I've been on the other end when I changed out a timing chain on a 318 back in 92, I could not get the damn thing to run right and tore the front end down three times to check and triple check my timing marks. When I took it to a shop for diagnoses, and I got ARE YOU SURE YOU DIDNT DO SOMETHING WRONG? The weirdest thing happened when they started the engine, it ran perfect! It turned out the crude early computer in the air filter had a bad connection. They found it in 10min, the problem started in a minor rough running engine, classic worn timing chain and was worn when removed. It never ran better after that, we bought it with 90K miles on it.