The rough idle might be something else other than the delayed injection pump timing after remembering that I did not comment on it. The best thing one can do when faced with an engine or performance that is substandard to the average example is a general tune up. By systematicaly doing it you tend to pick up faults along the way and many small things combined can lead to really poor performance. Or as in your case sub standard milage. My gut feeling is put your pump where it belongs after establishing your chain stretch is reasonable and correcting the cam timing with an offset key if required. It is very important to inspect your chain guides as well. Then do a compression check. Get your injectors checked and matched if you do not know their status. If compression check is good and cylinders are reasonably close . Compression tester is as cheap as 9.95 at hudsons I am told so there is no longer a reason not to have one if you cannot borrow one or have one already. Grab a digital meter that can read milli volts.. Read the output of all your glow plugs. If they are equal you should be good to go. If they are not then swap the glow plugs around to verify the plugs are not the factor. If the mixed voltage readings continue there is most likely a problem with the pump. Next a volume flow test to make sure pumps output per fuel element is simular. If volume test checks out you probably have a sequential pump timing problem or enough of an indication that the pump needs retimed to itself not in refference to the engine. Of course before any of this the first thing to always do on one of these engines is to set the valves. They are famous for all kinds of symptoms that are valve clearance related. If an engine makes all these tests and adjustments with reasonable equal compression it's going to hum like a pussycat. It's also all in the archives. Basically as soon as I hear rough idle in refference to these engines
I know it's time to do a total tuneup if I have no knowledge when it last had one. You of course can stop as soon as you have located the problem but for maximum performance and milage you should do it all. Your airfilter is reasonably clean by the way? it will cause reduced milage combined with excess smoking. I would expect that a rough idle may also be caused from too much air entering the fuel system as well. You have to keep an open mind. Most of the above proceedure is time rather than money. It is my hope we can restore or improve sequential timing errors related to aging and people adjusting things they should not have. But first we have to establish without a doubt that is the problem plauging the engine and be prepared to have a pump shop do the corrections it if we fail. But once the fault has been absolutly isolated there will be no reason not to try to improve the situation and my guess is it will turn out well for us once understanding and application are understood. It is not rocket science and we want to properly repair these old cars at minumum cost. The last thing we want to do is spend money where it is not needed as in most cases it will not be hard to find another area of the car that can absorb it. By the way if you want to do a rough check of your injectors at little cost. Just move one after another into the same cylinder hole. Read the milli volt output of the glow plug. If they are say all within .2mv you kind of know they are all the same. Thats basically what you are sending them out to find out. Also if only one injector gave a large variation to the voltage reading that is the one to send out. The others are known good by dynamic checking. All the above mentioned items pertaining to using the milli volt reading approach are a probe into the unknown at this point to some extent. So rather than what to do are more food for thought but I have passsed the point myself of only considering them as abstact Ideals. These tests will function well I believe if the user understands exactly what he or she is doing. Also there will be initial resistnce and that is normal. But to enjoy these cars that are on there way to becoming antiques and keep maintenance costs in line we have to prove the validity of cheap and effective tests if possible to keep our cars running as well as possible.