Dear Readers:

No letter for this article, so I will relate an actual neighborhood happening that I would have called a tall tale had it come in a letter.  But it is not a tall tale as it happened right in front of me in my driveway.Lawnmower

It started when I heard one of my neighbors fire up his lawn mower.  It sounded like it was running awful fast, but I was deep into my own project, so I ignored his mowing.  About a half hour later, he came pushing the mower up my driveway to my little workshop.  It was still running way too fast, and now it was also knocking fast and loud.  He hollered over the noise of the mower that he couldn’t shut it off, as he worked the kill switch off and on.  Then he really blew my mind when he reached down and showed me that he had pulled the sparkplug wire off of the sparkplug--and it was still RUNNING way too fast.  I had never seen a gas lawn mower engine run with the sparkplug wire disconnected.  After standing there for a moment with my lower jaw down aroundt my shoelaces, I grabbed a sparkplug socket and got ready to pull the sparkplug. As I got close to the engine I could feel the heat radiating from the engine.  As I reached down to unscrew the sparkplug, the engine finally shuddered to a stop.  I pulled the sparkplug out anyway.  We were standing there looking at the mower in disbelief when it started to dawn on me what had happened.  My neighbor spoke first and said that the throttle cable to the carburetor had broken the last time he used the mower, and he had not gotten around to fixing it, and he thought he could mow with it that way.  I removed the air cleaner--almost burning my hand--and found the throttle jammed wide open.  The governor could not slow the engine down.  I reached down and picked up the socket with the sparkplug still in it and did burn my hand.  The sparkplug was so hot it had heated up the socket to the point where the socket burned my hand.  We backed away from the mower to let it cool down while we popped a couple of cool ones to cool ourselves down and to treat my burned hand.  Purely for medical purposes.  At that point I was able to put together what had happened.

With the throttle jammed wide open, the engine was getting way more fuel than the engine was designed to handle.  The excess fuel had caused the engine to get way too hot.  It got so hot that when the piston compressed the extra fuel, it would ignite just like a diesel ignites its fuel--by the heat of compression.  Like a diesel, the mower engine was so hot it didn’t need a sparkplug!

Lawnmower EngineNow a lawn mower gas engine is not designed heavy enough to run like a diesel, but it did and did not blow up, which is what should have happened.  A diesel engine requires a strong steel connecting-rod between the piston and the crankshaft to withstand the loads placed on it.  The mower engine has an aluminum connecting rod between the piston and the crankshaft, and it should have been looking like a pretzel and broken!  It wasn’t!

Now for the really strange part.  After letting it cool for an hour, I checked for spark by cranking the engine over.  There was no spark, because the engine had gotten so hot it melted the insulation on the coil wire leading to the kill-switch.  After taping up the wire AND un-jamming the throttle so that the governor could work properly, the engine started!  Not only did it start, there was no knocking.  I told my neighbor that the engine had gotten so hot that the piston rings had probably lost tension, and as the engine warmed back up, it would probably smoke and use oil.  With that, my neighbor went home with a running lawn mower so he could finish mowing.

Had I not seen it with my own eyes, I would never have believed that a gas lawnmower would run with the sparkplug disconnected.  There was another neighbor with me, so there was another witness. 

No longer will I call them Briggs & Scrap Iron Engines!

Herr Professor Nuzanbolts