Nissan NS5B what the heck is this button??

[rbc]

New to me NS5B year 1990.
Front op panel has choke, kill button(not emergency off switch) empty hole with rubber plug then a push button (missing rubber cover). When I push button gas leaks out,push again and it stops. The button leads to a fuel line that goes to a Y connector. One part of Y is this fuel line,other part leads to fuel filter then to carb. The leg leads to a manual shut off at the fuel tank. I have looked in parts manual but it does not show this button? What the heck is this button?
Cleaned carb but couldn't get screw out that holds float. Tried to start engine but it will only run on the fuel I spray into intake. I assume I need to take carb apart again and clean float valve.
thanks for any help
RBC

 

[Sea Rider]

Post pics for better clarification. Is the screw head that holds float's pin severely damaged or only locked in place ? Is that a dual external and internal fuel tank version. If so has to have a front plug with a middle pin where you connect external 3 gal fuel tank to.

Happy Boating

 

[pvanv]

Sounds like you have a 5BS, which can also use an external tank.
What method did you use to "clean" the carb? You Must completely disassemble it (including float valve, jets and emulsion tube nozzle), and do a 4-hour soak in real carb dip, followed by a liberal blow-out with generic carb spray.

 

[Sea Rider]

Yep, being a 26 year oldie will need to a make a full carb clean to immaculate cond as when new in order to work as intended. Once carb is immaculate clean, run bowl dry after outing..M5B is also a dual tank model with fuel **** located at lower internal tank, what's the difference with the M5BS model ? .

Happy Boating

 

[pvanv]

The 5B is the internal version. The 5BS is the dual-tank model.

 

[Sea Rider]

M5B is a dual tank model, so with S ending is same ? Probably for other markets ?

Happy Boating

 

[pvanv]

US 5B models were internal tank only.