Small Engines (Lawn Mowers, etc.): Go Kart, manco kart, plug sockets

Question
The go kart was bought new Dec 2004, and has had minimal use.  I was thinking of a little gas in carb, than thought I'd pull the plug first.  Wouldn't you know, it's buried in there, and none of the 3 spark plug sockets I have fit it...  It's slightly bigger than a 3/4 inch socket.  Would you know what size socket fits a spark plug NGK BR-6HS?  It was getting dark, so figured I'd deal with it tomorrow.  I just noticed there is a spring hanging loose, on the engine throttle control.  So now I'm thinking it's probably that.  All I have to do is figure where it hooks onto, since it's not in the schematics for the go kart.  The pedal pulls a cable, that works the throttle on the engine.  The spring, appears to have been hooked on the opposite side of the triangular widget, that the throttle cable pushes.  Assume the gas pedal pushes it, and the spring pulls it back?  If that spring was disconnected, would that cause the backfire?  It's a Manco Kart Modle No.606B.  Engine is a Robin Subaru 6.5 hp, EX13D/EX17D, EX21D/EX27D.  I figure it's something minor, so don't waste your time looking it up.  Thanks!
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The text above is a follow-up to ...

-----Question-----
My sons go cart has not been ran since April.  Today we went to start it, and it doesn't start, when you pull the cord.  After several pulls the engine makes a loud pop, bang sound, and black smoke comes out, but doesn't start.  I thought is was maybe bad gas.  Drained the tank and added fresh gas, but still has the same problem.  Please advise..
-----Answer-----
 Hello Art:

 What is the Compression? From your Description it Sounds like a Valve is Stuck or the Spark Plug is Fouled. Have you Tried Pouring a Small Amount of Gas into the Carburetor Throat and then Start the Engine? If so, What Happens? Send me the Model, Type and Code Numbers Off of the Engine, Please. With these, I can Locate a Breakdown/IPL of the Engine and Better Assist you. Hope this Helps. I am here if you Require more Assistance. Thanks.

 Good Luck

 Respectfully

 John

Answer
 Hello Art:

 The Plug Socket is a 13/16". The Throttle is Supposed to Pull the Spring and the Spring Pulls the Governor Arm and the Governor Arm Pulls the Carburetor Throttle Plate. To Much Gas in the Cylinder will Cause the Backfire. Hope this Helps. Thanks

 Good Luck

 Respectfully

 John