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(From: Mike's Small Engine (smallengineparts@yahoo.com).)
Nowadays, a lot of mower/equipment is using OHV (OverHead Valve) style engines. This in itself causes many starting troubles. These engine use a compression release on the camshaft to allow for easy starting. Once the engine has started and reaches a speed of over 700 rpm, the weight on the compression release swings out and allows the engine to gain full compression and full power. After awhile, the valve lash/clearance begins to widen and the first thing that goes wrong is the compression release doesn't work properly causing the engine to drag which may sound to some as a dead or dying battery. I'm sure there are untold thousands of people that have replaced batteries, solenoids, starters, switches, etc., only to find themselves going to a repair shop and letting a trained mechanic do what is a simple maintenance adjustment.
Once you have exhausted these obvious problems, determine if gas is reaching the cylinder as follows: Perform the normal starting sequence and then, assuming it shows no signs of wanting to start, immediately remove the spark plug. If fuel is reaching the cylinder, the spark plug should be damp with gas and there should be a very distinct odor of gas from the spark plug hole. If there is none, then there could still be a blockage in the fuel line or the carburetor may need cleaning.
A flooded engine, most likely due to extended unsuccessful attempts at starting or a defective carburetor (float valve stuck open or gas-logged float) will result in inability to start as well and a distinct odor of gas. You might find raw gas coming our of various orifices - air filter as well as exhaust. (Note that in severe cases, enough gas gets mixed in with the oil to significantly increase the level in the crankcase and reduce the effectiveness of the oil. This will require an oil change.
To much air results in a mixture that is too lean, burns too quickly, and can result in engine damage over extended periods of operation.
Too little air results in a mixture that is too rich - there will be loss of power and possibly black smoke from the exhaust. This could be due to several factors:
WARNING: make sure there is no gas in the vicinity when performing the following test!
Remove the spark plug wire and insert the blade tip of an appropriately sized and well insulated (plastic) screwdriver inside the boot or clip in place of the spark plug. While holding the *insulated* part of the screwdriver, position the metal part of the blade about 1/8th inch from the block or frame.
An alternative technique is to use an old, but good, spark plug whose gap has been increased to about 1/8 inch or one specially made for exactly this purpose. In this case, simply connect the spark plug wire to the test plug and hold its threaded part against the cylinder head or other part of the chassis (away from the gas tank!!).
Note: Just positioning the spark plug wire a short distance from the spark plug terminal is not recommended as the results of this test will then depend on the condition of the spark plug as well since the spark will have to jump two gaps.
Have a buddy crank the engine at normal starting speed so that you will be able to hold the screwdriver or test plug steady and be close enough to see any spark clearly. Shield the gap from the sun or bright light if necessary.
You should see a nice healthy spark jump the gap several times on each pull (actually, once per rotation of the crankshaft/blade on both 2 and 4 stroke engines). Note: 4 stroke engines ignite the air-fuel mixture on every other rotation of the crankshaft. The extra sparks fire harmlessly into the exhaust gasses and are wasted. Can you believe it?!
CAUTION: if you are not well enough insulated, *you* will jump several times per rotation of the crankshaft/blade if the ignition system is functioning properly! Hey, that *is* a valid test!
If this test confirms the spark, it is still possible that the spark plug is fouled or bad. See the section: Checking the spark plug.
If there is no spark, then there is a problem with your ignition system.
However, a number of other problems can result in lack of spark:
Make sure stop switch/stop wire is in appropriate position - confirm with a multimeter, check that flywheel is being spun by starter and that flywheel key is intact to assure proper timing, check condition of points/condenser and setting (if applicable), test magnet (on flywheel) for strength, check the gap between flywheel and magneto core. If these are all fine, test or replace the magneto.
In more detail:
Items (2) and (3) are likely if your just attempted to move a curb with your mower blade (or if someone inadequately tightened the flywheel nut during some previous maintenance).
See the section: Testing the magneto.
First, check that the dead-man bar is properly disengaging the stop switch when pulled and/or throttle control is properly disengaging the stop switch when in the start or run position.
For anything beyond this, disassembly will be needed to identify and replace any defective parts.
If the no-spark condition happened after the blade hit an obstruction, (1) or (2) are likely. See the section: Lawn mower will not start after the blade hit an obstruction.
The best thing to do at this point is just replace it with a new spark plug and worry about the old one later. Actually, nearly every small engine maintenance book will recommend changing the spark plug every season anyhow.
In an automobile, the battery supplies the primary current; in a magneto, the magnet on the flywheel moving past the core at high speed acts as a generator and induces current in the primary.
As the magnets spin past the pole pieces of the magneto core, the points are closed and current builds up in the low voltage winding (and flux builds up in the core). At or slightly before Top Dead Center (TDC), the current (and flux) should be maximum and at this instant the points open. The flux then collapses (and the condenser (capacitor) across the points acts as a snubber allowing the current to bypass the open points and preventing arcing at the point contacts). This rapid decrease in flux results in coupling of the stored energy to the turn high voltage winding and results in up to 10,000 V or more at the spark plug.
(For EE types, this is somewhat similar in basic operation to the flyback converter in a switchmode power supply except that the moving magnet supplies the input power instead of the rectified AC line and the points act as the switch instead of a power transistor.)
