Look man, I’m sure this isn’t what you want to hear. But...
The spark knock you’re referring to can be caused by extremely lean mixture or heat, as well as old/low octane fuel. Highly doubt Your stock 11:1 piston (I’m assuming it’s stock) would be knocking around idle even with the lowest octane fuel you can get from the pump. Not to mention spark Knock (pre-ignition) will increase in likelihood with increased rpm. Not go away. Unless there’s something seriously wrong, valves will chatter, they won’t make a distinct knock sound. If it’s not the crank nut, the symptoms you are describing is a conventional big end rod failure and/or piston slap.
Think of it like this:
You checked the crank nut, and it’s tight.
You said it does it at idle, I’m assuming this is neutral. So very low chance it is transmission related especially because you had it happen at low rpms in different transmission selections applied.
So you’re only options left are something to do with the engine part itself. valves, pre-ignition, or rod knock/piston slap. Unless you have something odd going on.
You said it’s a knock, not a chatter, as long as that description is accurate a valve or valves out of adjustment can be ruled out (They could be out of adjustment still, but that’s not causing a knock) In theory, a valve that’s way out of spec can, in theory, lessen amounts of air and fuel coming in and out, so it COULD cause pre-ignition but it’s unlikely. I rebuilt a motor that broke the tappet bolt and wasn’t even opening a valve, it still didn’t have pre-ig issues. That leaves two options. You can go really high on compression and be fine at idle with 93 pump gas. Ive had 14.5:1 engines that I flushed out with fresh 93 pump at idle after running race gas. If you put fresh 93 in there and it still does it at idle, it’s not pre-ignition. Really sounds like it’s time for a full rebuild.
Also, keep in mind there’s two filters and two screens. Did you check all of them? Oil gets sucked up through the screens, metal that has fallen in the oil is going to be seen by the screens before the cartridge filters. I’ll also mention that just because there isn’t metal in the oil or screens or filters, does NOT mean you don’t have a serious issue. Things slowly wear in such a fine powder it’s not visible to the human eye. There is technically fine metal that comes out with oil every time you drain any engine.
Please don’t ignore the sound and ride with that knock going on, a blown up rebuild is usually way more expensive than a preventative one. When I changed oil on the motor I expressed previously, all the visible metal from the tappet bolt was stuck to the filter screens and in the oil/ignition magnet when I drained it. Almost nothing visible in the filters.
I know it’s not what you want to hear, but I’m 90% sure it’s time for it to come down for a rebuild.
