Old gas and detonation

Went for a 30-minute ride in a field near my house yesterday. Had a couple of gallons of mix in a cheap plastic Blitz gas can. It is seriously about a five-month-old mix. Used to be 91-octane with maxima 927 in it, probably with up to 10 percent crapanol in it too, since all gasoline is hard to find around here. Bike started fine and warmed up fine, but once warmed up, whenever it was under a load, I got some obvious knocking. Relatively cool, about 57 degrees at the time. Engine was smooth at all settings, other than the knocking at mid throttle under a load. I could back off or open the throttle and dump a bunch of fuel and it went away. So I finished my ride using a lot or a little throttle to stay away from the detonation and went home. I was having a blast since I was actually jumping a couple of hills on this little track in the field (love hearing that 7" of elsinore suspension bang when the forks bottom out--NOT) and I normally try to stay as close to terra firma as possible.
Now I know gas in plastic containers is supposed to lose octane rating fairly quickly. And I'm an idiot for not thinking about that and getting rid of the old mix. Normally I never have any pre-mix sitting around, but was supposed to go riding in October and we got rained out.
My best guess--and the obvious place to start--is to dump the fuel in my container and my tank and get some fresh fuel (spend the extra 20 minutes to fine 100 percent gas, too).
Anyone ever experienced detonation, with no other symptom, from using old gas??

Yep clear detonation sound, no mistaking it. I think it is a combination of factors: 1) the old gas which is probably about 60 octane and contains water by now; and 2) I somehow put a rubber fuel line on it last summer instead of neoprene or vinyl. When I went to drain the fuel out of the tank last night, the interior of the hose is deteriorating into black goo. Of course my carb bowl had crap in it from this. Emptied the tank, put on new fuel line. Will clean the carb tonight or tomorrow night and check the plug. Then I'll get some fresh, non-ethanol, 91 octane and mix up a fresh batch. I'm pretty cognizant of sounds my engine makes, so I'm pretty sure I backed off before any piston damage occurred, but it wouldn't hurt me to pull the head and check it out too. Now where in the heck I got a piece of rubber hose, I'll never know......

Cleaned carb, checked piston, filled with fresh mix. Bike runs great now.
Piston looked good, although it's about time for a light hone and a set of rings or a new slug. Have about 30 hours on this top end so far.

Plug was good. It has about 30 hours too, so I might as well change it. I have about 20 extra new plugs but you'd think they were made of gold the way I hang on to them.