The oil cap doesn't need the nipple it was an aftermarket buy and would allow you to use a more open air cleaner style like the one in this picture. It shouldn't have any negative affect as is though. My car was running similar to yours recently part of it was a cracked exhaust manifold the other was a bad carb. So besides replacing /rebuilding your carb check around for intake/exhaust leaks and double check your pcv valve.
Also you might want to move your fuel filter so it isn't sitting directly on the motor like that.