Kody Heglar

I have a 1990 Honda crx base 1.5L Straight drive. I had the bottom end rebuilt due to bad rings. After I installed the engine back in, the car never ran right. Before I would get 30-35 mpg, but now probably 15-20. There is a check in light that comes on, but once I get out of first gear. No check engine light when I crank it, only when I drive. I've tried to read the code but no luck since it's not on during idle. The car runs really slow and sluggish until I get to higher speeds. Take off is terrible. What could it be? Fuel? Anything will help! Thanks in advance!