G'day Unless I am much mistaken, I think you will find your OS 25 FP has an air bleed carby. This is NOT a problem but it is a bit different.

As Jeffie said, if you start with the high speed needle about 2 to 3 turns open it will start and be rich. The idle adjustment on the air bleed carbys is a small screw which partially blocks a hole in the front of the carby body. The initial setting is to have the hole half covered by the screw. BUT, unlike twin needle carbys, the air bleed adjustment works in reverse. To make the idle mixture richer you CLOSE the hole by screwing the screw clockwise and to lean the mixture you open it.

Most FP engines had ABx cylinder/piston sets but some early ones had iron/steel. The ABx type should be run at a rich two stroke for running in usually for just a few tanks, the iron/steel pistons need a long running in of very rich short runs with the mixture gradually leaned out over about an hour or so.

There is nothing wrong with a well designed air bleed carby. My Saito 40 has one and it is one of my best running engines in a very aerobatic model.