When you are trying to adjust the valves you will have to rotate the engine either by a Hex Key in the alternator bolt with the engine in neutral, or the engine in gear, higher gear works better, and rotating the rear wheel. Which ever method is used remove both spark plugs so you aren't rotating against engine compression.
I know you are keeping your eyes out for some loose wire, dirty connector, solution to this problem But in the mean time we would like you to put the bike in a state of basic good tune. It's been said before, (1) set the valves, (2) Adjust the timing, (3) adjust carbs on a warmed up engine.
So far we have said a lot about adjusting the valves. This is a critical adjustment for you to get right and to get it right it must be done right. So I hope you get this part. For timing adjustment you will need a timing light and know how to use it. Have you got the timing light? And the know how?
And for the 3rd thing I'm going to give you the basic settings for this engine and the carbs on this engine so it will run after you have done the other two things. The carbs on a 1976 R90/6 are the Bing 64/32/11 & 12. I don't think at this point we are suspecting any problems inside the carbs like plugged jets or bad/maladjusted floats? What you need is the basic setting for these carbs that will make the bike run well enough to get it warmed up. This follows;
There are two screw adjustments on the underside of the carbs. If you are not working on a bike stand but the bike is on the ground you will have to lay on your back on the ground to see these screws. The smaller screw that is inside a shrouded part of the carb body is the mixture screw. For this bike the mixture screw is one turn out. Turn the screw in gently till it seats. Do not turn the screw in tightly. It can be damaged or, because the screw is steel and the seat is Aluminum or Brass, the seat can be damaged. Turn the screw out some and in several times but don't seat it too tight. Then after you have found bottom turn this screw out One (1) Turn. This is the basic setting for these carbs on an R90/6. Other years and other carbs may have a different setting so if your buddy with an R80 G/S tells you he uses a different setting don't worry. Different bikes, years, carbs, different basic settings. I get this information from the Bing manual.
The other screw on the bottom of the carb is the idle speed screw. It is larger. It has a spring around it, that you can see, the mix screw also has a spring on it but it's inside and not visible. The idle speed screw opens the throttle butterfly, but just a little, to allow the engine to idle. It is really hard to see with the carbs on the engine but the idle speed screw should be about one turn in from the point it first contacts the lever it pushes against. I get this setting from an old friend of mine, it's not in the book.
If no other problems with the carbs this will allow you to ride enough to warm the engine and then be able to properly balance the carbs. We will maybe have to do another whole couple of pages on balancing but I think that can wait till you report back on progress.