any1 know how to time an87 s15 2.5???

[rydafoo]

ive an 87 gmc s15 2.5l runs a little rough so i dicided to set the timing but whn i didi it idled smoother but backfire and very slugish. on my 91 s10 blzr theres a bypass wire i jus disconect, set timing, reconect but thers not 1 on the 87. VECI says ground diagnostic connecter locted under dash, set timing, then unground. im not sure what to do n i dont wanna fry the ecm by jumping pegs on the connecter. any ideas would be greatly appriciated