil on contact points that slide like hood hinges and etc.

Next Step: drain the crank case oil and refill with new oil. If it has a filter change the filter also. We do this to remove all the contaminants from the crank case, which could damage the internal engine parts from setting idle over the winter. If the engine has a foam air filter you can wash it with soap and water. You can dry the air filter by putting it in a clean dry towel and squeezing it dry with your hand.
Most all foam filters have to be oiled, you do this by putting a little pre-oil or 30 w motor oil on the filter an lightly work the oil through the filter. I usually put the filter back into a clean towel after putting oil on it and lightly squeeze to remove any excess oil from the filter. If your mower has a paper element filter then just put it back in for now.


Next Step: Start the mower and let it run. This will let the fuel stabilizer we put in the tank distribute through the fuel lines and the carburetor.
Make sure the mower is in a safe place with the deck up (if possible)engage the cutting blades to distribute the grease we put on the spindle bearings(grease fittings) in early steps.
Yes, we are getting close to being done so stay with me.

Next Step: In the earlier steps we put fuel stabilizer in the tank, now if your fuel tank is easy to remove, do so. Pour the gas out of tank into a clean approved container. If you can,t remove the tank try to siphon the gas from the tank into a clean approved container.

Next Step: Restart the mower and let it run until the engine goes dead or stalls.
If the tank is difficult to remove or siphon from, then fill the tank completely with the fuel/stabilizer mixture. (By filling the tank to the full level you are eliminating the air space that causes moisture).
You can remove the spark plug and add a little oil through the plug hole(about 50ml) to the cylinder and spin the engine over. I like to spin the engine over by hand or the crank rope if possible. If the mower has a electric starter just use it it briefly spin the engine over. This lubricates the cylinder wall to prevent it from rusting.

Next Step: Fill up the tires with air and store your mower in a dry place away from the weather.
Neglecting to empty the gas tank and perform other end-of-season care can shorten your mower’s life considerably, and can lead to costly repairs.

When grass cutting time gets close next spring, I will have a post telling you how to get your air cooled mower ready to mow.

A lot can be learned about your engine condition as well as what it
sees by properly reading and “understanding” your spark plugs.

This page will show you pictures as well as explanations on cause and
effect of what you see when viewing your spark plugs. Look at the spark plug porcelin for the color and condition of it.
Plug colors can tell you how the engine is running and can be very important. Plugs change colors and the different colors can explain such things as if the engine is running too hot or if the engine is worn.

The color should be read by looking at the porcelin insulator and compared to the plugs shown on the next few pictures.

	A tan colored plug means that the engine is running normal and the air/fuel mixture is correct. This is the correct color a spark plug should be and it tells you everything is fine with the engine. You would install a new properly gapped plug. When installing a new plug, replace the old one with the same heat range. This plug shows normal wear in the center electrode. A new plug would have square edges that helps the plug fire better.
	This plug is worn out from being used for a long period of time. Notice how the center electrode is round and worn from use. A spark plug that is worn takes a lot more voltage to fire and can cause poor engine running.
	This plug shows what can happen when something hits the spark plug. Something from inside the engine has hit the plug and this problem must be repaired before running the engine further. Make sure the spark plug is the correct length for the engine.
	Excessive detonation has caused the porcelin on this plug to break away. If this engine is allowed to run, engine damage can occur. Make sure the fuel octane is high enough for the engines requirements.
	A white colored plug is caused by engine overheating. Failure to repair this engine will result in severe engine damage. Common causes for this are: Incorrect spark plug (too hot heat range) Low octane fuel. Timing is not set properly. Cooling problems, (dirty cylinder fins, no or low water if water cooled, low or no engine oil). Carburetor air/fuel mixture is too lean (too much air). Leaking crankshaft seals, no oil, base or head gasket leaks, or crankcase leaks on two stroke engines.
	This plug has ash deposits which are light brownish deposits that are encrusted to the ground and/or center electrode. This situation is caused by the type of oil used and adding a fuel additive. This condition will cause a misfire. This can be also caused by changing oils in midstream.
	This plug is oiled fouled, caused by poor oil control.
	Pre-ignition, which will usually look as a melted center electrode and/or ground electrode. Check for incorrect heat range plug, over advanced timing, lean fuel mixtures or even hot spots or deposit accumulation inside the combustion chamber.
	Sustained Pre-ignition, which will usually look as a melted or missing center electrode and/or ground electrode as well as a destroyed insulator. Check for incorrect heat range plug, over advanced timing, lean fuel mixtures or even hot spots or deposit accumulation inside the combustion chamber.
	Splashed deposits look as if they are small islands of contaminants on the insulator. This is usually caused by dirty carburetor bores or air intake.
	A black dry fluffy colored plug is caused by deposits from a carburetor that is running too rich (too much gas), or excessive idling on some engines. Black smoke coming from the exhaust is a sign of a rich air/fuel mixture. The rich air/fuel mixture must be repaired before installing a new spark plug. Common causes for a rich air/fuel mixture are: dirty air filter. air mixture screw or carburetor needs adjusting. choke is sticking. carburetor float height is out of adjustment or float is sticking open.

