Agree with RR an auxiliary gauge on the carb inlet line is an excellent diagnostic. A lot of the things mentioned (fuel pump, pushrod, vapor lock, gas cap) would show up as low fuel pressure. If the fuel pressure holds pretty steady then drops off as you approach 5000 RPM then you have narrowed down the problem. If not, keep looking. Your local AP store should have a suitable gauge. If you were nearby I'd loan you mine.