Okay so I have a Mikuni V38 on my bike. Really a nice carb but lacks in the ability to tune as far as I can see. I can only really find a idle mixture screw on the carb. How else do you adjust the mixture on the carb. I could rejet I suppose but as far as I can tell from the other threads in the forum is that I am running the correct jets and needles. Anything else that can be done with it?

Just a general question.

 

Different taper needles and needle height adjustment.

 

That VM38 Mikuni can be tweaked endlessly. There are 4-5 different "cutaways" for the Throttle Slide, there are about 20 different slide needles, and there are about 20 main nozzles (the fitting the main jet threads into). That idle mixture screw you are referring to is called the "air screw". It regulates the AIR only for the idle circuit. The other screw adjustment is for idle speed. Keep in mind this carb has been used on 2-stroke and 4-stroke street, dirt, and race bikes plus snowmobiles.
I had a "3.5" cutaway throttle slide that caused a really bad flat spot right at the "off-idle" point. After looking into the documents available it was determind that I needed a "2.5" cutaway to richen the transition from the pilot circuit to the needle. I gave up on tweaking in theMikuni and went back to the 'ol S&S "B" because of the price of the numerous parts I needed to get the Mikuni dialed in correctly (those throttle slides are $30/ea!). It can be very painstaking to get that Mikuni dialed in but once you do, it will do a beautiful job.
Rather than type a novel, you can do a on-line search on the VM38-Mikuni and dig up a wealth of tuning info on your carb.