Most Engine Starting Problems can be divided into two areas: Fuel Related or Ignition Related.

• Be sure that the machine has an ample supply of fresh fuel. For 2-cycle machines that require it, mix the proper amount of 2-cycle oil into the gasoline and shake to mix completely. Never use old or leftover fuel. Store fuel in an airtight container in a cool, dry place away from sparks and flame, and keep all vents on the can closed . Bad gas is more common than you may think, especially when starting a machine for the first time in a season. Do not store gas over the winter, pour it in your car and start fresh next year! 
• If the engine is a 4-cycle, be sure the engine oil is the right type, is clean, and is at the proper level.

   

Next step is to check for spark:

• Make sure that the ignition switch is “on”, and all attachments are disengaged and that the transmission is in neutral. Most machines have safety mechanisms that will not allow them to start or even crank over (if they have electric start)if this step is not taken. Consult your owner’s manual for the starting procedure for your machine.   

• Next- check for spark by removing the spark plug and grounding the “hex” part of the spark plug to a bare metal part of the engine or buy a Spark Plug Tester (you can get these at your local parts house cheap). Remove any spilled gasoline that is nearby, then spin the engine by pulling the rope or turning the key. You should see a blue spark jump across the plug gap. If you have a good hot spark, skip to the carb section.

• If the spark is yellow and weak, or there is no spark then the first thing to do is try a new spark plug. Do not try to clean the old one.

  

  The way a Spark Plug should look

   Set the new plug gap to the proper spec (usually 0.030″) and check for spark again. If you do see a spark, install the new plug and try to start the machine. If you still have no spark next try cleaning and setting the points on older machines, or replacing the electronic ignition module on newer machines. Before replacing the old module test it by disconnecting all the  wires from the module terminal that connect to the kill switches and recheck for spark, if you see a spark a wire or switch is defective.  Also check the flywheel key. That affects the spark timing, usually the key will

  

  Worn Spark Plug

  need replacing if you strike something while mowing and the engine stops.

 Next step is to check the carb & fuel system:

For optimum performance, you should change the oil in your small engine after the first five hours of use and then annually, or every 50 hours of use (whichever comes first).

Step 1: Getting Started

Start the engine and run it until it is warm. Stop the engine. WARNING: Be sure to disconnect the spark plug wire, and secure it away from the spark plug to prevent accidental starting. Clean around the oil fill and drain plug area to prevent dirt and debris from falling into the crankcase. Remove the dipstick, if equipped.

Step 2: Draining The Oil

Tilt the mower deck with the air filter or spark plug side up, and position some newspaper and an oil pan or jug beneath the mower. Use a 3/8″ drive socket wrench and extension (no socket) to turn the plug counter clockwise, allowing the old oil to drain. If the plug also serves as a fill cap (normally yellow or white in color), it may have two prongs so you can loosen it by hand or with a screwdriver or hex key for additional torque.

Replace the drain plug by twisting clockwise and tightening with a box wrench or adjustable wrench. If your engine does not have an oil filter, skip this step.

Step 3: Engines with Oil Filters

If your engine has an oil filter, replace it at least once per season.

Replace the oil filter by twisting counterclockwise on the body, using a filter wrench or pipe wrench.
Examine the sealing surface on the oil filter adapter for debris or gasket material. Clean if necessary.
Lightly oil the filter gasket with clean engine oil. Install a new filter, screwing in by hand until the gasket contacts the filter adapter. Tighten the filter an additional 1/2 to 3/4 turn.

Step 4: Fill With New Oil

Pour in the correct amount and approved type of oil for your engine, based on this oil capacity chart , or your operator manual. If you have two oil fill plugs (normally yellow or white in color) on each side of the engine, either one can be used to fill the engine with oil to the correct level. Please DO NOT fill the engine with oil without using the correct measuring device as illustrated in your operator’s manual.

Oil Recommendations

To optimize engine performance, use Warranty Certified Briggs & Stratton Small Engine Oil. Briggs & Stratton offers a Synthetic 5W-30 oil that provides the best protection at all temperatures as well as improved starting with less oil consumption.

For optimum performance, you should change the oil in your small engine after the first five hours of use and then annually, or every 50 hours of use (whichever comes first).

Use Briggs & Stratton SAE 30W Oil above 40°F (4°C) for all of our engines.
Check oil level regularly.
Air-cooled engines burn about an ounce of oil per cylinder, per hour.
Fill to mark on dipstick. DO NOT OVERFILL.

Remmber, this is just a guide line so be sure to read your owners manual!