what is the screw behind the needle jet for on the carbuator on 1954 wg4

AJ series carburetor used on early Mercury and
Wizard models is pretty basic. Idle jet,main (jet)
adjustment and idle adjustment...at a guess your
question is to the spring loaded idle adjustment.
It is pretty hard to navigate without a parts manual...if you look at manual showing any of the
AJ series Tillotson Carburetor the answer will
become clear. I believe one of the New York chapters
of AOMCI.ORG posts manuals for several Mercury
models using AJ carburetors...OK?

the long jet behind the low speed jet that u cant get to unless u take the carb or tank off

Here's a link to the page with info on the Tillotson AJ carburetor.
http://www.maxrules.com/carbindex.html

The 8th and 9th one down gets into the meat of the AJ gravity carb and it's adjustment procedure.

http://www.maxrules.com/oldmercs/carburetors/1965c/5.pdf
http://www.maxrules.com/oldmercs/carburetors/1965c/6.pdf

Discount Marine parts has a page for the Tillotson carburetors and some kits for them too. Scroll down and click on your carbs prefix letters (in blue).