1985 nissan 720 4x4 rear cylinder valve knock NEED HELP

Hello there everyone I have a 1985 Nissan 740 king cab 2.4L 4x4 that I have recently bought I have drove it for several weeks now and yesterday I was driving and it started making a knocking noise so I pulled over checked oil it was fine drove it back I my house a mile or so away my dad suggested it was a spark knock. It was idling rough it comes out of a rough idle if u gradually increase the rpms and won't knock but if I were to rev it quickly from 1k to 3k rpms it has a knocking noise coming from the rear cylinder so we replaced oil and filter plugs wires cap and rotor hoping it was just a spark knock needless to say it didn't fix the problem it's still idling rough and missing at low eps and knocks from the rear cylinder when revved up it isn't a real harsh knock so eliminated the bottom end from the equation the old oil also had a hint of gas in it but we are clueless to what the actual problem could be and it's cause. Any input would be appreciated thank you

Also within the several weeks of driving it I have had no issues and it has ran perfectly fine it had 150k miles