i need help with my flat-4

hello everybody,
i began building a buggy from a VW Variant, before taking it appart
the engine was running exelently however the body was so rusted up it
was not worth keeping it.
i have finally gotten the engine in place and connected everything up
and i recently started the engine after about 18 months without running
is started pretty much immediatly however after about five minutes of
runnung the engine would try turning off. you have to pump the gas
vigorously to keep it running just.
im not much of a mecanic but i think that it is to do with lack of
petrol thus by pumping it the injection cuircuit in the carburettor
keeps the engine running...
any help would be greatly apretiated!!
P.S: i had to change the coil for a second hand one.. dont know if that
has something to do with it. i also checked the spark plugs and they
were quite black with carbon residue.
thank you again
Stefan