Lawnmower question, nothing to do with casting

[Patrick L]

I'm going to ask this here because you guys are a pretty smart bunch.

I confess to knowing almost nothing about engines. I was wondering why you disconnect the spark plug when you work on a lawnmower, like changing the blade? Is it true you can spontaneously start it by turning the shaft? Is is sort of like in the old WWI movies, where they start the old fighter planes by having a ground crew man give the prop a spin manually?

I'm serious. I'm pretty smart with guns, reloading, casting, etc. I just never was into engines.

[la5676]

Same reason ya never point even an unloaded gun at anyone. I'm sure there is someone somewhere out there that can say BTDT on the mower thing. It's prolly not something you would want to find out on your own, and it's a very simply thing to do as a prevention. And, yes, on many mowers, a simple spin of the blade could start the engine of a standard run of the mill mower with no safety measures on it, like blade clutch, etc.

Be safe, pull the plug wire.

[cwtebay]

Yes!! You absolutely can start one by turning the shaft. Think model T crank starter, push starting a motorcycle or pull starting a car.

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[flyingmonkey35]

Yup

Just like pulling the cord your spinning the engine. It can start on ya.

Assume safety and common sense in all things.

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[Misery-Whip]

Yes your correct, your mower can start like a WW1 airplane if the conditions are right.

The blade is connected to the crankshaft, so is the rope starter, just on opposite ends.

[Patrick L]

Thanks I see now.

Please don't misunderstand, I am not looking for a way to get around disconnecting the spark plug. I just wanted to understand the "why" of it.

Am I correct in assuming that this instruction comes from the days when there were not so many safety features on mowers? I would think this would be unlikely on my modern mowers, which require me to squeeze down a bar on the handle to start them? If the mower were flipped on its side, and I was removing the blade, the safety bar wouldn't be depressed.

Again, i'm not going to try this.I WILL ABSOLUTELY CONTINUE TO DISCONNECT THE PLUG WHENEVER I DO ANYTHING. Just continuing my education.

[rancher1913]

most motors will not start by turning the blade, yes the older ones would but they have not been around for ages. the airplanes and cars that started with a crank usually had a magneto system and not the battery and coils modern stuff have. if it has a key and said key is in the off position then the motor is grounded and will not start. should you play it safe and pull the plug wire--absolutely. look for the rubber thingy that protrudes from the engine, usually between the fuel tank and air breather, might have a "t" tip to give you better grip for pulling, it will be very snug so it will require a fair amount of pull. do not grab with pliers as you can break the spark plug. search YouTube, thats what a lot of back yard mechanics do, I had to watch a video of how to tension the tracks on a small trackhoe as I had no manual for it.

[Elroy]

Yes,you are turning the magnito when you turn the crank,and it don't take much of a turn,and that spark can jump as well,so make sure the terminal is not even close to the plug.It may be better just to pull the plug..We used to check to see if our lawnmowers,or dirt bike were firing by holding the wire,and giving the motor a little kick.We hated doing it,and we had a political incorrect term for doing it having to do with a certain European nation.Every once in a while we could trick some one younger into doing it,and that was good for a laugh.It happened to us all ,so I chalk it up to more of a rite of passage than cruelty.LOL

[Doubles Shooter]

I had a 1970 Bolens snowoibile with a 440cc Kiekhaefer marine engine in it. 60,000volt CD ignition. I brought it in for an ignition problem. My dealer laid the surface gap plugs in the motor to check the spark. He braced his hand on the motor and didn't pull the rope 3".It knocked him flat on his back. Yep, They will fire.

[mdi]

I have worked on gasoline engines for 60 years, and a few magneto fired engines (like small single cylinder gas engines where the magneto is built into the fly wheel). But I have worked on many more that didn't (every engine that used a battery ignition). Ever push start a car with a dead battery? Very, very little voltage is needed to get one running. But back to the lawn mower; yes it is possible for a lawn mower to start when working on, turning, the blade. The magnet in the fly wheel only needs to pass the coil once to produce a voltage/spark. It is remote, but still possible. Just pull the spark plug wire and be sure (pull the plug wire and keep it away from your extremities. Possible surprise. BTDT:eek:)

[alfadan]

I cant remember if the hand lever grounds the magneto or only puts a brake on the flywheel?

