(From a post elsewhere)


"The Nature of Octane Numbers"
Some people believe that "OCTANE" is the ability of a gasoline or fuel to be more explosive - but the OPPOSITE is true.

The true nature of octane is:

"The ability of a given fuel to resist pre-igniton due to temperature and pressure"

And where that becomes important in high performance engines is because those machines exploit very high compression ratios (therefore more heat).

Visualise this - as gasses are compressed, the temperature of them rises, and add TO that the already high temperature of the engine itself. These two factors combine to set the absolute point where combustion will occur EVEN IF NO SPARK IS APPLIED.

To combat this, fuel additives are available to RAISE the octane (it's ability to NOT light off spontaniously from heat and pressure) so that ignition only happens when it is SUPPOSED to.

-This is why VERY high octane fuels are used in high-compression race engines.

Alternatively (and I saw this a lot when I was forced to fill up when visiting Mexico) if a very LOW octane fuel is used in a car - it can cause an unbelievable amount of "DIESELING".

Diesel engines are designed to have so much heat from pressure, that they are timed by adding the fuel at the top of the compression stroke. If the cylinders are at running temperature, or if they have a "HOTSPOT" (in the form of a glowplug) then the fuel ignites as soon as it enters the cylinders and mixes with the hot compressed air inside...

NOW- When an engine (gasoline type) runs backwards, it's usually because the fuel lit off before the piston reached the top of it's stroke. Often they run in the direction they are supposed to, but won't shut down because they are still drawing fuel into themselves.

I think from what I have read here, that is what your engine is doing - if it REALLY ran "backwards" it would back-fire through the aircleaner because the cylinders would be pumping in the wrong direction. And if that was true, only exhaust gasses would be drawn into the cylinders.
It can't run that way....
It's last kick might make it look like it, but that's as far as it goes.

And on a machine with a clutch you CAN set the brake and let the clutch out in gear to arrest it!

I would recommend trying a different gas station, or a higher grade of fuel as a first step.

There are other factors though -

If the upper chamber of the engines cylinders have collected enough crud, it can change the compression ratio. Char can also maintain a "HOTSPOT" much like a diesel glow plug.

One thing I have periodically done (believe it or not) is to dribble a little water down the carbuerettor throat while maintaining a high idle speed, so that the cylinders "Steam Clean" themselves. (have you ever noticed how shiny clean the upper chambers are in an engine that was sucking in radiator coolant?)

You should be very careful if you try that, but it can work like a charm.