Chevy Vortec 4.3l

[montanamike12]

I have a 96 chevy vortec 4.3 l that runs great. (at least at idle) I've given it a pretty good tune up but when you want to get off the line it is sluggish and slowly comes up to speed. it loves to cruise at 40 mph. also there is no power when heading up hill. was wondering if the timing had anything to do with it, but couldn't twist the distributer, is it electrically controlled?

Please help!