Ametek Tach

The old one, the needle bent and was sticking on the crystal occasionally. Then it got to a point that it wasn't working. The new one said to set s5 and s6 to on for all electronic engines. I started the engine and the tach redlined. So I tried the old ones settings, 1,5,8 on. It now idles at 500. First bump up should be to 1,200, tach is showing about 850 and it's dancing. Going down the road it looks like its about 300 rpm off, and the needle is jumping periodically up to 200 rpm.