998 bigfoot 50hp max rpms not reached

[wheelers4life]

I just purchased this pontoon has a mercury bigfoot 50hp. 50ELPT. It was in the shop when I bought it. New fuel lines, oil and plug changed, carbs rebuilt, new fuel lines and carbs sync. It does have a 4 blade prop. The shop said it was the norm replacement forbthat outboard. The outboard runs great except a little vibration in very low rpms. But I can only reach 4500 rpms. If i remember correctly the shoo said 5500 rmps at wot. I'm thinking about taking it to a different shop. But would like some ideas to look at before i spend more money on this boat. the prop is a solas md 19 12, then on the other side of the prop has these numbers 1413-133-13
thanks