Rough Idle & Delayed throttle response

I have recently rebuilt the carb on my 71 m38a1 and have noticed that it idles rough and when applying the throttle there is a large delay? Is this due to an incorrect carb setting? As well, what is the screw on the outside of the carb for? I originally thought this was an idle screw but seems like I was mistaken? I has a spring attached on the shaft of it and has a washer with a tab that hooks into the carb. Any help is greatly appreciated!

Welcome. That screw is indeed the idle mixture adjustment. When you rebuilt it, did the check ball or balls (forgot quantity) go back in the right places?

I believe I put everything back in as found just with the new parts. With that said i believe I only had a few left overs from the rebuild kit but I accounted for where they went. In regards to that screw, does the more you turn it in the more fuel to the carb or vice versa? I have found starting to be an issue with it as well which is most likely a related issue. If it dosn't start on the first go I have to use starter fluid.

In is lean. Could you have fuel contamination that came up from the tank and messed up your fresh rebuild?

Very possible as I did have a bunch of really bad gas go through before I thought to drain the tank (It was a stupid moment for me) Should I pull it off and clean it out or would a gas additive carb cleaner fix this?

I would drop the tank and do a good cleaning, I have and M37 and when i drained the tank I also needed to pull the sending unit and got a look inside the tank and it was NASTY beyond words. The tanks are easy t take off and reinstall and you KNOW what you have then and really besides a few hours you have nothing to lose!
Karl