There's a chance that your pistons, valves, intake have carbon build up which can create hot spots and create knock. Many on here clean carbon with sea foam. I did mine engine once about three years ago before I built it. Pretty sure I used a vacuum line to suck the sea foam in then shut the motor off for 10 mins or so then started it back up. It works very well. I forgot the actual procedure so do a search on here or google first.