[45-70 Chevroner]

This is actually a liability thing, and you can blame the lawyers for this. I'm 79 years old and I'm not stupid. I have been mowing my lawns for at least 55 years and have owned almost ever brand of mower available. It probably wasn't until the 1980's that warnings started showing up in instructions for tools and power equipment. I have never disconnected a spark plug on a mower to take the blade off for sharpening. To begin with you can't start a mower by turning the blade, mainly because you can't turn it fast enough by hand. The pullcord system has a gear ratio that allows the motor to turn fast enough to start. One other thing, when you take the blade off the mower you don't turn the blade, you hold the blade with one gloved hand and use a wrench to take the nut off with the other. Sorry guys but you can see that I disagree with you. There are a lot of dangers with power equipment but that is not one of them. Sorry for the rant too.

[murf205]

Be on the safe side or you could loose your trigger finger. (gasp). I was looking at a 25 hp Mercury OB motor one day and it was about 25 degrees. I pulled the rope to just check the compression and as soon as the piston flopped over 1 time , it started. The guy who owned it, and me, liked to have crippled each other trying to hit the kill switch!

[gwpercle]

Changing the blade .... Disconnect The Plug .
You can start a mower just like an old car with a crank start , or an old timey airplane ...by spinning the propeller blade or push starting a manual transmission car ...(man I'm showing my age) . You can crank the mower by turning the shaft/blade ...only trouble is you can't let go fast enough ... and the blade does a number on your soft body parts .
Don't be acting the fool ...unhook the plug wire .
Gary

[Conditor22]

The older lawnmowers motors were "always-on" electrically, they had a tab you pushed against the sparkplug to shut them off. The odds of starting a modern lawnmower by turning the blade by hand is extremely slight. First, you have to turn it fast enough to develop a spark, second the "kill switch" has to be disengaged (usually by gripping one of the handles)

I like to check the condition of the spark plug every time I sharpen my mower (maybe a couple of times a year).

[redneck1]

This is actually a liability thing, and you can blame the lawyers for this. I'm 79 years old and I'm not stupid. I have been mowing my lawns for at least 55 years and have owned almost ever brand of mower available. It probably wasn't until the 1980's that warnings started showing up in instructions for tools and power equipment. I have never disconnected a spark plug on a mower to take the blade off for sharpening. To begin with you can't start a mower by turning the blade, mainly because you can't turn it fast enough by hand. The pullcord system has a gear ratio that allows the motor to turn fast enough to start. One other thing, when you take the blade off the mower you don't turn the blade, you hold the blade with one gloved hand and use a wrench to take the nut off with the other. Sorry guys but you can see that I disagree with you. There are a lot of dangers with power equipment but that is not one of them. Sorry for the rant too.


I've replaced more then a few broken pull ropes and haven't ever seen any type of " gear " system in a recoil starter
A simple dog clutch and spring yes , but never a " gear " system .
You should pull one apart sometime and see for yourself

I tend to agree that starting a engine while changing a blade is very un likely , but to think it can't happen is not very conducive to keeping hands and fingers .
Playing it smart and taking ten seconds to pull a plug wire isn't exactly a huge burden.

I pull the plug wire myself .

One thing a person also needs to watch out for when replacing blades with hand tools is kick back if you bring the engine up on the compression cycle .. it might not cause you to lose and digits
But you can still get sliced up a bit or a good bruise .

[Patrick L]

Thanks for all the replies. This was quite educational!

[redneck1]

Duplicate post ..

[Elroy]

The old Lawnboys ,and some of the 4 stroke ones had teeth on the flywheel,and they did use actual nylon gears.It doesn't take a fast turn on the flywheel to make spark,but it is probably true that the chance of such a low compression motor starting from just a little accidental turn is slim to none,but I still don't risk it.I had an old Ford pickup that I rebuilt an engine on,and I put a kit from Petronics that replaced the point system with an electronic ignition,and that thing could sit overnight,and would hit as soon as the key was turned on without even turning the motor.

