If you're fairly certain we can rule out ignition timing and all things electrical that go with it, then it's time to consider carburetion.

► Start with replacing the rubber hose between the cylinder head and carb. Those need to be very supple to enable an air-tight seal. Over time they grow hard, and will develop tiny air leaks, which lean out the mixture.
► Next is the neoprene diaphragm under the top cover on the carburetor body. Those act as a seal between the slide (and its vacuum state) and the atmospheric pressure. Since they flex every time the slide goes up and down, they develop cracks and tears over time. An opening in the diaphragm means the slide won't raise and lower as it should.
► Replace the o-rings on the pilot screw and on the jets within the float bowl. If you take the old ones to the hardware store you can easily match them and buy a 10-pack for the price of 1 BMW o-ring. The one on the pilot screw is a #60.

Here's some helpful tricks you need to know....
► Any home handyman can can do the work described above. But after running together so many years, you don't want to swap carb parts around. Disassemble one carb at a time to insure that Left carb parts all end back up on the Left carb.
► Use of strong carb cleaners, especially Seafoam, advance the aging process of all "rubber" carb parts. If you continually run an additive to "prevent" issues, then it's better to use a name brand fuel that has the cleaners already built in.
► You might check out the complete carb kits from EUBMW for price and ease of ordering.

Hope this helps.

Thank you Wobbly. I really appreciate your suggestions.

You're quite welcome. Let us know how it turns out.

I decided rebuild the carburetors, replacing all the rubber parts and brass parts. The carbs were dirty and needed the cleaning. The main jet area in both carbs were congested with sand like material; how it got in there is a mystery to me. The big "ah hah" moment was when I took the right side carb and found the choke was difficult to turn and was not opening completely. Taking that choke apart, I found the o-ring had deteriorated and was jamming the whole works. I suspect this was giving me all the problems.

All is back together but since this is Alaska and the temps are currently in the minus zone, it's going to be a while before I can adjust and synchronize the carbs. But I am very confident my problems are solved and I will be riding many miles this summer.

I decided rebuild the carburetors, replacing all the rubber parts and brass parts. The carbs were dirty and needed the cleaning. The main jet area in both carbs were congested with sand like material; how it got in there is a mystery to me.

The o-rings and rubber parts would have naturally disintegrated over time. Nothing new there. I know you're glad those were discovered and renewed. That part is probably good for another 40 years !!

However, the sediment needs further investigation. That could ultimately be the main issue. Some small amount of sediment is always going to be there from gas stations. More than 1/4 teaspoon would be unexplained and could only come from 2 places. Both need to be looked into...

• Fuel Tank. Any sediment in the float bowl most likely came through the tank. Either wind-blown sand from a fill up during a sand storm, or rust/ other compounds originating inside the tank. One thing for sure, if contamination entered that way, then you no longer have the stock internal brass mesh fuel screens installed. You either need to renew those OR install lawn mower style in-line fuel filters.

The BMW tank has a wonderful internal coating to prevent rust, so if you detect rust it's a sign of improper winter storage. The old method used to be fill the tank and install the fuel cap, but not any more. The ethanol in the fuel now attracts water. You are much better off rust-wise and fire hazard-wise to completely empty the tank, remove the 2 float bowls, and prop the gas cap open... allowing all the fuel to totally evaporate. That will remove the source of water.

If there is rust in your tank it will continue to fleck off and clog filters and float bowls. You may wish to investigate tank sealers from Bill Hirsch Auto or Caswell Plating. Absolutely DO NOT under any circumstance use Kreem tank sealer.

• Air Filter. Sand drawn into the intake could possibly bypass the air filter and enter the float bowl through the float bowl breather. It's well worth your time to remove the LH air filter cover and make sure the air filter element makes contact all the way around on both LH and RH covers. An older filter (although technically not dirty) could have rubber end caps too hard to effect a good case seal. That and you have to feel up inside the LH case during installation to make sure the filter gets fitted over the 3 guide posts.

Removing both LH and RH air filter covers and then spray washing the top of the gearbox along with the insides of those 2 covers will probably remove several tablespoons of sand and is always worthwhile.

Hope this helps.

Thanks Wobbly, I truly appreciate your guidance. I've owned the bike for less than a year and a half. While the PO claims regular maintenance, I've found evidence to the contrary. It looks like someone rebuilt the gearbox but put a wrong rear seal into it and then used a silicone sealer to keep gear oil from leaking; this was found by Ted Porter who rebuilt my gearbox after it starting leaking out the rear seal. Given this I am leery of the quality of any work done before and besides, the bike has over 50K so I am rebuilding major components during my long winters. Last year it was the forks and front brakes. Over the summer I had Ted rebuild the gearbox and correct the circlip problem. The clutch looks close to new and the driveshaft is like new, further evidence that someone was in the gearbox. I've taken the petcocks out of the tank and inspected/cleaned the screens, replaced the air filter and am current with all the fluid changes. There are in-line fuel filters in place and they look good. So I think the tank is good, the screens are good, and I've got a clean air filter installed. All this is to say that I don't have confidence is whatever the PO may have done but I'm bringing the bike to a good base level of maintenance. After my work on the carbs, I'll ride for the summer and take the carbs apart next Fall. If there is more sand in the main jet area then I'll know I still have something askew.

One thing I have been wondering, I've read where some riders include cleaning of their gas tanks as part of the winterizing process. I prefer your process of emptying the tank and letting it dry out but if I have to clean the tank, how is this done?

The only tank cleaning you need is related to the long term (more than ~8 weeks) storage of ethanol-loaded fuels currently for sale. Most fuel system cleaning can be avoided by simply buying Chevron, Shell, or one of the other name brand fuels with the built-in cleaners. But regardless of brand, all ethanol fuels will get you into trouble if allowed to sit. This is not only true for motorcycles, but any type of small engine... weed whackers, blowers, generators, lawn mowers, chain saws, etc, etc.

If you must keep a can of fuel around, say for instance for an emergency generator, then the best stabilizer seems to be StarTron. But even then you have to "rotate your stock". So the best plan is to have one can of fuel which feeds all your engines and, due to constant use, is being re-filled on a regular basis. Then, always run those small engines completely out of fuel before putting them back into storage.

http://www.starbrite.com/startron

The problem is that the ethanol is mixed with the gasoline, but never really goes into solution. When allowed to sit, it starts to separate back into gasoline and alcohol. The alcohol then starts to attract water. BTW this can happen just as easily in a gas station underground tank. Buying your motorcycle fuel at a station that doesn't sell a lot of high-test can mean you might be getting more alcohol than gasoline.

The water issue is somewhat better with non-ethanol fuels, but they still retain the very short shelf lives. The 8-10 month shelf life on fuels we had back in the 1970's is gone forever.

Maybe more than you asked for, but you need to understand the basis of the fuel issue. 😉

I live in Alaska and the bike is stored from October till late April at the earliest. I've taken your advice and emptied the tank, the carb is empty as I rebuilt them, and the tank cap is open to evaporate left over gas. Again, thanks for the guidance, it's truly appreciated.