W/ full tank 1200 gs runs 2or3 seconds then dies _ wont restart

what year, ??????????? 1986? to 2003,. called XII (mark 12)
on reserve or not;.?
petcock filter /screens dirty. or gunk/trash in bottom of tank.rust?
it is staving for fuel , find that and is cured. (simple but reality is a beast)
why post to cars?, and not motorcycles. only.
dirty carbs will not like weak gravity fed tank pressure (natural)
dirty float needles/?
I bet all the above, that told
or just bad fuel.(age old)

Float adjustment and/or fuel flow from fuel tank.

the fuel gauge sender is a float in the tank running against a coil of wire if the float has filled with fuel it wont float,first to check two wires connected to sender,pull off and touch together that sould read full on gaugeif not its the gauge at fault,if its ok youll need to get sender from tank its normall held with a big nut half turn to release,check for corrosion breaks or damage good luck

The most likely cause is the fuel pump controller. It is positioned under the cover just left of your instrument cluster, on top of the fuel tank. These units are not repairable and it will need to be replaced. this is what it looks like.

Most likely your fuel pump regulator that is malfunctioning.

It is located under the LHS top panel, more or less in line with the instrument cluster.

chegk out "R1200GS.info" tell's you everything

I have been told by my dealer that the 2005 GS fuel gauge reads full until it the tank becomes 1/2 full then it drops to 1/2. I have had a similar problem to yours but it cleared itself. I keep an eye on my miles riden per tank as BMW will not admit it has a sending unit that is not up to par.

Bonzaii

Your problem is the pump pick up in the tank is either broken or it has been bent up at some time, and it should be almost touching the bottom and yours would be about 2or3" up so when its still 1/4 full you cannot pick it up .
So take out the pump and fix the problem, or take into the garage and tell them what to do ..
NOT unless you have a faulty gauge sitting on 1/4 and it is EMPTY.
But thats one of the two problems it can be .. Have a goodday Ron

Check and clean your gas tank. If still a problem there might be dirt in the carb.

Air filter clean?
Air filter port set for summer or winter ?
Check the fuel filter inside the tank to ensure that it is not dirty or hasn't fallen off.