Good day I'm new here. I have a 1992 gmc sierra with the 350, 4x4. 9 months ago the truck started back firing when accelerating. I checked the plugs and replaced them all and no issues until recently. Now when the truck is in any gear other than neutral, it lopes quite noticeably. I'm thinking its a fuel issue as I take my air filter off and nothing changes. Either that or a vacuum leak somewhere. Any help would be appreciated, thank you.