The diaphragms, are they Bing parts, or aftermarket pattern parts ??
Hmmmm...interesting question. This one took a bit of digging, but here's what I used:
https://www.eubmw.com/collections/carburetor-kits/products/bing-carburetor-kit
I can't tell if it's OEM Bing or not.
And it doesn't matter a damn if they are or are not OEM. providing of course that they are good quality and seal correctly.
The "non-OEM" diaphragm thing started a few years back when some one had difficulties after rebuilding their carbs and assembling them wrong. They tore down and rebuilt, but with OEM diaphragms and wonders of wonders the bike ran correctly - obviously it was the diaphragm's fault.
The myth persists.......
Thing is, if you look at the vacuum system in a Bing carb it is immensely powerful (relatively speaking) a "stiff" diaphragm (within any rational meaning of the term) can do nothing more than slow the rate of change, it cannot alter the amount the slide will rise because vacuum is vacuum.
I have been using non-OEM diaphragms since 1980, and I have never had a problem. Initially Stromberg diaphragms, but more recently whatever Moto-Bins is flogging as the OEM alternative.
I have miss-assembled carbs on occasions and I have sucked up the fact that I made a mistake and fault diagnosed till I found and eliminated my own mistake.
In no particular order I set out below a list of things that can cause your problem (or similar) for you to check.
Diaphragms seated correctly in both carb body and slide.
Dome seating correctly and no leak at the top (if there is the 'approved" fix is the BMW badge that is the same diameter and epoxy - if you are in the USA you can send the dome to Bing USA for removal of the metal "bung" fittment of a new one and re-crimping, but be aware that the price is pretty much the same as just buying a new dome.
Slides have full, free and correct movement available, all airways (vacuum ways I suppose) not clogged
Start enrichment circuit assembled correctly and all screws tight
Emulsion tubes clean and correctly mounted to jet assembly
Funny looking flat washer fitted to mainjet - you would be amazed how many people remove this and either do not replace it, or replace it with a much smaller one - the odd looking oversized washer is the correct one and does an important job.
Float valve, needle and seat and floats not binding and flowing fuel freely. It is possible to miss-assemble these so that they can "hang up", but usually the lack of fuel in the bowls is a dead giveaway.
Intake trunk rubbers on the cylinder heads in good condition and snug tight to both head and carb
Check plastic intake trumpets and make sure any rags you stuffed in there on disassembly were removed when you put it back together (don't ask me why this one is included).
Airfilter, clean and good quality (which includes not being an oiled fabric piece of rubbish) - sometimes OEM, or a good copy, is best.
If you had the throttle plates out are you absolutely sure they went back in the right way round?
Fuel tap in god condition and flowing correctly? There is a filter built into the tap, it is known to clog.
Fuel lines in good condition and not kinked anywhere? My personal trick was to trap the cross feed pipe on an early model R65 with clam shell aircleaner - it flowed enough to trick me for some time, except that the RHS carb would run dry quite quickly at WOT or thereabouts. I felt a 1st class dill when I eventually found it.
Recheck every jet, a minute piece of lint will change the flow characteristics for the worse.
Once you are absolutely sure that your carbs are OK, start checking ignition components - on the well established principle that if you *think* it is fuel it will be electrical - and vice versa.