[gumbo333]

Wow! I turn the key off on my riding mowers when I take the blades off to sharpen. 2 bangers, never take a plug wire off. If your mower has a Kohler engine it probably won't run with the key on.

[rockrat]

If your riding mower is like mine, it uses a magnetic clutch between the engine and mower deck, so the mower is disconnected from the engine.

[flyingmonkey35]

This is actually a liability thing, and you can blame the lawyers for this. I'm 79 years old and I'm not stupid. I have been mowing my lawns for at least 55 years and have owned almost ever brand of mower available. It probably wasn't until the 1980's that warnings started showing up in instructions for tools and power equipment. I have never disconnected a spark plug on a mower to take the blade off for sharpening. To begin with you can't start a mower by turning the blade, mainly because you can't turn it fast enough by hand. The pullcord system has a gear ratio that allows the motor to turn fast enough to start. One other thing, when you take the blade off the mower you don't turn the blade, you hold the blade with one gloved hand and use a wrench to take the nut off with the other. Sorry guys but you can see that I disagree with you. There are a lot of dangers with power equipment but that is not one of them. Sorry for the rant too.You ever hear of the guy who took a air wrench to the bolt holding on to the blade?

Vroom. Redneck haircut.

I worked in a ER for 8 years and I saw some oddball stuff come in.

My best was the guy weed whacking with baling wire. And straight in the eyeball it went.



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[Patrick L]

I'm dealing with basic Toro and Craftsman self propelled walking mowers. Typical suburban lawn commando stuff. I'm gonna walk while I mow as long as I'm able��. I'm only 54.

[Winger Ed.]

Not so much starting when your in there fumbling around with the blade as they can 'pop'.

You could be able to turn it through the compression stroke, the plug can fire,
and it could 'pop' like a back fire, and whap! ya real good.

[rancher1913]

anybody ever hand started an old johnny popper. some days you could walk right up, spin the flywheel and away you go but if you ever really needed it to start, you would wear yourself out spinning the dang thing. the pony motor sure was nice when it came along.

[Petrol & Powder]

I think it has all been said but I'll toss this out there as well.

A lot of the old Briggs & Stratton engines that had the throttle control directly on the carburetor had a "Stop" position for that lever to shut off the engine. There was a detent for that "Stop" position and in that position a wire would ground out the magneto, stopping the engine. After they got a little age and wear on them, about 90% of those controls would move beyond the stop position. When the lever was moved beyond the "Stop" position - the engine would still run. So it would look as if the lever was in the Stop position but the engine would run just fine.
Since we're dealing with things that can cause serious and often permanent damage to humans - a little extra caution is a good idea. Kind of like pulling the bolt completely out of a rifle before sticking your eyeball over the muzzle to inspect the bore. Or putting jacks stands under a car supported by jacks before getting under the car.

[charlie b]

Pull the plug. It is simple to do and there is really no reason not to do it.

It is hard for me to believe that anyone working on an engine like this would not remove the plug wire. Probably have been a couple of Darwin awards for stuff like this.

Yes, the principle for pull starting a lawn mower is exactly the same as for the old aircraft engines, crank start cars, or kick start motorcycles. The pull start is just easier to use on these lower compression engines.

Yes, some engines have safety systems. Are you sure it works?

Yes, you are not supposed to move the blade when removing it. But, when the bolt is stubborn and you are pulling hard with both hands engaged can you guarantee that the blade won't move? FWIW, there used to be some 'jigs' out there just for this. Have not seen one for a while.

Yes, I have had an engine kick over with the plug wire removed. I had forgotten to put the plug wire back on and pulled the rope. Engine kicked a couple of times and stopped. The rubber cap was old and was left touching the side of the plug. The spark ignition is a very high voltage which can do strange things.

Yes, this is a rare thing. The engine has to be in the proper rotation spot. A charge of fuel/air mix has to be in the chamber and valves closed. The condenser has to be charged. How (un)lucky do you feel?

