QuestionI just changed my cars spark plugs and it backfires.
It never has done it before.
AnswerMy first guess would be that you mixed up the spark plug wires. If it runs at all, you probably have most of them right.
Follow the plug wire routing around the distributor cap - they should be plugged in using the firing order, which is 1 8 4 3 6 5 7 2. The cylinders are numbered 1 3 5 7 down the driver's side from front to back, and 2 4 6 8 down the passenger side, from front to back.
If those are all correct, start putting the old wires back on, replacing one at a time until it runs right, then you know you have a bad replacement wire.
If you've disturbed the distributor cap itself, make very sure you have it back on straight and clamped down tight.
If you still can't make it run right, post a follow up question and be sure to tell me what you mean by "backfire". Everyone I talk to has a different definition for that word. To be sure, just tell me what the engine does, forget the lable!
Good Luck.
Dick.
