Hi!
As has been said earlier the OS FP series of engines uses an airbleed carb.
That means the carb has a highspeed needle...and a lowspeed screw . The lowspeed screw is a small screw that regulates the incoming air through a small (around 2mm in diameter) round hole up front on the carb.
The standard setting for this low speed screw is to have the hole half way open, but most of the time a little more is needed.

All OS air bleed carbs/engines are very reliable and very easy to set and they are excellent engines for scale airplanes and twins where outer most reliability is required.

Any enlarging of the air bleed hole is not necessary...at least in my 31 years of using these engines have I ever encountered such an engine.

Regards!
Jan K
Sweden