Determining a course of action after a Lawn Mower wont start-blade contact can be frustrating to attempt, it helps to have guide as to where to begin.

Determining a course of action after a Lawn Mower wont start-blade contact can be frustrating to attempt. It helps to have guide as to where to begin when trying to diagnose this problem. 

I have found this section of a much larger article covering many aspects of small engines to be very helpful. There is a lot of good information here for a Lawn Mower wont start-blade contact. You may want to bookmark this page.

Notes on the Troubleshooting and Repair of Small Gasoline Engines and Rotary Lawn Mowers 

Version 2.39 (28-Aug-07)Copyright © 1994-2007 Samuel M. Goldwasser 

--- All Rights Reserved ---

Instant troubleshooting chart - most common problems and possible causes


The following chart lists a variety of common problems and nearly all possible causes. Diagnostic procedures will then be needed to determine which actually apply. The 'possible causes' are listed in *approximate* order of likelihood. 

While this chart lists many problems, it is does not cover everything that can go wrong. However, it can be a starting point for guiding your thinking in the proper direction. 

(Portions of the following from: Chilten, Small Engine Repair 2-12 HP, (1).)


Lawn mower will not start after the blade hit an obstruction


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 spark plug, 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.

Flywheel removal


Remove the shroud (blower cover) if you have not done so already. This is usually fastened with 4 screws and hopefully does not involve any head bolts - if so, you will need to tighten them to the proper torque using a torque wrench once you have remedied the problem.

You may need to remove the fuel tank (if you have not done this already) and other trim pieces as well. You should now see the top of the flywheel. In most cases, a large nut fastens the flywheel to the crankshaft. (However, in some designs, part of the starter mechanism is actually used and this is supposed to require a special wrench to remove.

However, using a piece of wood as a buffer and tapping the ears in a counter clockwise direction will work also. Refer to your engine manual for details.) Use the proper socket to loosen this nut (counter clockwise).

It may be necessary to brace the flywheel securely to gain enough leverage. Make sure this is done against something that can stand the force. Once loose, remove it by hand and then remove any washers or other parts that are under it.

Make a note of how these were positioned including which side is up on some cupped washers.

You should now see the key way. The slots on the crankshaft and flywheel should be aligned. There are two common types of keys:

• A rectangular or D shaped piece of soft metal that locks the flywheel and shaft. You should be able to see if the two identically sized slots are still aligned.

• A piece of soft metal with an L-shaped cross section. The slot on the crankshaft is narrower than the slot on the flywheel and is slightly offset (thus, the L). Again, it should be obvious if the two slots are still aligned.

You may even find that the flywheel is relatively loose on the crankshaft if rotating the blade while holding the flywheel stationary is possible. Either the blade key or the flywheel key or both are broken in this case.

You will have to remove the flywheel to replace the key if it is broken or damaged. If the flywheel is loose at this point, then the following will not be needed as it can be lifted off.

There are several approaches to flywheel removal:

• The best way by far is to use a special puller designed for your particular engine. Briggs & Stratton and Tecumseh flywheels usually have 2 or 3 holes placed around the center of the flywheel which are used with special puller blocks.

These have self tapping bolts which you thread into the holes and then tighten down nuts to pop the flywheel off of the crankshaft. I have made my own blocks for this purpose from scrap steel. If you have a drill press, it is not difficult.

Alternatively, you can purchase these from the engine manufacturer. The use of a puller really does reduce the use of 4 letter expletives and virtually eliminates the chance of damage to the flywheel or crankshaft by the alternative techniques.

Bolts are screwed into holes in flywheel. Then, plate bears against the flywheel nut (slightly loosened) and the nuts are tightened alternately until the flywheel pops off.

WARNING: do not use an ordinary gear, clutch, pulley, bearing, or other puller unless this is specifically mentioned as a recommended technique in your engine manual. The flywheel could be damaged - possibly not immediately obvious - but the result could be catastrophic failure once the engine is put back into service.

• A 'knock-off tool' is a special closed-end nut that you thread onto the crankshaft in place of the normal flywheel nut. You then are supposed to pry under the flywheel with a pair of large screwdrivers while tapping the knock-off tool with a soft hammer.

Aside from the fact that as described, this requires 3 hands, this may or may not work easily. Depending on conditions, the flywheel may pop off at the first tap or may stubbornly refuse to budge no matter how much you whack.

If not done properly, it is possible to bend the crankshaft - very expensive. Some people also worry that the shock will damage internal parts or even partially demagnetize the magnet on the flywheel. Thus, my preference for the puller unless the first couple of taps releases the flywheel.

