240sx, Runs lean, loud knocking

[dareo]

1990 240sx, SOHC KA24E engine, 178k miles. (my red one, not the grey 89 in my sig)

After a lot of abusive drifting, my red 240sx starts to knock. Its got a loud knock going, but it will go away if im in 5th gear and on the gas a lot, but only going like 25mph. So i think its running really lean. What should i check first? The idle is messed up too, it goes 1500-2100. This happened just before we started to drift it, then the knocking came around. Yeah i know i probably shouldnt have drifted it with an idle problem, but this car is my rusty beater/drifter. But it still needs to run cause i got a guy that really really wants to buy it.

I've had the battery disconnected overnight, to clear ECU. I will check and see if its still messed up a little later.