CZ 70 and small question.Help!

I put a 5 inch CZ coil on my CZ&0.I can't get it to ground balance correctly.When I bob the coil annd adjust the GB knob It goes as close to quiet as it gets at 10.It ground balances OK with the stock coil at a lot lower setting.I have found bare spots with no metal in the ground in at least 10 different locations with another detector in all metal.It always GB's at 10 with the small coil.Is it possible the 5 inch coil is bad?Any ideas,suggestions or help would be appreciated.I have a spot to hunt that is super trashy.The newest coin out of there so far is a 1908 Indianhead penny.It's an old roadhouse from the local stage coach line.Thanks Bill

Will Bill I think it would be the 5" coil, if the 8" coil will GB, But it maybe the ground you have and that is a big maybe.

Coil is impedance mismatched/out-of-tune. Try something. Take about 50 newer nickels (not dug) and see how the detector ID's them with the 5" coil.