Most engines and pumps have a sweet spot and if you cant get a good read on the RPM's try this. with the unit up and running with a glove or shop towel cover the intake . Now dont let the rag get sucked insp fold it over about 6 times and cover intake. Run the engine up and down to where it sounds like its not over speeding and is getting good airflow. Now see what you have for vacuum. Will it improve with lower or higher RPM's ?

I was a novice when I started and assumed that wide open on the motor had to be better for the vacuum levels. Thurs out with a 9 HP engine just above idle was best for the pump also. Piston pumps dont rely on speed to seal the internals. Vane pumps need a certain RPM tp get the vanes to seal and yet over speeding hurts them because of the heat generated with less cooling air. Yes Gast vane pumps can be dead headed BUT that wont really show its true capability. Plus whit a oil vane pump you want the oil to lubricate the vanes against the drum surface and over speeding will cause them to hydroplane and not seal as well.