Years ago (early-mid 80's), I was very involved in the rc air hobby -- mostly pattern and sport acro. I left the hobby for years, and have returned with a vengence over the last few of years.
One of my favorite planes was the built-up Super Sportster (don't ask), and I had a particular fondness for the SS 20. That plane was powered by a ST .25, which had a very aggravating problem. It would invariable quit about 5 minutes into a flight. It didn't sound lean, rich, or seized, it just QUIT. You could run it on the test stand all day long, and you could also run it in the plane on the ground all day long -- in mixed attitudes and throttle combinations. I learned a great deal about spray bar positioning, low and high needle setting, and even managed to get past the ubiquitous mid-range stumble (or at least thought so). The 2 ST .61's I had, after their initial teething, were great engines. The .25 ALWAYS suffered the sudden inexplicible sudden stoppage about 5 minutes into flight, but ONLY during flight. Eventually, that stoppage killed the plane when it happened in a low-level maneauver over rocks. The engine was cleaned up, after-oiled and stored.
I was very recently given a SS .20 from an estate that had a used-up OS on it (no compression). Initial thoughts went to trying to find some way to use the ST .25 (other than as a boat anchor). Have any new thoughts about the older ST's come up while I've been away? I tried multiple different glow plugs, the carb settings, replacing the fuel, clunk, filter, and all tubing -- all to no avail. I've tried many different fuels, and while the ST .25 seems to prefer the 5% nitro, even at out high field altitude (>3000 feet). The one thing that I never remembered trying was going to a larger-than recommended prop diameter.
I susupect these answers have been dealt with years ago, and from the old comments I've found, it looks like ST's may be more suited to decorative uses than others. The engine has been torn down, with no signs of wear or corrosion, and there is no evidence of carb leakage. Fuel is pressurized using muff pressure. Compression is good. Starts easily. The glow plugs look okay. Not burned out, and the catalyst looks okay, with no obvious burning/flaking. Would a Perry carb be likely to solve the problem? If there was a good chance of it doing so, I would be willing to go that route. The ST .25 was a great match for the SS 20, at least when it was running.
Should I even attempt to resurrect this engine, or would I be best in pursuing another brand? Whilst on that subject, what do you think about the GMS engines? I have a couple of larger GMS's I'm pleased with, but nothing this small. Just for sake of principal, I'm inclined to spend more and order an Irvine (or GMS) from Just Engines, before payig less at Hobbico/Tower.
I have several other reliable .25's, but these are all on funtional combat ships, and I hate to start stripping them as combat is a nearly weekly thing here with a couple of local friends. On a few occassions we actually have some out-of-town guests and have a real "group".
Sorry for the long post. I have no local resources. For years, I was the only on in this region interested in RC, and even now, with the friends I've interested, our "club" numbers 4, and I'm the "old-hand" (scary, no?) The nearest real club is a bit over 100 miles away, and aren't particularly friendly to non-locals (even if you're a dues-paying out of town member, you're welcome to watch at their flights or meets, but they don't really want you flying, or even appear to be interested in mentoring). The "LHS" is the same way, a bit over 100 miles away, and not too knowledgeable at that. Basically carry Hobbico/Great Planes and that's it.