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Yes, you want to check it on level ground.
Retarded spark would not cause spark knock. Too advanced spark could cause spark knock (i.e. the mixture in the cylinder is being ignited too early in the piston's travel and fully explodes before the piston is at top dead center. That's one cause of spark knock.
A second cause of spark knock is having spark plugs that have a heat range that is hotter than the engine needs. How this works is that the spark plug sticks a little farther down into the cylinder and thus absorbs more heat from the combustion gasses. They were used in cars, years ago, when a car suffered from fouled plugs caused by incomplete combustion.
A little more insight is needed here. Does the noise (I'm going to pursue spark knock here) happen when the engine is cold? That is, when you first start off and accelerate hard, do you hear the noise? If not, that suggests that the timing is OK (most likely), and that the Knock (pinging) is caused by a hot spot inside the engine. Such as a red hot carbon deposit on the cylinder head, piston, spark plug, or exhaust valve. More later after another coffee. lol
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