Sounds more like a vacuum leak to me. I've had timing 180 off plenty of times, and I've never had it backfire through the exhaust.
I have had a nice vacuum leak on the EGR valve, which would do exactly what he said. The car would start, and run, but if you got it over 2k rpm it would backfire.
Piston/valve theory sounds extreme, I don't think it would be that.
On a side note: check to make sure you have the kit to prevent the power valve from being blown. If you don't have it, you need a new power valve.
Keep us posted. Would like to hear what it is/was. Good luck.
[ 29 April 2002, 03:07 PM: Message edited by: kenny ]