Bought a older 89 trx350 4x4 accelorates good but sputters when running a constant speed unless its under heavy
load, i can find one adjustment screw on the very bottom of the carb (you have to take the carb off to adj this). The
screw was bottomed, does anyone know the proper setting for this one?

 

Sure its an 89? I only find the 250 made in 89. Anyway, its most likely the mixture screw, which only effects the idle mixture...once the throttle is opened up, the needle and main circuits take over.

Its funny that itll idle with the screw all the way in. But a good intial setting is about 1 3/4 turns out.

How old is the gas in the tank?

The carb may need a good cleaning, and it probably wouldnt hurt to clean and reoil the air filter, and change the spark plug.