[C320-list] C320 Prop Question

[Frank Reid]

I'm in the process of buying a Catalina 320 and the current owner changed the prop from the 2 blade 13 inch (measured tip to tip) with a 3 blade 15 inch.  During our sea trial while under high RPM the engine was expelling unused fuel out the exhaust.  If we place the transmission in neutral and rev'd it up there was no fuel expelling.  I suspect the prop is the wrong size.  It has the 30F engine.  Does anyone know for sure what size 3 blade prop is correct?