Last, please do not go out and 'hand prop' your mower to see if it will start. When hand propping an aircraft there is a very specific method used to make sure there are no injuries, including a particular way to grab the prop and swing. Hand crank cars required a specific method of holding the crank so that 'kick back' would not break bones. Kick start of large motorcycle engines sometimes resulted in leg injuries.

[Elkins45]

I cant remember if the hand lever grounds the magneto or only puts a brake on the flywheel?

It probably varies by maker. On my Lawn Boy there's a ground switch as well as a brake.


This is actually a liability thing, and you can blame the lawyers for this. I'm 79 years old and I'm not stupid. I have been mowing my lawns for at least 55 years and have owned almost ever brand of mower available. It probably wasn't until the 1980's that warnings started showing up in instructions for tools and power equipment. I have never disconnected a spark plug on a mower to take the blade off for sharpening. To begin with you can't start a mower by turning the blade, mainly because you can't turn it fast enough by hand. The pullcord system has a gear ratio that allows the motor to turn fast enough to start. One other thing, when you take the blade off the mower you don't turn the blade, you hold the blade with one gloved hand and use a wrench to take the nut off with the other. Sorry guys but you can see that I disagree with you. There are a lot of dangers with power equipment but that is not one of them. Sorry for the rant too.

I must disagree about not being able to spin them fast enough to start. There's no gear ratio built into wrapping a rope around the flywheel and giving it a yank. I have personally pull started my old Johnson V4 85HP outboard by wrapping a rope around the flywheel and giving it a yank. I did it literally dozens of times because the starter on that motor would overheat if you cranked it more than a few seconds. The circumference of a big flywheel actually slows the rate of spin, it just makes it easier to pull, so if slow flywheel spin can start it then so can a slow spin of a blade attached directly to the crankshaft.


Wow! I turn the key off on my riding mowers when I take the blades off to sharpen. 2 bangers, never take a plug wire off. If your mower has a Kohler engine it probably won't run with the key on.

I doubt anybody does on a rider where the deck and the motor are separate things. As long as the key is off you're safe. I'm pretty sure the OP is talking about a push mower where the blade is bolted directly to the crankshaft.

[JoeJames]

When I was growing up we had a Mighty Mite tractor. It did not have a starter but just had a pulley wheel with a notch on it on the front of the engine at the end of the crankshaft. You took a knotted rope and wound it around the pulley wheel to start it after the gas cock was turned on. Learned to be very cautious about backfires which could pull your hand into the pulley wheel.

[gwrench]

I agree with those that say it's highly unlikely to start, but it's not worth a risk. Modern stuff has safety stuff that SHOULD prevent a start if it hasn't been defeated or changed. I can understand why companies say you should never do it. They just can't have the liability.

Sometimes an engine will surprise you how easily it will start.

Years ago I used to ride dirt bikes a lot, usually miles from anywhere in the mountains of Colorado. One time our group was getting ready to start riding after a few minutes of rest on the side of a really steep hill. It was in such an awkward spot I couldn't get a good kick on my bike (Yamaha IT 490) so I kinda wiggled my foot and ankle to get it past compression and the thing started right up! It never ever started easily but it did that time! My BIL thought I was showing off and just shook his head.

[mdi]

In my entire time working on small engines that used a pull starter, I have never seen a gear reduction/multiplication type pull starter. Every one was a system of "dogs" or similar system that when pulled would engage the flywheel. and when released, would retract free from the flywheel. Speed of magneto is a minor factor as physics don't pay much attention to RPM. While remote, all that is needed to produce voltage is for the permanent magnets in the flywheel to pass the induction coil. Early hot rod engines used magnetos because they were lighter and more efficient than a battery, ignition coil, points system...

[Elkins45]

When I was little (1960s) my dad had a Montgomery Ward mower that had a folding crank on the cowel above the flywheel. You would unfold it and give it several cranks which wound a coil spring, then fold it back town and twist a trigger on the side that released the spring and cranked the engine. As I recall it gave the engine a much faster spin that you could ever get using a pull rope starter.

