i've got a 94 s10 blazer 4.3 mfi. it seems to have an odd fuel mixture. no smoke at all out of exhaust, although it does smell a little rich. i'm only getting about 13 mpg. i have less than 140k on it. wondering what i can do to correct this. any help would be appreciated.

also i get a herendous clunk from the back when turning semi-fast/semi-sharp. i was told by the old man at work that it was the diff. and i could put some magical fluid in there to fix it... but i looked at the bolt/plug in the differential and it was stripped out so badly you could never get it out. so there goes that idea. again any help would be great. thanks