1995 gmc 1500 305 cid replaced timing chain and sprockets replaced plugs wires cap rotor unplugged bypass time set and set timing to 0 btdc runs great until I plug in bypass time set then it runs terrible Misfiring terribly bad

SOURCE: Timing off, advancing 20' 95 GMC Sierra 350 cu.in.

after connecting your wire try resetting your distributor or pull timing chain cover and ckeck timing marks on gears

SOURCE: runs terrible, p1391 is the

since you've replaced all the expensive components 1st.i suggest you look towards clogged fuel injectors next.try some sea foam,add it to your fuel tank to clean the fuel injectors.your computer tells the injectors and the crank sensor tells them when to pulse. you may only have partially clogged fuel injectors or water in the fuel.the seafoam will dry up any moisture in your fuel tank and also unclog fuel injectors. lucas fuel injector cleaner is what i usually recommend,but your issue call for extreme cleaning. try the seafoam,it costs $7.99 per bottle and treats 25 gallons of fuel.follow the instructions on the bottle.but if you suspect alot of water in the fuel tank,i suggest clear rubbing alcohol !/2 bottle to your fuel tank directly.it will not hurt the engine,but will remove excess water from the gas tank. thank you,for choosing fixya.com!

SOURCE: 1994 GMC 1500 Sierra 4.3

The distributor gear. It wears down and causes rough idling to no start. Pull the distributor, look where the rotor button is pointing as it needs to go in the exact same position. you can get just the gear and replace it. it is held in by a drive pin. Push it out and swap gears and reuse the drive pin. hammer it in.

SOURCE: 95 GMC K1500 350... replaced distributor...timed

ok ran great before changes. right. you say unpluged computor to set timing.. yes.. do you mean the plug on the firewall passenger side that looks like a fuse holder kindof..?? (set timing connector ) (tan /blk wire)
did it run good during timing proceedure??? wont be MAP that is engine vacume has nothing to do with what you have replaced.
proceedure is to set timing . shut off engine , plug in.. then restart.
yes timing moves as per ECM specs. check that timing did no get out one tooth when you replaced distributor. retime . restart