I've got a 1990 16v GTI with serious performance issues. acceleration is slow and it tops out at about 90!!! i've had it up to 120 before!! not on uk roads obviously

seems to rev ok between 5-7,000, slow to pick up before that tho.
I think the car is running rich. which way do i have to turn the screw to fix this?? getting a lot of blowback from the exhaust. not sure if its sealed properly.
any ideas guys??