I have a 1973 ranchero with a 351c i recently converted it to duraspark and hooked up a tach. now i know i have the tach hooked up right i've checked and rechecked that it is a sunpro supertach III it is set on 8cyl. my problem is that it reading way high it is correct at idle but anything off idle it just shoots up to about 6 or 7000. it is about to make me pull my hair out..

thanks