Rich exhaust (out one side)??

is it fuel rich (burns your eyes when you are behind the car) or "oil" rich? if it is oil, i would suspect a bad intake gasket sucking oil into the intake from the valley. (ask my how i know about that one!)

There are a lot of reasons this could be happening. I doubt it's a valve problem. Have you done a wet and dry compression test?
Has this car ever run correctly while you've owned it?
Did the problem occur after installing the Carburetor?
Have you made any other changes that may have caused this? Tuneup, plug wires, etc?
Have you checked for vacuum leaks (most likely on the right bank)?
Is your PCV and valve cover breather plumbed, and working correctly?
Is the rich smell only at idle?
If you have a power brake booster, can you hear a hiss at the booster?

That's a lot of questions, but if you can answer all of those, it will assist in pointing you in the correct troubleshooting path.