What else could it be?
 

The 12 HP in my 128 backfires sometimes when I turn it off. So far I have rebuilt the carburetor and adjusted it, set the timing with a timing light, put a new plug in, and adjusted the governor linkage. After all that, it still "popped" today when I turned it off.

Do you have power to the coil when the key is turned off??? sounds silly but there could be spark still after shut down??Is the coil "hot"/powered all the time,this I dont know!!
How hot is it running??? who knows but something has to be igniting the fule after the key is shut off and power removed.maybe!!
Im working on a crapsman for a friend that does this,great big jump out of the seat POP!!

Do it all again ,carb adjustment,timing.
Maybe the points sliped bit.

I know not much help,and Im no expert!!! the expert guys will come along,LOL

Rember when we were kids and we would shut off the key in the old stick shift carburated trucks,pump the gas,leaving it in gear,an then turn the key on to get a big old back fire!!!
the raw fule was pulled through the engine into the exaust where it would ignite!! I blew apart a muffler that way!

Good luck CBH

Jason

How long do you idle it down before you shut it off? If you don't idle it down for at least a minute or two, it'll throw gas in the hot muffler, which can cause that BANG you hear. All of my cubs (except the diesel, of course) will do that unless I let them idle for a bit before shutdown.

There are a lot of different theories to this, one of them is;

When you turn the key off, electricity stops going to the spark plug, but the fuel continues to flow into the combustion chamber, the piston continues to move up and down, the valves continue to open and close until the engine stops turning, thus filling your hot muffler with unburned fuel. The heat from the muffler will cause the fuel to ignite, causing an explosion.

I do let it idle down, but I may have to let it idle more before shutting it down. It doesn't do it all of the time, but my other Cubs have never done it. It's just kind of annoying to not be able to correct it. :bash2:

CBH, My good friend's 129 does it all the time too. My (cough) Murray (cough) does it most of the time. I hear you on the annoying, nothing like getting the bageezus scared out of you as your getting off the thing :biggrin2.gif:

I have only one that will and it burns RICH and I mean rich but it runs soooooo good I'm not screwing with it. Matt you are correct, I idle mine down too. Smart aren't we?

Scott

Oh BTW I bet it burns a little rich.

How is your head gasket? A lot of carbon build up in the head, or a blown head gasket will cause that problem. Usually with a blown head gasket you will see some black soot material on the cylinder below the head.

I like to idle mine too because these kohlers get nice in toasty when it's hot outside.

Today I pulled the head and de-carboned everything, but it was actually more like soot than any carbon build up. Installed with a new head gasket, mowed for about 1/2 hour,... stopped and let it idle,... turned it off ....BANG!
If it is gas getting into a hot muffler, I'm wondering if an exhaust stack style muffler would keep it from banging. What do you think? :bigthink: