Hi, no such thing as a Rebel prop for the I-3 and going from a 19 inch to a 17 inch will not increase RPM. The I-3 wants to see 5000-5200 at WOT with normal load and it must be a very light weight boat if a 19 inch whatever pulled 5400 and 52MPH. I think once you have confirmed that he has done nothing wrong, the plugs are the correct QC12PEPs and index right, and the engine is reaching correct temp at low speed, than the fact that you are seeing 5100 now with the 17 inch at least it is in its idel rev range. Did you get a history report you could post, because the EMM will keep a history of RPM and temps.

Cheers,

Huey.