So, after a bunch of questions this weekend about it, yet again, "setup instructions" for the 20FP and "new" 25LA. Maybe it is so simple no one can believe it, but run the engine stock with all the original stock parts from the OS Factory. DO NOT REPLACE ANY OF THE PARTS with "better" parts and the various Dremel-tool-and-a-devil-may-care-attitude grinding or drilling modifications.
Common advice for either engine (profile mount assumed):
OS 20FP-S ABC, 20FP-SRN (rear needle), or OS25LA-s RN ABC engine NO MODIFICATIONS OF ANY TYPE OR NATURE.
Stock OS E2030 muffler, also stock including internal conical baffle. NO MODIFICATIONS OF ANY TYPE OR NATURE.
APC 9-4 prop. This is the normal one with the usual APC scimitar shape. Only modification is to sand the TE with 240 grit sandpaper or similar until you can flip it forward without any chance of cutting yourself.
Any suction tank (not uniflow, at least not to start with) where the outer edge of the tank is more than 1 3/4" from the mounting plane. Suggest Sullivan SS-4 clunk tank with "bottom" (wider) up against the fuse side
Powermaster "Air" (basic rc sport fuel), straight out of can. 10-15% at sea level, 15-20% at altitude (for example, Tucson). The one time I ran the 25LA at Tucson, I ran YS-20/20 because it was the only 20% fuel I had. That just about made up the altitude difference between sea level and 2300 feet at 70 degrees. Other fuel (like SIG) might work as well, just do not use lots of castor oil. GMA is probably OK, Fox Superfuel IS NOT OK.
Attach a pressure line from the STOCK muffler tap to the suction tank vent. Plug the overflow with an airtight plug. You want it to run on a suction system with pressure, not uniflow with or without pressure.
Start engine, lean out to a 2-stroke, then slowly lean it out until it is absolutely as fast as it will go, and one click more makes it sag. Then, back off needle until you just get a distinct RPM drop, just a bit off peaked out in a 2-stroke. If you get it rich enough to start misfiring in a 4-stroke, even occasionally, you have gone *much too far*. This is about 5 clicks open from peaked-out lean using the stock OS front needle, about 5 clicks open using the 20FP rear-needle (all-metal on a metal bracket). It is about 3 clicks open from peaked out lean using the stock 25LA rear needle (gray metal needle in a plastic backplate mount). You do not need a tachometer, but it will be 2-300 RPM from peak.
Launch the airplane and it should get slightly richer-sounding in level flight, and then peak out in the maneuvers. If it peaks out and then sags in the maneuvers, you are too lean, open needle *1* click for the next flight, try again, until it does not sag lean in the maneuvers at any point. If it ever hits a 4-stroke, you are too rich, close the needle one click and try again.
Unfortunately, due to painful experience, YOU WILL FAIL if you make any of these modifications, so DO NOT MODIFY OR "IMPROVE" any of these items:
Venturi: The stock CL venturi is the correct size, DO NOT CHANGE IT
Spraybar Assembly: The stock OS spraybar assembly is the correct part, DO NOT UNDER ANY CIRCUMSTANCES CHANGE IT FOR A "BETTER" type, specifically, DO NOT USE ANY OF THESE:
Supertigre
Supertigre clone
PA
RO-JETT
McCoy
Fox
Merco
Stalker
Como
Kirn-Kraft
You name an engine manufacturer's needle, I will tell you not to use it. The ONLY ACCEPTABLE SUBSTITUTE is the Enya copy of the stock OS assembly commissioned and sold on eBay by Shtterman. Replacing the spraybar is by far the #1 most popular way to f*ck up your engine, because in addition to losing 30% of the power right off the bat, you also have to drill out the case to fit it, which means you have semi-permanently destroyed another nearly irreplacable part, and presumably the venturi, too. If you replace the spraybar, do not come to me complaining that it does not work - because I can tell you right now that it WILL NOT WORK, at least not the way I am trying to get you to run it.
Muffler - DO NOT REPLACE THE MUFFLER WITH A "lighter/better" one, DO NOT RUN A CHIP or "TONGUE" muffler. This is extremely critical, IT WILL NOT RUN PROPERLY WITH ERZATZ mufflers. If you airplane is nose-heavy, put lead on the tail, do not remove the muffler to balance it.
Head Gaskets/screws - DO NOT CHANGE OR ADD HEAD GASKETS. In fact, don't even buy any "spares" because if you don't have any, you will not be tempted to use them. While you are at it, the 20FP comes with a very nice Allen Wrench. That doesn't mean you are compelled to use it!!! Don't take the head off at all, do not replace the screws, just leave it alone. It is not going to come loose.
The 25LA uses JIS Crosshead screws, they are also OK, do not remove them or replace them with "better" socket-head screws. Along with that, if you have a screwdriver from any common US manufacturer/brand name (Craftsman/Proto/Williams, etc) and use it to remove or "adjust" the head or backplate screws, you will very likely damage the screws because those are Phillips, not JIS. If you absolutely must have a screwdriver, a Vessel or Hozan #1 JIS crosshead screwdriver is the only thing you should use in the exceptionally unlikely situation that you need to take it apart. DO NOT try to really crank down on the 25LA backplate, it need only be snugged up, not gorilla-tight, and if you tighten it too much you will either cause a leak or crack it. It is otherwise perfectly OK, until you break off the corner in an inverted crash. LEAVE IT ALONE.
You would think this would be self-evident, but apparently it isn't - DO NOT GRIND ANYTHING, CUT ANYTHING, or DRILL ANYTHING!! Neither you, nor your flying buddies, or anyone else knows how to improve it by grinding it. Even if they did, I can assure you that if you *follow the directions*, it will be better, stock, than any other engine you and your buddies have had for the the typical 35-sized profile models like the Skyray, Flight Streak, etc.
"old" VS "new" 25LA. I know of no way to tell whether a particular engine is the "old" version or the "new" version. It may or may not follow the paint- all of the "old"/lower-performance types I have tested or have been tested by qualified observers were the original blue-paint type, and the "new" higher-performance types have all been bare, but I can't say for sure that this is a bulletproof test. Unfortunately, the only reliable test is to run it. On the stand, the "old" types will be peaked out around 12,800-13,200 RPM with a 9-4 APC, 10% fuel, at sea level. The "new" types will spin up to around 14100-14400 on the same prop and fuel. It is not a subtle difference, and please don't tell me that "they are the same", because they are not (although I do not know what the internal difference might be, and neither will you, because, again, *you are not going to take it apart*, right?). The older types are usable but not nearly as good as the "new" type, at least for stunt.
25FP - the 25FP will work, too, and is much more powerful than the 20 or the 25LA. This is almost a negative, because on the typical airplanes, they tend to be too fast, leading you to try to needle it down, which then makes it run less good than the others.
Note also that the same sort of advice applies to the 46LA - dear GOD ALMIGHTY, just leave it alone! You buddies don't know how to improve it, either. It is extremely good right out of the box, the problem being that it, too, tends to be far too powerful for the sorts of airplanes people want to use it in. Figure it to be an upgrade to an ST46 - more reliable and much more powerful. I think Tim Wescott flies one of Paul Walkers old Bad News/Impacts which originally were designed around either the ST60, 45 FSR, or 40VF.
Brett