A bit off the main direction of this Club...
I fly mostly CL Stunt - properly called Precision Aerobatics, or CLPA. Precision, yes. When you fly between 5' above ground and the length of the flying lines + your reach height, you are never three mistakes high.
The 'figures' almost all include segments that use that ~5' altitude. With no carb, airspeed is not flier-controllable, but does vary from drag loads in tight cornering, and from progress up and down.
Carbs may be a chore to set, but it can be done. It is more 'interesting' to set a fixed-choke engine to perform consistently and correctly, for a g-exposure range from 1g in level flight to over 25g in sharp turns, and to handle the effects of gross load angles (the resultant of gravity, maneuver radius load momentary directions, and model roll attitude at various 'heights.') There is also the roughly 3g outward from centrifugal force at most modest altitudes.
The Fox 35, still in occasional production, if the most hopeful word from Fort Smith is correct, became the essential engine for the early decades of CLPA. Newer designs outdid the Stunt 35 in various ways, but where are they now? Of course, today, we DO have better made, better designed engines that are less particular in use. Progress, after all, since 1948 cannot be denied. Still, it seems to be a shifting environment - new engines appear and become de rigeur for a season or three, then fade away somewhere.
The Fox 35 Stunt, for all its warts, quirks and age related liver spots, STILL does what it did when it laid out the basic definition of stunt engine performance. The most significant change is that the 4/2 cycle break is no longer held in awe. Newer engine designs have a very broad RPM range in 2-cycle mode, so prop, plug, fuel and setting can be studied to get the optimum flight performance.
Why the Fox Stunt 35? Iron (Meehanite) piston in leaded steel sleeve (don't know if the sleeve is still 'leaded' - sounds poisonous!) Plain bearing shaft and rod. Specs that hit a wonderful middle ground range - almost all Fox 35s, unless brutalized criminally, run much the same. The combination of metals benefits from a high% castor fuel...
I've "cleaned up" and "modified" Stunt Fox engines for over 30 years, mostly for my on use, but also for a few dozen friends. At times in the past, Fox QC was not the most effective. That has improved enormously since the introduction of schneurle ported Foxes in the 1970's...
My mods are to make an optimum engine out of the parts in the box. I mike the parts to determine how much, if any, production tolerance 'this one' has. Oddly, these engines are match-fitted so that the spec deviations are compensated to a great degree. An undersize sleeve is matched to an undersize piston, for example. They still run very much the same!
Cumulative spread of spec variances can cause significant results. I've reworked some that measured under 0.350 displacement, and some that were near 0.360. Design specs would result in 0.352 cu in... The accumulation of spec variances can change sleeve timing drastically. E.g., cylinder flange thickness, casting shaft center to deck height, crankpin radius, cylinder port (opening) distances from flange, piston height, piston head distance above piston pin center, rod length on centers.
I've measured stock-in-box samples from the worst years with port timing variances of 10° to 30° from intended spec! Again, this is much improved in recent decades...
I developed spreadsheets to allow me to analyze what a sample is, and how it needs adjustment to my preferred numbers. E.g., 'blowdown' - the total difference between shaft degrees of exhaust port open and bypass port open - relates to the RPM for best efficiency. The greater the 'blowdown,' the more it suits higher RPM. I prefer to think of the lag between exhaust and bypass port opening - i.e., half the blowdown duration. THAT lag time governs whether flashback of burning charge may obstruct bypass flow, or at the other side, whether useful 'time' is forfeited from bypass flow.
I've settled on EX=115°/ BY=120° B/ or A/TDC port openings/closings, and can almost always achieve that despite cumulative tolerance errors. I also partial-balance the shaft:- its mass is a bit eccentric - metal removed for the shaft inlet port is 'to one side.' Judicious 'carving' on the crankdisk almost always allows the shaft to balance on knife edges with the crankpin vertically above the shaft centerline. THAT eccentricity is reduced significantly. As stock, the tendency would be to 'wipe' around the main bearing...
"MY" Fox Stunt 35s, if I'm satisfied with them, vibrate noticeably less, are a bit more economical of fuel, and have slightly more power than stock. They start and run dependably. I admire some other modified Fox 35s - notably the L&J Foxes my friend Larry Foster turns out - but if I've seen them honestly. they clearly produce more power, consequently consume more fuel, and vibrate noticeably more. Personal preferences...
Not all Fox engines are RC. This is a review of the engine that started the successful history of the Stunt 35 and all the other great Fox engines over the past 66 years. Consider: - the Fox 40 Schneurles are based on a Fox Combat Special of the 1970s. The Fox 15BB Schnuerle has been and still is a powerhouse in production since the early 1970s. The 25BB Schneurle is a fabulous engine, dismissed as a Fox by those who dislike the brand. Only an ABC version could spread its excellence beyond this Club's circle.
Oh, by the way, if you hadn't noticed, I DO like Fox engines. I don't have enough fingers and toes to count how many I have, and use...