Welcome Ian, "something isn't right" seems to be the case here.

Am I to understand that your throttle cable set up is a single pull cable from the handle bar that leads to a Bowden assembly from which the Right/Left throttle cables are pulled?

If so, is it possible that the "too short" cable is not seated in the disc inside the Bowden cylinder?

Might it be possible to switch the cables, R/L, to gain the slack needed?

Is there anything that can be done at the anchor point under the fuel tank to gain some relief?

Would it work if you were to fit 2 of the long lower cables?

How do the new cables compare next to the old ones?

I must admit that I am not familiar with the particular details of the G/S models, so I am just tossing out speculative possibilities.

In fact, the only experience I have with Bowden cables was installing enricher cables on my 1985, R80RT. 

Thanks for the ideas, yes, one throttle cable, bowden junction and two lower cables. To make a long story short, I need to get the bike out of the van, back on the workbench, compare the old parts and see what's up. The flopping of cables has also occurred to me... the parts sites do not provide guidance as to which is left or right.

The last person who balanced the carbs on this bike did so nearly 20 years ago and it has run so well that I've never had to touch them again.

• Look straight down on the engine. One cylinder is physically set behind the other. The cylinder that's set to the rear uses the longer cable. It's that simple.

• Since both carburetors are alike (which you should definitely check), then BOTH throttle cables should have the same length of INNER cable exposed when laying on the bench. One cable being physically longer for the rear-most cylinder makes ZERO difference to the exposed inner length. 

• I'm with Jim on this one. I think the crimped-on nib on the end of the inner cable is not fully seated inside the junction block. I also get my cables from Max BMW and had 1 or 2 of their clutch cables that the nib had to be filed down quite a bit in order to fit inside the clutch lever barrel. The nib is the correct diameter... until it's crimped. The issue is the distortion if the nib caused by the crimping process.

Hope this helps.

Follow-up on this... have not yet found an authoritative answer as to the correct lower cable for my 40mm carb.

Going back to my old parts, both lower cables were the same length, and the same length as the new left lower cable.... and the bike ran fine prior to all my work.

The new right-side lower is indeed longer than the left, but the inner cables both project from the end of the outer cable the same length... and as close as I can measure, the same length as the old cables.

I got into the bowden junction, everything is properly assembled.

However, the adjuster for the upper cable was out a fair piece, so I threaded that back in. Now... with throttle closed,  the left lower cable  is loose enough, the right cable is slightly looser. Using the ferrule adjusters, I tightened the right ever so little, adjusted the left idle screw, then the right (while running) and things seem right. Comes off idle smoothly and is fairly balanced. 

Going to try a first ride tomorrow. I don't have all my balancing stuff out and hope I don't need it. The last time my tech balanced the carbs on this bike was 20+ years ago and it has never needed it since!

Thanks very much for the advice.

Posted by: @ian-schmeisser

Follow-up on this... have not yet found an authoritative answer as to the correct lower cable for my 40mm carb.

Should I take that as a vote of "no confidence" on my advice ?

Going back to my old parts, both lower cables were the same length, and the same length as the new left lower cable.... and the bike ran fine prior to all my work.

The new right-side lower is indeed longer than the left, but the inner cables both project from the end of the outer cable the same length... and as close as I can measure, the same length as the old cables.

I got into the bowden junction, everything is properly assembled.

So what you are saying is that the problems you are experiencing are completely of your own doing. 

However, the adjuster for the upper cable was out a fair piece, so I threaded that back in. Now... with throttle closed, the left lower cable is loose enough, the right cable is slightly looser. Using the ferrule adjusters, I tightened the right ever so little, adjusted the left idle screw, then the right (while running) and things seem right. Comes off idle smoothly and is fairly balanced. 

Going to try a first ride tomorrow. I don't have all my balancing stuff out and hope I don't need it. The last time my tech balanced the carbs on this bike was 20+ years ago and it has never needed it since!

With all due respect, here is the proof of my second statement...

The carb-to-cable adjusters are not some after thought on throttle cable installation; their adjustment is always an integral part of new cable installation. And throttle cable installation ALWAYS requires resetting the carb-to-cable adjusters. 

While physical idle screw positions will generally not be affected, the fact remains that no 2 cables can ever be the same length. Thus installation of new throttle cables will always have an effect on throttle opening; that is to say the RPM range (roughly) between 1100 and 2500 RPM. The RPM range required for smooth "take offs" from standing starts.

Thus, installation of new throttle cables should always be accompanied by 1) balancing of the carb idle speeds, which is immediately followed by 2) balancing of the carbs at an engine speed around 1800 RPM. Idle speed is accomplished by the Idle Speed Screw, while balance at 1800 is accomplished by cable adjustments at the carb-to-cable adjusters. BOTH adjustments should be done with the aid of carb balancing instruments with the engine hot. I can hear it with my ear, but for most casual home mechanics an instrument aided adjustment is advised. 

Furthermore, throttle cables will change lengths over time simply because they are being placed under tension. Thus for your happiest riding experience, it stands to reason that this balancing process should be followed up once a year. So going "20+ years" between adjustments is something less than a point of pride.

I sincerely hope you'll dig out your carb balancing instruments so you can finish the job. The instrument readings alone will prove whether I'm giving you the "authoritative answers" you're seeking.

All the best.

You know, I am sort of sorry I asked. Came here for part numbers, not a lecture. Peace, man.

Brother, I'm not lecturing. I'm simply giving you all the facts so you can make informed decisions and take wise actions. There just happen to be a 'dump truck load' of basic knowledge about throttle cables that most novice repairmen don't know and thus never consider. And a lot of these guys end up 'walking home'. 

