ok i searched the fourms and found similar problems and found no help. so i have a 1988 t2 fc shell and i got an 88 engine from someone he said there were no problems with the engine , and it has been maintained very well, when i got it, it has been sitting for a couple month. i put the new engine into my shell hooked up all the wiring and fired her up. she started but only revs up to 3000 rpms then starts to die. i can give it gas and itll stay on a little bit longer but start to die and you can keep on giving it gas but eventually it will die anyways.also it used to i have checked the afm with a multimeter and it passed, i have fixed all vaccum leaks i am aware of. test the bac valve it passed. tested the coils it passed. did i get a bad engine any help would be appreciated. thank you

 

Still sounds like a vac leak to me.