There is nothing wrong with an 850 DP with stock jetting as new! Carbs are calibrated to give 12.5 to 13. to 1 AFR at wide open throttle and 14ish at part throttle. carbs do not know what is under them only how much air is being pulled through it.

Anyone who washes the walls has problems and no crab tuning and fixing skills. washing the walls would indicate floats to high, blown power valve, bad gaskets and so on.

870 is a good choice if you intend to do some racing then I would lean to a 850 DP. new or at least properly rebuilt and set up.

many get a carb then their expert buddies talk them into jetting it up 10 sizes cause it's a "race motor" I could tell you stories.