When I asked for help with the diagnosis I didn't expect to get so many responses. Thanks to all of you. You have certainly given me a lot to consider.
I was wondering about the compression differences and whether that could actually be a cause for it to run rough. I guess I still have some diagnostic work to do.
The motor is not EGR equipped but of course that doesn't mean there can't be an issue with flat spots on the cam.
BTW, I verified that the engine is a 1972 model.
As far as I can determine the problem can be in one (or more) of three areas. Ignition, fuel system, or mechanical. With the tools and resources I have available I have decided to approach this methodically and try to eliminate one thing at a time.
Since I have replaced the points distributor with an HEI and still see the same problem it tends to make me think that it's probably (I do say "probably") not the ignition.
That leaves me with a carb/fuel system/vacuum related problem or a mechanical problem. I think I will install the new carb and manifold that I have in the shop and see if that improves anything. If it does, great! If not, then I move onto the mechanical possibilities.
Hmmm...mechanically probabilities probably point to valve train issues. Could be: flat cam, the timing chain had jumped a tooth or two (I have never had that happen but I guess it could be theoretically possible), bent pushrod, or a problem with one or more valves, rocker arms or valve seats. I'm not getting any funny valve train noises that I can detect.
It I can eliminate any possibilities other than mechanical (I guess I should say "probabilities") then I have to determine whether I want to tear into this motor or not. I don't want to get into a total rebuild which is where this could lead and I don't want to throw money at an already tired motor by patching it up. You know...like going through the heads and doing a valve job without doing the bottom end. Frankly I don't want to spend that kind of time or money right now. If it's an easy and inexpensive fix then OK but if not then I have a good running 283 waiting in the wings. I'd just like to have this one running right if at all possible.
I guess my questions now are: Does my thinking seem logical? Is there something I'm not considering?
I should have that new carb and manifold on in a few days but before that I will pull the valve covers and do a visual inspection of the valve train. I'll keep you all posted.
Thanks again guys!
