1 – Low quality Fuel
2 – Wrong Engine Timing
3 – Too Lean Air/Fuel Mix
4 – Bad Knock Sensor
5 – High Compression Ratio
6 – Improper Spark Plug
Let’s talk about No.5
Can be difficult to diagnose but easy…
If you carry out compression test on the cylinders and notice the compression value is higher than what is stated in the repair manual like AllDATA, then possibly you have issues of carbon deposit on the piston head which usually is caused by oil leak into the cylinder via the piston rings, which is one. The other cause can be carbon deposits on the valve stems, 
when they open to allow Air in or out (exhaust gas), the deposits fall on the piston head in the cylinder.
That’s why when you remove the top cylinder head, you see deposits on the cylinder.. Always scrape them off when overhauling or tuning up the engine.
