What brand of fuel are you using? Do you have stock needle setting specs to start from? Have you tried any other kinds of glow plugs? O.S. #8's and A3's always work well for me. I'm guessing the engine that appears to have been abused is probably junk. A nitro engine in the wrong hands can last as short as break in and be junk. If you're tight with cash you could see what a piston and sleeve for the engine costs, but I find it's usually more economical to buy a whole new engine in most cases.