What airplanes do you test these engines on?
All of my LA25s are apparently the bum low performance models.
Most of them were on a SIG Skyray 35 with various degrees of deviation. Most of the original tests were in the early 90's, the LA tests were fist in the early 2000s when the FP was replaced by the LA, then a few years ago till now. Also included at varioius times were two different Dick Mathis Coyotes. Auxiliary tests were done on SIG Twisters, Fancherized Twisters, Top Flight "Green Box" Nobler, a SIG Banshee, and models like that. We (Jim Aron and I) have tested the Veco 19bb and 20FP on a Ringmaster (RSM Box Art Model, the 20-pointer from VSC), and he has tested a large number of variants on his Ted Goyet Stunt Feno.
In almost all cases (the Skyray in particular), the main problem we had was keeping the airplane from flying too fast while maintaining the good run characteristics they have at high RPM. The only case we had a problem with "too slow"/not enough "power" was Bob Hazle's Fancherized Twister with a 25FP at the NATs. It had been too fast in terms of air speed or too slow in terms of RPM all week using 10-4's of various stripes. So we got a Brodak BY&O 10-3, Even set to supersonic RPM, the airplane was too slow (maybe 5.6-5.8 seconds). Knowing what I do now, we probably could have solved it with an APC 9-4, allowing enough revs for fuel suction which enough inefficiency to keep the airspeed under control. He also used the accursed ST spraybar - with a stock spraybar it probably would have been OK.
The only problem I see on a routine basis is *not running the stock parts*, particularly the ST or other spraybar assembly, but sometimes the stock factory muffler (which is CRITICAL to making it run correctly) is discarded to "save weight". Or, alternately, trying to run it on improper propellors like 9-6 or 10-6s at low revs. With the stock parts, it *will not* have enough fuel suction for reliably run at 10-6 revs (maybe 8500-9000 in the air). You might get it to be OK with the larger spraybar at those revs but that probably cuts the power in about half, at which point, you might as well get a McCoy 35 Redhead, and a time machine to take you back to 1956.
Changing them from rear to front intake is unnecessary but doesn't hurt anything in terms of performance.
The other issue is a psychological phenomenon, where people hear a 2-stroke, and thing "that's going to BURN UP!". Just unwilling to peak the engine out for fear of damaging it. Thats a legitimate concern with a Fox (although people run them pretty hard in Foxberg events), but this engine was intended to be run by RC guys in "squeeze it till it screams" mode. You can't easily destroy it by setting it too fast.
Assuming STOCK parts in all areas, don't give up. Trying to run a 9-4. you might not be able to get enough revs. So, go up a little in pitch and diameter. A Bolly 9.5-4.5, adjust pitch to get the desired speed at a decent setting. The intent is that the engine be running in a medium 2-stroke in level flight at normal flight speeds, and have it pick up "power" in the maneuvers. The only thing you don't want to do is attempt to run at low RPM, because it *will not* suck fuel sufficiently as it comes. That's why the APC 9-4 is better than other 9-4s and why it is usually better than any 10-4 - the smaller diameter allows it to rev up more while having low enough efficiency to keep the airplane from going like a bat out of hell. But you can use the inverse if the airplane is too slow, or can't rev as high, add diameter or pitch *in small amounts* to increase the efficiency just enough to get it off of peaked out in level flight.
Brett