timing belt issue

[jdstealth66]

I replaced my timing belt a few years ago on my 1990 300zx n/a everything went fine but lately ive noticed a hollow sound coming from the exhaust like the exhause valve is opening to early. I have just checked the timing o it and everything lines up correctly. I also checked the timing and its running at 30 advanced instead of 15 deg. I cant get it to 15 by turning the cas there is not enough adjustment and the engine dies when it gets near it. is there anyone that can help me out?