engine problems
 

I have a 1987 S10 blazer with the 2.8 engine in it, and I was working on some problems. I ended up fixing a lot of mechanical parts in it such as the cam, lifters, pushrods, etc. I also gave it a tune up. Well once i got the engine reassembled and got it timed I took it out for a little run to see how it was. After not too long the blazer broke down again. I found the resn it stopped working was because the coil on it was bad. I went and bought a new one, and once i attached it in, the blazer started right up. So i went to time the engine again and the engine stopped starting. for some reason it started right up out of time, but when we tried to time it it wouldn't go. Also it seems to back fire when I give it gas and start it now. Any suggestions on what might be wrong here?