[Stubby79]

If not pre-ignition, then why the stuttering under load in lean burn? Why seem less with better gas in it? Unless that was entirely in my head.

On the other hand, if it was firing the spark plugs a bit too early, due to the cam chain, I suppose that might cause it.

Hows about the non-indexed spark plugs? If that caused some of the mixture to burn, but not all, the rest of it might get ignited by the build up of pressure and suddenly what's left self-ignites right before TDC and causes a jerk, like, presumably the timing being too far advanced.

Of course, I haven't heard any pinging in the exhaust. Maybe you won't with engines that don't open the exhaust valve until BDC. Didn't notice it on my Metro with the XFI cam, no matter how low of an RPM you got when upshifting.

*scratches head* Now I'm just confusing myself, or at least feel like I'm missing something. We'll see what happens as I check off each variable until it goes away. With the fuel, that might take a month or two before I can fill it with all fresh...