You haven't made any mention of your low speed needle. Magnums tend to be very sensitive to the low speed needle. You say the engine behaves on the ground, but don't specifically say what that really means. To check your low speed needle setting, let the plane idle at its lowest reliable idle for at least 30 seconds. Then punch the throttle to full. It should transition smoothly without hesitation. If it sputters and dies, the low speed needle is probably set too lean. If it hesitates and then goes to full throttle, its probably too rich.

If you adjust the low speed needle, make the adjustments in 1/8 turn increments or less. Then reset your high speed needle again before checking the low speed needle again. If it engine doesn't improve after a full turn on the low speed needle, go back to the original setting and adjust it in the opposite direction.

Another thing to check is to completely disassemble the carb to ensure all the fuel passages are clean and clear. Make sure the spray bar isn't varnished. Its amazing how a very small speck of plastic, silcone, or dirt will affect the mixture settings. Dissassembly and cleaning only takes a few minutes.

Finally, I'm confused by the pictures. You had a remote needle set up on the back of the engine, along with the needle mounted on the carb? Or was there simply a plug installed in the carb where the normal needle valve is now installed? If you had two needles in the flow path, you were probably creating too much resistance to flow for the engine to properly draw fuel. If you had the forward needle turned out far enough to preven this, there was probably air leaking in around it.

Brad