Impeller tuning?

[Digdog]

I bought a Solas CD15/21 impeller but it has a little too much bite despite it being recommended for the stock engine, is there a way to take a little bite out of it? I need to gain 400 rpm

 

[Seadoodoc]

I ran into that on my boat. Lazy... low rpm ....so I had Impros repitch a 255 pitch to a guestimated 275 hp range. Boat went nuts. Actually banging rev wot and super agressive midrange.

 

[Seadoodoc]

1521 ismod engine tall gearing I thought

 

[IDoSeaDoo]

You can change pitch yourself. Go to Harbor Freight and get one of the larger adjustable wrenches. You'll have to rig up a way to hold the impeller vertical. I think used a bolt through the impeller, held in place by a vice. You will also need impeller pitch gauges. I found a jpg image of them online and had them printed out at a local office max. Use their paper cutter to cut them precisely. Once you have the impeller in the vice, get an initial reading using the pitch gauge. Then clamp the adjustable wrench to the outside of the leading edge and bend down to lower the pitch. Make small changes and check with the gauge often.
There are YouTube videos and forum posts about doing this. I believe I have some pics of this when I had to do it for my boat motor swap (link below).