1985 nissan 720 4x4 rear cylinder valve knock NEED HELP

[Hsanders843]

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

[Hsanders843]

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