Carb help

My opinion of the "cavitate feeling". I believe the engine is running lean, as Frnknsteen noted. Another possibility is too much timing, either attributable to the vacuum advance and mechanical advance. If the carb is just open slightly, it is possible that the carbs transition circuit is to blame. As said previously, look at the spark plugs to get an idea how the engine is running. Paper white center electrode insulator indicates a lean condition. If so, try increasing the primary one step richer. Or increase the carb's fuel kevel slightly, giving the effect of increasing (richening) the fuel mixture. Disconnect the vacuum advance and see the effect. Another possibility is there is too much cam overlap allowing exhaust gas dilution of the incoming fuel charge making it harder to ignite promoting a miss fire condition and a stumbling feeling....especially in the range noted. I'm sure others will have their own opinions and advice.
Bob Renton