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Then you checked the valves...they were OK. As part of the valve clearance check (for which you had the alternator cover off), when the cover was reinstalled, the points wire was pinched/severed. That has been replaced. Along with new points...even the older points hasn't cured the problem.
So, the 22 mpg problem is still there because you're troubleshooting the tendency of the bike to stutter and backfire. No real road time with the bike because of this problem.
Since you've changed the points, condenser, and are quite sure the timing is correct, I just can't get past the carb issues with 22 mpg. Seems like running the bike that rich for a period of time can't do the carbs any good let alone the engine with a super rich mixture in there, possibly fouling the plugs.
I'm not seeing anything wrong with the electrics...I'm wondering about the carbs.
I agree. I'm an amateur wrench, but when I got my current bike 3 years ago- it was getting 22mpg. (1974 R75/6) I had the heads redone and hardend valve seats put in, and I re-built the carbs myself with all new everything. Then I put it back together. It was still getting 22mpg. I talked to one of the pros at Max BMW and he told me to drop the needles in the carbs. Problem solved! Mileage immediately went up to 40 and the bikes been great ever since. I would double check the needle position in your carbs referencing the Bing book.
Tried two new sets of points. No difference. Same symptoms with old and new condenser.
Yes, I am absolutely certain that the points gap is right. Double checked. I use the Paul Tavenier tool, as I did before this issue, and the gap checks. Paul's tool requires the advance mechanism be removed, the tool is inserted emulating the maximum points opening position and the points are adjusted using a feeler gauge. Far more accessible than with the advance unit in place.
Repeat Q: Could damage - if any - to the diode board cause my symptoms?
If it ran well before - albeit using too much gas - and refuses to run for more than a few seconds now (so I cannot do the coil test described) I'm inclined to suspect the diode board. But that rather defeats logic as I understand the function of the diode board is to keep the battery charged by rectifying AC from the alternator to DC for battery charging. Or does the diode board feed current to the coils for spark? If the latter then the diode board is the only additional variable I can think of.
Any <b>diode board experts</b> out there?