if it runs it wouldnt be a fuel pump, i dont think anyway. is it carb or TB? if carbed its most likely that. if its the carb do a webber carb. my 1987 GL is carbed and sometimes it will shutter or lack power at cold temps and if i gun the throttle. saving for the webber...
gonna leave the top part incase its carbed. not sure what TBI means. but if its throttle body then it has to be a sensor at the TB or a O2 sensor. melted cat or something.
hope that helped any.
87roo.