Alright guys here it is. I have a 79 mustang 302 hatchback ,and when I hook the factory tach up- in drive and flooring it ,when it hits 2500 Rpm’s it stutters and back fires and the tach won't go past 3000 rpm . When I remove tach wire from negative side of coil it revs just fine passed 2000 Rpm’s, no stuttering or backfiring.