Seriously, that’s not normal at all. Are you saying it starts and runs fine but just flattens out in top gear and won’t rev anymore at 160 kph ? First would be check fuel flow. Do you hear your pump running when you turn the key on ? If you’re getting decent fuel flow at the carbs, do a tune up. Check for a clogged air filter. Run some SeaFoam through it. Remove your air filter and make sure both carb slides open when you turn the throttle with engine running.