I want you taking informed courses of action based on all the knowledge so that you can enjoy your ride. All the way back to your home. 

► If at any time you'd rather NOT know, then just say, "Please shut up. I don't want all this information. I enjoy walking." 

Cheers !

@wobbly I agree with you on this Mr Whatley. You can't come in here and ask for advice and be upset about what you receive or lack thereof. I know a ton of folks here always throw in their 2 cents worth and are usually spot on in most cases about it. A good person asking the question should take all the information and make the best judgement for them. To your point, I guess he rather walk....

An update on the question I originally asked... it turns out that there is no part number for my carbs, at least for my model of bike and design of its throttle.

A knowledgeable friend tells me that bikes with 40mm carbs had two throttle full throttle cables, no bowden junction arrangement like the stock R80G/S.

I've converted several R100 bikes from 32 to 40mm carbs. The earlier bikes that ran 40mm carbs used a different throttle grip housing and the internal gear wheel/chain combo isn't compatible with most of the later twin. You can verify this for yourself by looking at the part numbers at realoem.com. Functionally, the throttle arms on the 40mm carbs are longer than those on the 32mm ones, so BMW put a bigger lump on the throttle gear to provide more travel in the cables as the grip is turned. I've had some success cutting new cam lobes and epoxying them in place, but if I had to do it again, I'd just pick a bike that used 40mm carbs and get the cables and throttle housing from it to use on the later bike. You can look in the tech article section for my write up of the conversion of my R100GS to get a better idea of what was involved.

Posted by: @8166

I've converted several R100 bikes from 32 to 40mm carbs. The earlier bikes that ran 40mm carbs used a different throttle grip housing and the internal gear wheel/chain combo isn't compatible with most of the later twin. You can verify this for yourself by looking at the part numbers at realoem.com. Functionally, the throttle arms on the 40mm carbs are longer than those on the 32mm ones, so BMW put a bigger lump on the throttle gear to provide more travel in the cables as the grip is turned. I've had some success cutting new cam lobes and epoxying them in place, but if I had to do it again, I'd just pick a bike that used 40mm carbs and get the cables and throttle housing from it to use on the later bike. You can look in the tech article section for my write up of the conversion of my R100GS to get a better idea of what was involved.

Is a 1986 (essentially a /7) a later twin?

After a ride, I think my problem is that the left cable is not allowing the throttle arm to fully return. The right cable is slack with the ferrule adjuster all the way in. At speed the engine is smooth as glass, idle is inconsistent (too high, stall, lumpy...)

I don't think there's a situation where the carb throttles not fully lifting when the loud handle is WFO, although I only ran the bike up to about 75 and didn't check top speed. Even with the bike's Wüdo taller 5th gear, the rear drive is short, so it's doesn't have a lot of top end. The bike was build for low and mid-range torque for twisty mountain roads.

Posted by: @ian-schmeisser

1) After a ride, I think my problem is that the left cable is not allowing the throttle arm to fully return. The right cable is slack with the ferrule adjuster all the way in.

2) At speed the engine is smooth as glass, idle is inconsistent (too high, stall, lumpy...)

1) After a ride should be no different than 'before a ride'. Cable slack/tightness does not change just because you took a ride.

Both cables should have ~1.5 to 2mm of slack. This 'slack' is what allows the cable return springs to pull the Idle Speed Adj Screws hard down against their stops. This in turn is what allows consistent idle speed. Idle speed is NOT set by the cable, but rather totally dependent upon the Idle Speed Adj Screws.

2) At speed the CV pistons are lifted (actually "flying") solely on a column of air flowing across the carb venturi. As such, each carb is independently allowed to ingest the exact right amount of air/fuel mixture as determined by the individual cylinder. If the running above 1/4 throttle felt "lumpy", then that would reflect the condition of the carb diaphragm... NOT the cable adjustment. This is why (as previously explained) cable adjustment is not needed above ~2000 RPM. This wholly independent lifting (basically "self-adjusting") of the carb slide is the major advantage of using the CV carb. 

With all due respect, what you are saying means you do not understand the operation of the carb, or the interplay between the carbs and their cables.

I fully understand you are having to use 38mm cables on 40mm carbs, but there are 'work-arounds' available. What you'll need to do is change the ratio of internal cable to external cable housing. This is done by snipping away independent coils of the outer housing. Slide off the end ferrule, cut back the black plastic outside sheathing with a knife, then snip off 2 coils of housing with VERY sharp cutters (without nicking the internal cable). Put the end ferrule back in place and then try it. Repeat as required. This technique is shown in this video starting at about 2-1/2 min. 

https://www.youtube.com/watch?v=2qVjTXC1Bno

Hope this helps.

Since this discussion is on Throttle cables I would like to add what I've experienced regarding wear on my 1986 R80RT Converted to RS by the original purchaser when bought.

I've Changed the Cables out at these mileages:

76K (I presume these were the original cables) due to squirrel caging

108,949 - (32,949.00 ) Barbacks were installed... Longer cables installed.

164,888 (55,939) due to squirrel caging at the normal cruise setting at the carb where the cable enters the sheath.

I recommend that the cables be changed out at probably 50K Miles for prevented MX in that not to be stuck somewhere.  I happened to see this when I was doing unrelated MX on the Ignition/Starter system...  I was surprised to see these in this condition (both sides) as I didn't notice any issues with control and the cables run smoothly with no kinking or any other sort of reason to cause this wear.  I think this is just due to the normal vibration just as the throttle gear teeth wearing in the particular spot on the cam...