Am I experiencing pre-ignition?

This is related to my experimenting with E85 fuel, so this is not related to normal operation on gasoline.

As I've mentioned before, I've been running my bike on E85 and over time I've been working out the kinks. I've gotten it to run just as well except for one problem. I think I'm having pre-ignition.

When I start out on the bike, everything works fine. The idle works fine and the engine can be killed instantly.

Then after I've been riding for about 5 minutes, the engine gets to a point where when I disengage the clutch, the idle speed is way out of control. It revs extremely fast and the kill switch won't kill it. The only way to kill the engine is to turn on the choke and turn the fuel off(I have to do this quickly so I don't eff up my engine). I know that the kill switch isn't the problem because I once yanked off the plug wire and the engine somehow kept going.

I know it's not the throttle and I know it's not an air leak. I'm using an NGK B6HS spark plug.

My only guess is that the engine gets hot enough after a few minutes to light the fuel.

Anyone have any suggestions as to how I can solve this?

^^ Thank you for your insightful input.

Oh okay, I guess its dieseling.

I don't think it's the spark plug because I've tried different plugs. It might be carbon deposits, but the last time I opened the head there wasn't much carbon. I'll still open it up and take a look though.

I am thinking it might be caused by running lean. I richened the mix a little bit and I didn't have the dieseling problem anymore during my 25 minute ride. Though it did start dieseling right as I got home. I don't want to run it any richer because it seems to decrease power.

I haven't taken any temperature readings. Perhaps I will.

How would I check to see if it's leaking air into the crankcase? The intake manifold is gasketed and sealed, and I don't see how it would lead to anywhere else other than the jug.

I drilled the jet with a #67 drill bit, which is a 15% increase. I also have the c-clip at the 4th position, which is where I've averted most of the dieseling but it seemed to start doing it again after 25 minutes. I may try drilling it with a #60(40% from original size) since that's the only other small drill bit I could find. Optimally, I'd like to find a #64 bit which would be a 30% increase which is what I think the size is that's usually used.