I just acquired an FP 25 without a carb.
I will make a venturi but I'm not sure of the inside diameter.
The spray bar is .133 inch diameter.

Any suggestions for the venturi size?

   Bob Z.

Despite what Tower says on their web site, every FP 20/25 venturi I've bought from them measures 6.5mm ID.

   And that is .256". That's perfect for the 20 and marginally too large for the 25, as long as you don't bog it down with excess prop diameter.

     Brett

Brett, can you suggest a reason why the 6.5mm venturi is right for the FP 20 but marginally too big for the 25 which is 20% larger displacement?

Also, can you suggest an ideal ID for the FP 25?

  See the other thread for the full answer.  But who ever said there was a direct relationship between the displacement and the venturi size?

   The correct (as defined as WC-winning) venturi for a PA75 has less choke area than the 20FP, 40VF, PA61, or RO-Jett 61. I am currently running a #5 AWG venturi with a spigot in the RO-Jett and David ran something like a #13 with a spigot to win the WC - WAY less choke area.

  To first approximation, for a given airplane,  you have to suck in the same amount of air regardless of the displacement. A bigger engine can suck that amount of air through a smaller hole.

    Brett