my guess and im sure one of the pro's will jump in. Your'e probably getting a false pressure and when you loose it your engine is going lean. I would try running a tank out on the ground with maybe  an ounce less in the tank and see if your needle changes throughout the tank.

Tom

That "false pressure" thing is a good theory.  It happens when a bladder is inflated in an under size compartment.

Another common failure mode is under revving the engine.  If an engine is under revved, it will unload and go lean in the air.  The fix: use a smaller prop and let it howl.  It might still bog down and go a bit rich in maneuvers, which might be exactly what you need.

Thunder Tiger Pro .25, on bladder. Runs the first minute fine then begins to pulsate on and off, until empty. Changed bladders, changed plugs. Just checked, all screws are tight, head and back-plate. What do you think?

My bladder stock is a few years old.

I used to have the simular issue with my ST G21.35 /Nemesis and a TD.15 MK-IIS / F2D,  They both would run normal at first, then it'll start to pulsate on and off sounding like a mistuned diesel.  Long story short.. the problem was actually in the "Needle valve assembly".  as it got half gummed up with probably castor oil from sitting.  I fly these planes like once in 2-3 month, so they sit around alot.  After I started doing the routine flush prior before flights..  (with fuel or 99% isopropanol alcohol)  It has been running good right from first flight on.    

Good point on the NVA Joe.  All the common needle valves use too coarse a thread, even Supertiger.  The only readily available ones with a fine enough thread are the OS CL needle valve sold for the 20-25(Tower Hobbies LXCR49), and the OS 1a(LCXR44, LXCR49).  These have a .35mm thread compared to the more common .5 mm thread.

Y
Also you can still get the old OS 10 NVA on e-bay

To quote from Flyinglines.org, Northwest flyers.

http://flyinglines.org/buzz.Fx25.html


"Since the original writing of this article, we have noticed that the engines have a tendency to go lean. Actually more than a tendency, the damn things go lean. These are running on 10% nitro. Increasing the nitro to 23% helped but did not eliminate the problem. Don McKay was having more than his share of problems. I did have problems with the engine going lean if I did not let it heat up and then set it off with a slight gurgle. I removed the venturi and the engines ran better but still had a went lean.

At the Bladder Grabber this year, Mike Moreland, Jeff Dawson, Jeff Rein, Don McKay and myself discussed the solution. It comes down to engine setting. You have to let the engines get warm. Start the engine and set the needle so you have the rich gurgle. By the time you walk to the circle (approximately 14 seconds), the engine gurgle will have disappeared. Reset the needle to gurgle and then launch. If you are competing where the pilot starts their engine and then dashes to the handle, you need to wait for the gurgle to go away and let the pitman reset the needle before launching. You risk not being the first up, but you should have a good setting."