I finally put an OS 91 FX on my Swallow. After struggling with the engine for a couple of flights I finally sorted it out. I was using an Ironbay fuel regulator running from muffler pressure. It seems that the pressure was not enough and the engine died when pulling the nose up. I didn't want to drill and tap the backplate. I will try an OS 140 FI style pressure nipple. I think the OS Surpass 52 fuel inlet with feet (3.5mm thread size). Anyhow, I removed the fuel regulator and checkvalve and the engine runs smoothly . Iam still running it (only 3/4 of a gallon through it so far). I am using an APC 13x8 propeller and the regular muffler and it has enough power for long verticals and big loops. It doesn't have the power for prolongues vertical figures (i.e. vertical eight with rolls, figure M with rolls, etc.) I will try a tuned pipe later (together with the crankcase pressure?).
The only problem I found with the plane is the landing gear. The plane bounced in every single landing (5 so far). It ended in the grass last night and the right leg broke down. The fuselage took the beating with no problem. I am considering retracts or an aftermarket landing gear right now. I haven't weighted the plane but it is built stock with the lightweight OS 91 FX, two Futaba S9101 in the ailerons, two hitec digitals 5625 for rudder and elevator and a micro hitec 81 for the throttle.
Rudder looks impressive in size, but it takes some effort (and throttle up) to rotate the plane through a stall turn. The plane tends to fall from the sky if it slows down too much in vertical.