y do i have to keep my gas petal pressed to start it and as soon i i release it it dies any ideas

 

Probbaly a vacume leak. Check to see if the vacume line from back of manifold to brake booster came off.

 

Also the tps sensor could also be shot do a search on how to test it with a multimeter cheap and also easy to do if you have the meter that is. Is this a 2.5 or a 4.3 not sure what they had in the blazers back then however if its a 2.5 let me know i use to have one of those in my 88 s10 for years i cant remember what the same problem was im leaning towards something vacuum like stated already but im unsure i believe it was a vacuum leak i know when i had my 2.5 i cleaned the intake out alot and it ran like a champ.

 

its a 2.8l carb with the vacuum leak thing i have a chiltons and haynes manual and my daigram for my truck is not in either of them

 

85 is carbed and doesn't have a tps. 4.3 wasn't an option in s-series till 88.

Most likely a vac leak and/or the carb needs rebuilt. When you have the carb off if it still has the screen grid under it take that and throw it in the trash. That thing goes bad and causes problems most of the time.

 

i bought a new one and those thangs are not cheaP

need a vacum diagram UDW cant find it in any manual nor a ood enough 1 online for me to understand