My 40 has a bit of a pop to it. That is why they are called Johnny Poppers. My MT before that did as well.
Now if it is backfiring with a bang like a gun going off that is different and something that can/should be addressed. Have you ever checked the timing? If it is backfiring I would start with making sure it is timed correctly. The best way to do this is to obtain the owners manual and follow the instructions outlined in there but use a timing light. All you need is a cheap timing light which you can find on ebay or amazon. If your 40 is 6 volt you will need a 12 volt battery to run the timing light. I have used the battery in my truck or other 12 volt tractors to do this.