Well I finally had the car running really good by advancing the timing a lot and by leaning outh the carbs, so I decided to take the car on the freeway for the first time and after flooring it for while (about 85mph) it kind of started to miss, then it back fired through the exhaust. After that I got off the freeway and it was runnning like crap, So I took it home and checked the plugs and the 3 back ones on each side were badly fouled.

What could cause this?
how do I fix this?

thanks, will