u sure its at 400-500? cuz like rich said...the stock tach starts off at 500. plus, the stock tach isnt exactly accurate..its about 200rpms off... i used to think my idle was really really low too and sometimes it even sounded like it was going to stall... i got it checked and it was at a rock solid 700 and when i put on my SAFC, it proved the "inaccuracy" of the stock tach.