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)

[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.