I have an 83 K5 Blazer, i recently rebuilt a 350 and installed it, worked fine for a while but then had starting problems,Sometimes it will start and run just fine but then i can go back out a couple hours later and it won't start. it turns ovoer but never fires, any ideas?  

Check for ignition "fire" when it does not start.  

in the distributor is a control mod. when it gets hot sometimes it will quit working. you can take it into napa carquest or whomever your prefered parts house happens to be and they can test that for you...if that is the problem, make sure to put some dilectic grease on the bottom of it, heat is a killer... also you could check for a cracked fuel line. if there is one it will sipion back and can take a lot of crancking to prime the lines again