1985 Monte Carlo
 

I have a 1985 monte carlo and I have put a new cap, Wires, plugs, fix Vaclum hose , and converter on mufler and when I give it gas it backfires I need some help. When I go down the interstate and give it gas it backfires, also it has been harder to start.

Retarded timing or worn timing chain,check distributor timing with an appropriate timing light.

Bad ignition module or distributor.

I would start with the basics, check the timing, make sure no plug wires are crossed. Does it have the factory carb? It could be as mentioned above, a bad timing chain, or maybe an intake leak, bad carb, etc.... Those symptoms can occur with an extreme lean condition. I would suspect the timing though.