The secondary will always be accessible for testing but the primary of an electronic ignition may be not be due to the electronic components:
Wires can break due to corrosion or vibration. This would result in an open winding - infinite resistance. Shorts can develop between adjacent windings or to the core. This may be detectable as reduced resistance but without knowing exactly what it should be, there is no way of knowing if a slight discrepancy represents a problem or just slight variations in design or manufacturing.
A more complete test would involve checking the 'Q' or doing what is called a 'ring' test and even more for an electronic ignition. This requires special equipment. Therefore, it is best to swap in a known good unit. They are not that expensive.
Apparently, some mowers are made with swing tip blades:
(From: Roderick Carmichael (carmic@nex.net.au).)
"I use a top notch Supa-Swift with a gravity cast alloy chassis and swing-tip blades (Australian invention, no bent cranks on our mowers mate!)"
Why are these not common in America? Conspiracy to sell replacement parts by the small engine manufacturers? :-). You would have to really work at bashing a curb to bend a crankshaft with such a mower.
Another possible approach - applicable for both new lawn mower designs as well as field upgrades - would be to replace the heavy steel blade with one made of nylon with a molded-in steel edge. An auxiliary flywheel might have to be added (under the deck) to provide the needed inertia (normally supplied by the steel blade) for the engine to start and run properly and to help the mower plow through tall grass. However, since this additional flywheel could never be stopped abruptly due to hitting a rock or curb, its inertia would never contribute to a sideways bending force on the crankshaft. The greatly reduced mass and increased flexibility of a reinforced nylon blade should virtually eliminate the possibility of a bent crankshaft from such unfortunate incidents. See the section: Why you really don't want to attempt to move an immovable object.
(From: Mowerman (mowerman2687@my-dejanews.com).)
I have been reconditioning lawn mowers for many years and find it fascinating what design goes into some of this equipment. Like arrangements to prevent the crankshaft from bending if the blade hits something. Most engines have an aluminum key at the flywheel but this will not stop shaft bending most of the time. Mower manufacturer uses many methods. Lawn boy has a tapered shaft at blade level and so the shaft has no key and the blade slips if hit. Some other mowers had steel washers and fiber washers nearest the blade like a clutch I seen this on some antique mowers. The best idea is disc with small flail blades or short blade with flail at end. Manufacturers in New Zealand and Australia use this method on their domestic mower mostly and it works well. Victa of Australia was one of the first ones around here with this idea. I am sure you have them over there to no doubt. I hope this information will be to some help.
I recently came across a description of a blade assembly similar to the one suggested by you to make bending the crankshaft difficult. This is in the Royal Horticultural Societies' "Encyclopedia of Gardening". In the tools and accessories section, under the "lawn mowers" subsection, a caption says: "Plastic disk. This cheap replacement part cuts the grass by rotating horizontally". The inset shows rotary, cylinder, and "Flymo" mowers. A picture of the part shows a disc almost the diameter of a regular blade, with one blade joined to the circumference by a pin so that centrifugal force keeps it radial. This blade is short and narrow compared to the plastic disc; no telling if it is plastic or metal. There is no further reference to this part in the text. In my translation, it is on page 465.
If only they had included a name or supplier!
I have recently purchased a new rotary lawn mower and appear to have started wrecking it in the first two weeks of use! Problem is, my property has lawns that run alongside a gravel driveway, and its often very hard to guarantee there are no stones lying in the grass before you start mowing. Not surprisingly I often hit small stones. These usually cause no harm, but today some teenagers were mowing my lawns and hit a real monster that measured approximately three inches by two by one, and weighed more than half a pound! Although the mower seems still to work OK, the impact has created a three inch long tear in the mower's cast aluminum body. Not a nice thing to have happen to a new machine!
Despite the stone's size, I was surprised at the size of the resulting tear in mower's more than 1/4 inch thick aluminum casing. I hadn't imagined a rotary mower blade powered by a 5 HP Briggs and Stratton motor could produce such force!
It would be interesting to hear from others who have survived similar experiences and to get an idea from any budding engineers on whether its perfectly reasonable for a stone this size to do such damage to the body of my mower - or whether it's more likely the body casting had a manufacturing defect that made it split prematurely?
The primary cause was likely a lack of oil - I should have checked it before attempting to run the engine for more than a few seconds. I have no idea whether someone had actually drained the oil for who knows what reason or it was just very low. In addition, I may have accidentally put the governor link back in the wrong hole permitting the engine to run at an abnormally high (and dangerous) speed.
There was no warning. The rod cap just exploded into e pieces (and this was at normal speed) and took a nice chunk out of the interior of the crankcase. Based on a post mortem of the rod, it appears as though one of the cap screws just loosened and backed its way out totally - there was no evidence of thread damage that would be expected if it were ripped out - and fell into the sump. With only one screw holding the rod and cap together, eventual failure was inevitable. Due to the offset design of the cap, this probably worked for a while since most of the force is on the rod.
Discoloration indicated excessive heating but no obvious bearing damage was evident that could be attributed to the lack-of-oil condition. The bearing was not in pristine condition but the type of scoring seemed to be more due to just poor general maintenance - lack of regular oil changes - than to this incident in particular.
Lessons: Check the oil level no matter what if there is any question or you are working on an engine of unknown history. Double check the governor linkages - take notes during disassembly - and be aware of what a normal speed sounds like for your type of engine (2 stroke or 4 stroke). If in doubt, install the link in the hole that would result in lower RPMs - closer to the carburetor. You can always move it later.