If an engine dies when you remove the glow plug battery, you can have a bad plug, bad fuel, or a bad carburetor setting. A too-rich setting will cause it. So will fuel that's water-contaminated. It will take some detective work. If all else fails, you may have to put the engine onto a test fixture and run it with a propeller to see. With a good setup, good fuel, and a good plug, you should be able to locate the difficulty.

One other thing: if the engine's been run a LOT, it may be starting to wear out. When the compression fit starts to go, low-end running is the first to suffer. Dirt ingestion over time will also erode the compression fit so that you have poor compression. Heli engines are prone to dirt damage because they operate a lot at very low altitudes.