Starts and runs great but backfires under a load?

Hi,

A backfire is usually the result of either a lean or rich fuel mixture or ignition timing.

Take a look at this link:

https://www.2carpros.com/articles/engine-backfires-while-running

You mentioned the fuel pump was replaced. Did you check to make sure the fuel pressure is within the manufacturer's specifications? Have you checked for engine vacuum leaks?

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

If you pull the vacuum hose off the fuel pressure regulator, check to see if there is any evidence of fuel present. Also, with the engine running, make sure the hose has a good vacuum.

Let me know.

Joe