• Many engine books will simply recommend threading the flywheel nut back on flush with the end of the shaft and tapping this with a hammer as above (with the 3 hands). The risk here is that the threads may be damaged in addition to the possibility of bending the shaft or causing other damage.

Use a piece of soft metal - aluminum, brass, or lead - to protect the end of the shaft and nut. In any case, only use this approach as a last resort.

Flywheel removal on Briggs & Stratton enginesThe following applies to most B&S engines with the square starter shaft. This procedure will be needed to gain access to the points and condenser (on engines without electronic ignition), oil seal, etc.

The square shaft which turns one direction but not the other is the recoil starter pawl. This can be pulled straight up from the round cup that it sits in. There should be a couple of steel balls inside.

Some models have a large internal snap ring that holds this in the cup. Remove ring, and pull it straight up. Use a magnet to remove the steel balls. Once you have this starter pawl removed, and the steel balls, hold the flywheel, stationary, and using a block of wood, bump one of the lugs on the outside of the starter cup in a CCW direction.

Once you break it free of its torque, it should be able to be spun off CCW. Remove this starter cup, and the beveled washer under it. Now you’re ready for your flywheel puller. A strap wrench can also be used to turn this starter cup loose. When installing the starter cup, make sure it is clean and free of grease and oils.

A drop of oil or a light coating of grease on the crankshafts stub end (over which the starter pawl goes) is usually recommended. The starter cup only needs to be snugged down, and you don't have to go overboard trying to torque it to ungodly tightness. Snug is sufficient.

Drop in the starter pawl, into the cup over the stub of the crank, and drop in the steel balls, replace larger flat cover over this assembly, and insert snap ring if there was one.


Flywheel inspection


Once the flywheel is off, inspect the key way on the crankshaft and flywheel for damage. Serious damage will require replacement of the affected parts. Slight burrs can be removed with a small file.

If there are any cracks in the flywheel radiating from the hole, the flywheel MUST be replaced as this is a serious safety risk - the flywheel could literally explode when run at full speed. However, don't be concerned by surface flash - lines that look like fine cracks resulting from the molding process.

To confirm that these are not cracks, there will be no visible penetration inside the shaft hole and fine sanding will quickly remove all traces of this flash.

Assuming there is no serious damage, a new flywheel key should be all you need - about 25 or 30 cents. To confirm that this is all you need, replace the flywheel without the key but line up the two slots as they would be if a key were present.

Tighten securely (but it doesn't need to be to the full torque as this is just a test). This should permit the mower to start and run normally but I would not recommend using the mower to actually cut grass until you replace the flywheel key.

To install the new one, insert the key into the slot in the flywheel first and then slip the entire affair onto the crankshaft (I like to use a bit of WD40 for protection as well).

The flywheel should seat securely with no detectable free play - it should be on straight and not rock back and forth at all. If this is not the case, the key may be in upside-down or there may be something or some particles of dirt or metal blocking it. Replace the washers, dirt screen, etc., and then hand thread the flywheel nut as far as it will go.

Tighten to the specified torque (typically, 30-33 ft-lbs). Note: There may be a cupped washer between the nut/screen and flywheel. This must be installed cupped-side facing the flywheel or else you will probably be replacing the flywheel key again very soon.

WARNING: Do not install a hard steel key in place of the recommended flywheel key as you will lose the protection that the soft metal provides and the next incident may be the last...

Then, replace the shroud, fuel tank, etc. If head bolts had to be removed, it is probably a good idea to slightly loosen all of the head bolts and then re-torque them to the proper value in the recommended sequence for your engine.


Why soft metal keys must be used


Normally, the soft metal keys lock the blade and flywheel to the crankshaft. However, should the blade strike an obstacle and stop suddenly, one or both key(s) will shear and reduce the likelihood that the very expensive crankshaft or other parts will be damaged.

The reason is that the substantial inertia of the crankshaft and that of the flywheel will tend to try to keep them rotating. Something has to give and you want it to be the 25 cent key and not the $75 crankshaft!

Note that the soft metal flywheel key can also be damaged without totally shearing which may result in slightly incorrect timing. Symptoms may include a mower that is hard to start, runs rough or lacks power, or cannot be restarted when hot. Therefore, always replace the key if there are any signs of damage or wear.

If the previous information was of help to you and you would like to view the entire article please click here .

By having a basic guide when you’re Lawn Mower wont start-blade contact you may save yourself many hours of anger and disappointment when you find out that it was something as simple as water in the gas that been preventing the mower from starting.

This information may also save you a lot of money by not having to take your mower right away to a small engine mechanic. Remember when ever you are working on your equipment think safety first.


“Keep It Simple to Succeed” lets get out there and make our lawns healthy and green!

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