I started a previous thread on this subject but thought I would start a new one to document the results of my experimentation (if you can call it that), with the OS 55 AX modified for stunt.
The modifications are simple and consist only of an extra head gasket (.008 thick), a venturi from a OS 46 SXS (.283 diameter), a Randy Smith Aeroproducts Needle valve assembly, and two layers of panty hose material (because it really needs about a .270 diameter venturi at this altitude).
It had a slight "charge" to it on down lines that tank adjustment wouldn't cure until the panty hose material was added and that totally eliminated it.
Smaller venturiis are on order. (My lathe is still non-functional after my move).
Muffler is RSM tongue for OS46 SX with a OS pressure tap I added. Pressure was added to eliminate richening going into wind. Pressure is to uniflo tube.
As mentioned in the earlier thread the engine is mounted in a Brodak SV11 ARF With an all up weight of 61 oz. Incidentally after some head scratching trimming this has proved to be a very good flying airplane.
Propeller that worked best so far is a 13-4.5 Bolly two blade...but a 13-4 APC worked almost as well.
Fuel is Powermaster 10-22 half and half.
Ground setting with either prop is 9400-9500 RPM and that gives 5.2-5.3 second laps on 67 ft .018 lines (center to center).
Plug is Thunderbolt R/C long. Tank is Brodak 6 oz wide wedge uniflo with Figg style vents.
This engine is an easy starter...usually on first flip after a couple of chokes.
All flight testing was done in Tucson AZ at 2200 ft altitude and 100+ degree temperatures (not so different than the midwest this year), huh!!
This thing is very powerful and easy to handle. Needle adjustment is non-critical and fuel consumption is 5.7 oz to fly the pattern with about 10 laps left over.
The run is fast 4 cycle with just an occasional beep at the top of some maneuvers and on overheads.
The power and stunt run is on a par with any of the dedicated stunt engines. (No...I'm not kidding)!
The only down side is the 14.5 oz weight of the engine but in this airplane is not a problem. (Yes it took 1 oz of tail weight to overcome it)
For anyone looking for a really good stunt engine for a largish airplane that doesn't want to spring for a PA or RO Jett, I can heartily reccommend this engine and setup. Price is about $160 at Tower.
I'm building a dedicated airplane for it.
Don't tell OS...they'll probably stop making it...

Randy Cuberly
PS: This is a good one, and will fit in PA or RO Jett mounting...