Go back and re-check your work. Make sure the timing belt is on correctly, the timing marks line up at TDC, and that the distributor is back in the original spot. Really inspect the timing belt and cam gear. When I built an engine for my last car, I had the exhaust cam gear (DOHC) advanced one tooth and didn't even realize it. Everything looked right and the car would run, I just couldn't get the timing within spec. Luckily I knew a guy that checked it over and found my mistake.