Any ideas on the rignt prop for an LA 25 on a 1970's vintage Carl Goldberg Shoestring Stunter? 
Rick

  9-4 or 10-4 APC.

    Brett

Thanks Brett,
What launch rpm?
Rick

   Try 13,500 to start with. A "new version" has no issue getting to maybe 14,200 or 300 with the 9-4. That will be a medium 2-stroke.

    A whole bunch of people have told me there was no difference, but on Sunday I acquired and tested one of the original early 25LAs (blue with front needle) and it would barely get to 12,000, the "new" (last few years, bare metal, rear needle) has no problem 14,000+ - tremendously more powerful.

    Brett

  For a little better pull in manuevers, try the APC 10-4 at anywhere from 9500 to 10500, depending on your model weight, lines and such. It's been covered a lot here on the forum on several different models, and I have had them on airplanes up to 45 ounces. Not the best in the world but wil fly them. Great on the Shoestring type airplanes and on the venerable ARF Flight Streak.
   Good luck and have fun,
    Dan  McEntee

 

    A whole bunch of people have told me there was no difference, but on Sunday I acquired and tested one of the original early 25LAs (blue with front needle) and it would barely get to 12,000, the "new" (last few years, bare metal, rear needle) has no problem 14,000+ - tremendously more powerful.

    Brett

That figures since I've got about 4 of the blue ones. 

I have an APC 9.5x4.5 on my Buster with a LA 25.The engine seams to be very happy with that prop.Trax

10/22 is what most of us run here on the left coast as well. Humidity maybe? I like the 10x3 APC on LA/FP25's, 9x4 BB setup on FP-20's. 

Here's a tip that the experts forget to tell the new folks asking for this type of advice:

The setting on the ground with a tach will vary a  significant amount due to the type of tank you're running, where it's mounted, venturi size, altitude density. etc.
If you're running a center mounted tank (full fuse), a conventional metal tank mounted outboard side,  a narrow clunk tank, or a clunk tank on the inboard side; the ground setting can vary a huge amount to achieve the same in-flight setting. It will also vary with muffler pressure, uniflo, or old school stunt venting.

So......on the first flight I recommend: with the prop you've selected, set the needle on the ground in a fast 4 cycle and fly the airplane. pay close attention to what it does in the air I.E. stays rich, leans out a little, leans out alot, etc. On susequent flights, adjust the needle (in small increments) to achieve the type of run you are looking for I.E. 4-2-4, all 4, wet 2, etc. When you get to that point, fly it again, don't touch the needle, and this time get a tach reading on the ground and record it. Now you have a useful number for that prop, fuel, airplane , engine combo. If you change the prop or the fuel or travel to a flying site at a significantly different altitude, you will need to go through the process again to determine the optimum ground RPM
There are just too many variables to count on using someone else's tach reading as a starting point for your set-up. As you can see from Brett's experience above with seemingly identicle set-ups, one engine reads vastly different than the other. Trying to get to the higher RPM figure with the slower engine would result in a very lean run. Starting out rich on the ground at first may waste a flight flying level but it won't damage your engine

   Try 13,500 to start with. A "new version" has no issue getting to maybe 14,200 or 300 with the 9-4. That will be a medium 2-stroke.

    A whole bunch of people have told me there was no difference, but on Sunday I acquired and tested one of the original early 25LAs (blue with front needle) and it would barely get to 12,000, the "new" (last few years, bare metal, rear needle) has no problem 14,000+ - tremendously more powerful.

    Brett

   Been thinking about this, and almost without having to check, the spray bar on a rear needle set up is smaller in diameter than a front needle valve set up spray bar. No needle threads to have to account for. The venturi might be the same diameter, I would have to check. I fell victim to this senario on a .40 FP for my kid once. Went from a front needle valve to a rear set up, and suddenly couldn't get the pattern in on the same amount of fuel, and needle setting was different. Finally figgered it out and resorted to the old stick in the venturi trick to get through the contest. Measured the spray bar afterward and the rear needle version was significantly  smaller, thus opening up the venturi. I may have both at home to measure and check.
  Type at you later,
   Dan McEntee

I ordered some 9x4, 9.5x4.5 and 10x4 APC props, which I received yesterday (well, not the 9x4's, they sent 8x4's instead).  I flew the Shoestring with the 10x4 on the LA 25 with the stock muffler today.  Launch rpm was 10,000 rpm and lap times were 5.4 sec.  The plane loves this combination!  Tons of power and no bad habits.  I'll try the 9.5x4.5 tomorrow and the 9x4's when I get them. 
Thanks for all the advice and suggestions.
Rick