What does this screw do?

[BradleyPLKD]

Having rough idle problems with my 1996 5.7L VP gas motor. Runs perfect at higher rpm. Rough idle or dies at idle. In checking the carb, I noticed a prior fuel leak on choke side of fuel bowl where paint was dissolved (see picture). It looked like the leak came from under a small bump which turned out to be black putty covering this screw head. What does this screw adjust (center of pic)? Does it have something to do with my rough idle? Mahalo!

 

[Idlespeedonly]

The screw is for the fuel level adjustment.
If the fuel level is to high it could flood the engine.

 

[Scott Danforth]

your rough idle is because you probably need to have the carb cleaned. build up of dirt and crud in the idle passages is my first guess.

 

[Maclin]

That is a fuel level sight plug, well that is what the threaded hole is for. Looks like someone lost the real sight plug and cobbled in a screw of some kind then when that did not seal they doped it up. The sight plug has a large head that does not fit flush, and seals to the bowl with a gasket that fits into a recess.

The sight plugs are most useful on carbs that have externally adjustable float/needle/seat type fuel bowls. They are still useful for internally adjusted float levels (can only adjust with fuel bowl off the carb) to see what the level is while running. When fuel level is correct the fuel in the bowl should just be coming out the very bottom of the hole when the engine has been running a bit and the plug is removed to check.

 

[BradleyPLKD]

Thank you everyone for the replies.