I have a 1976 R90S with a Boyer Brandsen ignition. I had been having carb problems -- I don't ride the bike often and every time I did the carbs would be clogged even though I drain the float bowls after every ride. So I took the shotgun approach: I sealed the tank with POR-15 (that seemed to go well), I Chem dipped the carbs and replaced all the seals, o-rings, pilot jets and accelerator pump pistons (I had replaced the needles and needle jets recently). The timing is good and so are the valve clearances. It starts right up on the choke and it runs fine at mid and full throttle. I took it for a 20 mile ride yesterday (it was hard to keep runnng at stop signs) and then synched the carbs using a flow meter. It would only idle with the idle screws turned all the way in. Also it ran best with the air screws turned all the way in. I've checked the float heights using Snowbum's method and they're good. I've checked the choke cables and they have play in them. I went out to try it today and gave up tying to go another twenty miles because it kept stalling when pulling away from a stop.
I'm at a bit of a loss. I'd appreciate any suggestions before I start tearing down the carbs again.