OK here's the situation I just replaced my carb with a rebuilt adjusted/tested stock carb from national carburetor. The timing is dead on(0 with vacuum lines disconnected and plugged), and it idles smooth(850) and runs awesome. The exhaust is all new except the cat which is very much empty. When I let off the gas I get really bad afterfiring. Could this be caused by not enough backpressure on the exhaust without a functional cat? Maybe a plugged vacuum line? That little thing labeled "jet" towards the center top does not allow airflow in either direction. Is that a check valve? If so which direction should the flow go?

EDIT: Figured out one of the vacuum lines on the distributor vacuum advance is not doing anything. I assume both should cause some movement.

 

That was it, the distributor vacuum advance. Now I don't feel paranoid putting in a brand new catalytic convertor. I set the idle as low a 700rpm and it still purrs like a kitten, with no major repairs at 235k miles I am guessing the timing chain is the next thing to go though. Which comes to my next question. Do I stay stock or move to a dual row chain when the time comes?

 

Double row?? Why. What she came with has lasted near a quarter million miles.

Think you'll last long enough to put another quarter million on??

 

Haha yeah could be another 230k before I need a timing chain, and I'll be 97. I hope my great grand kids can drive.