Bad catalytic converter too????
 

After finally plugging all of the holes in the exhaust on my 91 740 n/a, I'm thinking I have some flow issues. It started with a sulfur-y smell when I accelerated, and now it occasionally feels like someone jammed a potato in my exhaust along with the sulfur smell. I've used 3 different kinds of gas over the last few weeks, but nothing changes, I still get the sulfur smell. I drove it home from work last night and it was fine, smelly albeit, but fine. Then my wife went on a short trip a couple hours after I got off work and it felt gutless. From about 25-45 mph it just seemed to seriously lag on acceleration (didn't take it above 45). I think all of the lean intake/rich exhaust is catching up with it. Of course if i "replace" the catalytic converter I will be replacing it in the sense of fabricating a pipe to go from ahead of the test port/extra o2 sensor port to just behind the converter, with a custom welded o2 bung.

Well, yes, your cat could well be d.o.a. A simple test is to hold your hand open near the tail pipe and have someone blip the throttle to, say, 3-4k. A clogged cat will have appreciably less forceful exhaust than normal. You may very well have particles from the cat blow out on to your hand. Good time to change the O2 sensor too if it isn't new.