Today I was on my way back from dallas, which is a 3 hour drive, and I went to pass a truck. I stepped on it and it downshifted and only went to 5,000 RPM or so. When I let off it started making a popping noise every second or two. It sounded like a backfire through the carb. I slowed down and it happened every 5 secs or so as long as I didn't try to accelerate. I would think it was a carb problem, but this is the third carb that has done this. The only thing I can think of is lifter float because it only happens when I accelerate hard past 4,500 RPM. I looked inside the carb and there were no black deposites, and it still looks new. Any ideas?