i postedit here since it was a widely used motor. a while back it was missing real bad and ended up it was the distributor gear worn down that it was cross firing. but now it misses and runs really rough on start up. it just started doing after i messed with the oxygen sensor ( i dont have a cat) did i do something that could mess with the start up mixture. it doesnt do it all the time either and if i hold it to about 3 grand for a minute or two then runs perfect its in a 98 k1500 with almost 170000 miles

 

it could be a bad distributor cap & rotor