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rusty70f100

Two stroke weirdness

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Some of you may know that in the past I've converted 2-stroke lawn boy mowers to E85, and have experimented with various oils.  The best oil I found was Klotz Benol, and I've had good results with Klotz Super Techniplate.

 

Anyway, some time ago I got another free Lawn Boy mower.  You can never have enough of them.  ;D

 

I did the natural thing, converting it to E85.  I drilled out the jet, and adjusted the idle.  Runs great.  So I take it out back and mow a small area with some tall grass with it.  About half way through mowing, it starts running real smooth.  I mean, like, model airplane smooth, speeding up, and making gobs of power.  I could go through real heavy grass without slowing it down.  Also, when this happened, it stopped making any form of smoke.  Gee, I can live with this.

 

The problem came when I got done and went to turn it off.  Simply put, it didn't want to turn off.  I grounded out the coil with my (temporary) jumper wire.  Buzzzz... kept running like nothing had happened.  So I figured my jumper wire came off the coil.  Alright, well there's a big plastic guard on top of the coil... I'll just pull the plug wire off the spark plug.  That will stop it.  So I pull the plug wire...  Buzzzz...  No change whatsoever.

 

It took me about 1/10th of a second to realize what was happening.  The plug had gotten so hot that it was acting like a glow plug.  I now had a nicely running Lawn Boy model airplane engine.  ;D

 

The problem was, how in the %@#$ do I stop it?  First I tried disconnecting the fuel to the carburetor, figuring it's got to stop without fuel.  After I did that and held my finger over the hole in the fuel tank for a few seconds, visions of fuel on the muffler bursting into flames or the engine exploding and taking my hand with it convinced me that was probably not the thing to do.  So I quick plugged the line back in.

 

I stood there for about 20 more seconds, mower still running with the plug wire off and several inches away from the plug.  Then I saw the choke control that this carburetor thankfully had.  I push it to full choke.  Buh.  Mower quit running.

 

I waited several hours for it to cool down, then removed the plug.  The very end of the grounding strap is blue, otherwise normal.  It still has good compression.  I haven't run it afterward though.  I figure it was running too lean, and / or had too hot a plug in it.  The plug is one of those Champion easy-start plugs, heat range 14.  The fuel in use at the time was E85 mixed 16:1 with Klotz Benol.

 

Thoughts?

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The only thing I can think of is the engine was Dieseling. I have seen this in higher compression small engines in rare occasions. Usually related to higher temperatures. Prevention is a good question.

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"surface ignition" (not sure about correct translation)- ignition, started from one very hot part in combustion chamber, usually tip of spark plug.

 

when it starts, this tip do not become colder and this type of ignition is "stable".

only one way to turn off engine - cut off fuel :-)

 

so, at first check air/fuel mixture (maybe it is too lean), and type of spark plug - maybe it is too hot for this engine.

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Sounds like 'dieseling' for sure.  Possibly a glowing flake of carbon on the piston or combustion chamber?  You said this was a free mower, so maybe the previous owner didn't take the best care of it and/or used the wrong oil which led it to carbon up?

 

Funny you mention 'smooth like a model airplane motor' as that is how many of those things run, too...glowplug equipped.  Guess you lucked out and what ever 'timing' was happening from the auto ignition was about what the engine wanted for max power.

 

Is that the normal oil ratio?  The extra oil will definitely lower the octane, suspect it might lower the auto-ignition point, too.  More oil also means less gas for a given volume, so you're actually leaning out the fuel, too.  Guess if it runs really good, you could just pull the spark plug and replace it with a glow plug.

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