RE: Seagull Spacewalker II 120 Arf
RJ... in my opinion the large opening of a Yak cowl needs dampering (others call it baffling). There is so much air moving into the cowl that it would take a huge hole to exceed the intake and that is what is needed for the air exit size to exceed the intake size. Otherwise what happens is a high pressure condition exist within the cowl and that pressure changes depending on rpms. That screws with the metering circuit of the carb, which wants a stable atmospheric pressure reference to wit the needles can be set. If the metering circuit is constantly changing its pressure reference... setting them to suit the whole range of flight is difficult if not impossible.
For example, lets say the pressure increases in the cowl at full engine power. That pressure pushes on the anvil of the float needle opening it further causing more fuel to flow so to compensate and get a proper top end adjustment at full power, the HS needle is leaned to suit. At mid throttle the pressure in the cowl starts coming down so less fuel flow so the HS needle is now too lean and even worse at idle because the LS needle must have a proper setting of the HS needle and it doesn't have it. At idle what happens is that the LS needle gets set much to rich to compensate for the too lean HS needle at idle rpms so as to avoid spooling up sag. As you discover... the idle is too rich and if you lean it any, it likely won't spool up without sagging.
Two choices are left... damper the inflow of cowl air or solder a tube to the atmospheric hole and run a line inside the plane to get normal atmospheric pressure. For me, the better is to damper the inflow because that also insures proper engine cooling and no stagnant air pockets in the cowl to over heat.
Sorry for the long primer on small engine carbs... but hope the info will help. It explains why an engine runs fine on the test stand but not in a cowled plane. I'll enclose a pic of the cowl damper on my Yak. Note that the inflow is just where the cylinder and muffler are... the rest is blocked off. The exit area is about twice the intake area. I used 1/8 light ply and glued it in with silicone adhesive.