The fuel pump on my 87 samurai 1.3 has 4 hose fittings . I know where the 3 on the top go to  but what does the one under the diafram connect to? Is it just a vent hose to the vapor canister?

There is one tube on the Carbureted 1.3 mechanical fuel pump that curves downward and has no hose attached to it.

This tube acts as a vent for the back side of the pump diaphragm so that air will flow in and out as the backside of the pump diaphragm as it displaces air.  If the vent tube was not there, the pump would have to compress the air trapped behind pump diaphragm which would lower the efficiency of the pump.  It would also keep oil from being moved into the backside of the diaphragm to lube the pump rod.

On a side note, if you see liquid dripping out of that tube - or a wet dusty build-up on the end of it, you may have a broken diaphram!   That dripping moisture could be - SHUDDER - gasoline.  If it's gasoline, you have a BIG problem that requires you to replace the pump IMMEDIATELY!  You may be pumping gas into the oil which will thin out the oil to the point where internal engine parts will start to wear violently!

Been there, done that, got the replacement engine.

thanks for the info, I couldn't remember taking anything off there,but it's been a while since I tore that thing apart