I've replaced all of my rod bearings, and I still hear a knocking noise at high speed but not at low speed,and if I was to install a bigger air spacer between the intake and carb would that give me more horsepower or not,so if anyone could tell me what else the knocking could be and how can you check which piston is knocking.

What is the year and size of the endine ? Was the crankshaft measured for wear,before installing the new bearings?

Mechanic's stethoscope.