Well,
The (outwards) laying engine gives an unbeatable advantage: the hole on the spraybar (or end of the carburettor nozzle) is at least 1/2 inch outer position than normal inverted, or standing arrangements. By my engines this is more: ca. 20 millimeters (4/5 inches).
Who is ready to make 2 x 1/2 or 2 x 4/5 inches wider fuselage to achieve this???
And why would you bother, when the problem is effectively solved (to the degree necessary) with completely conventional means and inverted mount?
Don't get me wrong, I am impressed and fascinated with with your efforts, results, and your machine shop capabilities, and what you and Lauri are doing is very interesting. You don't have appearance points to worry about, either. But we haven't had these sorts of run asymmetry issues for a long time now, 15+ years in my case, and to be honest, it worked well enough in 1990 with a $200 production engine straight off an assembly line, if you set it up correctly. And I am particularly picky about it.
I see two differences - you (apparently) run at extremely low RPM, and don't use nitro. We use no plug shields, micron-fit piston rings, heat reservoirs, etc. We have completely conventional squish band/semi-hemi heads, glow plugs that used to cost 79 cents. As far as I can tell, there are three relevant issues:
proper carburetion/mixing (solved by spigot venturi and diffusers, perhaps by Belko's (effectively) plenum, too)
reduced fuel system flow resistance
internal gas dynamics around the ports and bypasses
I would note that I didn't discover #1 and #3 (which were at least first described in print by Frank Williams), and # 2 used to be mentioned in the Fox 35 instruction sheet! I just call up and order them from the manufacturer, anyone else could get the same thing. The only real cost I see is the heroic fuel consumption (6.7 ounces of 10% for the little 61, and 8.5+ for the 75), you definitely have that beat.
Note that I didn't mention pipe regulation, it resolves *other* issues like allowing you to run a superior prop as a more beneficial RPM, but I don't think it matters to this particular problem. I don't know for sure why anyone wants to run it at 8000 rpm with 6" of pitch, that is a different albatross. Maybe trying to run 60s-91s (10-15CC) engines, capable of maybe 7-8 HP in ideal conditions, at 1/2 horse, is a problem. It's not a problem for me, I am probably running 3/4 horse or so, 50% more, but still nothing compared to what it could do if pushed.
I would note that we had these sorts of problems, and they drove us absolutely crazy, about 20 years ago, so I get the motivation - it makes it much harder to fly stunt when the engine is changing all over the place. David, Ted, and I (and to a lesser extent, Paul Walker, I think) got on several engine manufacturer's semi-permanent poo-poo lists over the topic, and I got a particularly histrionic rant about it in person at the 2002 NATs, from someone whose engine screeched into a squeaky lean 2-stroke in every outside turn.
So please don't interpret this as a criticism, I get why you want to solve it. But I just don't quite get the elaborate lengths and highly unconventional solutions to what we managed a long time ago with pretty ordinary (by comparison) approaches.
Brett