Ferris/Briggs and Stratton Backfires

Good Morning All, I am new to the forum. Three years age I bought a Ferris IS700Z with a B&S 27HP Model 49T877. Recently the engine has taken to backfiring quite a bit. I have changed plugs and did a Seaford Cleaning but the problem still exists. Can someone give me advice on the next steps I should take to eliminate this problem.
Any help will be greatly appreciated.
Regards

C

Does it backfire if the mower is sitting still hand brake on and no one in the seat ?
Don't know the try it .