I recently purchased a 1984 w150 that was swapped from a 318 to a 360 tbi out of a 86. I’m not to familiarized with older motors and am looking for some guidance. When you first start the truck it fires right up and runs great for about 10 seconds then it dies down back fires and shuts down. If you keep you applying throttle it will stay running but it back fires and keeps wanting to die. I’m not sure where to even start with it and looking for advice.
If you were close by I would offer to teach you how to work on these older trucks. It is knowledge that is dying out and really should be passed on. Trying to do that by typing out instructions is near impossible. Find yourself a local old guy, car fanatic and see if you can get him to mentor you.
However, as a starting place, backfire is caused by too much fuel being put into the exhaust and ignited. Incomplete combustion in the cylinder. (Not enough fuel also causes backfirs but those are usually up through the carb not out the exhaust.) Over fueling is often caused by the choke being on or something off in the carb such as float stuck etc that is causing more fuel to be dumped into the cylinders than they can burn. The unburned fuel ignites in the hot exhaust. First place to try, take the air cleaner lid off and prop open the choke plate in the carb. See if that fixes your backfire issue.
Can also be caused by bad timing, bad plugs causing poor ignition and incomplete burn or bad spark plug leads, bad rotor or cracked or carbon streaked distributor cap. There are other causes that are a lot more work to fix so start at the easiest and work your way through those first.
