Help needed engine swap

[Emmalou]

Hi guys, husband has autism and is very much into his Jazz's, we have a Honda Jazz 1.4 2003 automatic, which is his first car and he loves it, the engine failed so I bought a replacement one off Ebay not realising it was manual.
We've got the engine in, the car runs better than it ever has tbh, swapped the cam sensor, checked injectors etc, checked timing. But when it starts the Jazz backfires slightly and runs like poop for a few seconds until it kind of seems to find it's rhythm then runs perfectly and I'm not sure how to fix this as everyone seems stumped. Any ideas of advice please

[CRC]

Did anyone check the condition of the eight spark plugs do you know?

From what you say, it sounds like there's not a lot wrong once the engine has started and been running for a short time. A backfire is caused by neat fuel and air getting into the exhaust because it wasn't ignited by a spark in the cylinder head, then igniting as the flame from a proper exhaust pulse meets it.

It's common for owners never to replace the rear plugs because they don't know that they are there.

Good place to start I would say.