[bayjoe]

You can check if you're getting spark by holding on to the wire while someone pulls the rope. Guaranteed to make your eyes sparkle!!!!!

[Plate plinker]

You can check if you're getting spark by holding on to the wire while someone pulls the rope. Guaranteed to make your eyes sparkle!!!!!

Co worker and I were lit up by a small engine motor with a crack in the plug wire. We were checking for spark and both had a turn at it. Spark was there all right!

At least I was first and not dumb enough to be the verifier.

[JRLesan]

Condenser has to be charged??? Condenser does not 'hold' the charge in the system as some folks assume...

[john.k]

My old rider mower needs a big electric drill to start,thats what I use anyway.....But that reminds me of the crazy old woman used to live nextdoor.....she was running up and down the slope at the back of the house .....trying to push start the mower....Or her indoors says .....yer always fooling with old motorbikes ,why dont ya fix that poor womans mower?....I says.....better fix our own first ,and I aint done that yet.

[samari46]

Reminds me of a certain old 1920's bulldozer that had the crank handle at the front of the engine. If you weren't paying any attention to what the engine was doing and it kicked over, usually a broken arm was the result. But once started ran like a top. real PITA to start in winter when the roads needed to be plowed. Frank

[john.k]

The Caterpillar D7 s had the crankhandle up through the top of the engine hood.....Half swing at a time ,but if the donkey was in good tune ,you d have black smoke in no time.....Use to love the sound of the old D7s working ,made me want to buy one when I was six.

[mdi]

You can check if you're getting spark by holding on to the wire while someone pulls the rope. Guaranteed to make your eyes sparkle!!!!!
When teaching me small engine troubleshooting my Dad showed me how to test for ignition spark. I removed the plug, but he suggested I hold the plug wire while he pulled the starter rope (and this was before retractable starters were popular, just a fat pulley shaped "cup" with a slot on the edge on the end of the crankshaft. Tie a knot in the end of a cord/rope and wound the rope around the cup and pull, and don't stand behind Dad when he's starting the lawnmower). Taught me a few things; even on a small engine, ignition voltage can very high ("shockingly high"), think about something before asking/trusting Dad, and always have clean underwear available. I was mebbe 12...

[nelsonted1]

I've pushed the blade around and have had it finish the turn with a big lurch. If the blade hit me it would hurt.

[nelsonted1]

The reason the world war two videos showed mechanics turning an engine over by slowly pushing the propellers was the oil would settle into the lowest cylinders on the rotary engines. If the engines were turned over with the oil at the bottom the compression may have bent the connecting rods. Long after beginning the use of an auxiliary starter- the big box pushed out to the plane engine along with the guy with the fire extinguisher- the propellers were turned over by hand first.

https://aviation.stackexchange.com/questions/29561/why-do-ground-crews-rotate-a-radial-engines-propeller-before-a-motor-cartridge

[country gent]

A motor can start by being turned over and may do so easily depending where it is in the stroke. A lot of motors were started by hand back in the day. The old hit and miss engines, early John Deere tractors were turned over by pulling the flywheels on them.( bigger JDs had a compression release on the cylinders) Cars and others had a hand crank that allowed the motor to be turned over. There was a art to this also as the crank handle was cupped in the hand not held with the thumb wrapped around, saved a lot of broken thumbs when a back fire occurred. Kick starters were also in use. Bigger motors may have had a spark advance also.
While it is rare for a motor to start with a wrench it is possible even more so a jump or lurch from compression stroke. A freshly sharpened blade can make a nasty cut if your hit by it.

[nelsonted1]

Grandpa and I were working on his lawn mower. He told.me to hold the wire while he pulled it over. Ziiiing! Ow!....Then he told me when I get older people will be trying to talk me into things. Don't let them do it. Use my brain first. Then he said "here, hold this"
When I was in junior high the geniuses were trying to talk me into smoking. Every time they came around I'd feel a jolt of electric zing up my arm.