John Deere X320 was running perfectly fine. I suppose this is a perfect example of not fixing something that ain't broke. Did complete maintenance kit with filters, sparkplugs, & all this time, rather than just oil change & deep cleaning, as technically it was past due. Engine is now backfiring a lot & does not seem to have the same power as before. Any ideas as to what the cause could be? John Deere said spark plugs are pre-gapped.