Yeah I run a TB 20mm Carby on my 88 and when I first started the bike up with it on it wasn't running well. You do have to adjust it a bit with the C-Clip on the needle and the air screw also. Good idea to test different idle settings on it too.
I reckon you should keep fiddling around with it before anything else.
Like Saga said, 20mm would be the biggest Id go on an 88 even with a Race Head.
I thought having a bigger Carby would create more bog? Haha