I have no idea what's under your hood, but I doubt it's a carb. Your car most likely has a Bosch fuel injection system. MB did not put carbs on these vehicles. You likely poured fuel down the AMM(air mass meter) as some call it. If that helped, you may very well have a fuel problem.
You're going to have to elevate the rear of the car to get any kind of sane access to the fuel filter/pump(s) close to the passenger side rear wheel unless you weight 40 lbs.
Here's a photo of a 300-SEL AMM. I "think" the 420 is similar enough for this to at least show what you most likely have. The photo might be expandable. Look at the gold disc. This is where you probably poured fuel.
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1EQ0PTV2S1H112V785&year=1991&make=MB&model=300-SEL-006&category=B&part=Air+Mass+Meter