I have a 90 rs which runs fine with 206k on it but idles at like 400 rpm in drive, is there any adjustments that I could do.

 

Chance are your tach is just off and your car is idleing fine. Does it hesitate or act like it needs a boost. The idle is set in the prom but you can do a few tweaks to raise it if you have to.

 

It is very hesitant and shakes alot while in drve.

 

there is a article in the tech article section about setting your idle. At the front right side of the TBI there is a hole with a cap pressed inside, get a screwdriver and pop out this cap, it will allow you to adjust the idle screw with a T-25 torx bit.