So today for the first time ever if your acceralting at aprox half throttle it backfires and a large cloud of black smoke I haven't done anything to the car in weeks other than drive it about 150 miles in a month its driving me crazy it'll start up and idle then about 10-15 seconds the tach will just drop and then start working but it's not just the tach the entire engine stumbles to thr point that it'll shut off if you hold the acceraltor at about 1500 rpm for 20 seconds or so it stops dropping off but while driving obvious lack of power too much throttle will cause back fire and blacksmoke....