emjay
+1y
I need a bit more information to help pinpoint the problem. You say that you have a carb'd 2.6, is it the factory carb or is it a Weber aftermarket carb?
If I had to guess right away I would point my finger at a rich running condition, possible fuel restriction (either a partial clogged filter, crud in a carb jet, or a weak pump.) What is the potential for bad gas in your area? Maybe one of the hoses is brittle and causing air to seep in?
It could also be a timing issue, maybe the distributor cap is cruddy and providing weak contact? Maybe your ignition coil is going south, or your plug wires are falling apart?
When I went with a gutted exhaust system I would get periodic backfiring due to the lack of back pressure. What condition is your exhaust in?