1978 R 100 RS- looking for 40 MM Bing advice.

[jsey]

Hi All
I have a 1978 R 100 RS with the 40 MM Bings. I completely rebuilt them including new diaphragms. Here's the problem.. I've sync'd them and the they idle perfectly, the idle jets are backed out 1-1/2 turns and the engine has sluggish acceleration. In the rebuild I didn't change any jets or position of the needle jets. Before the rebuilt it ran strong. I rebuilt them for the first time since 1978 thinking it might be a good idea. I think I'm running 145 mains and 60 idle jets. Timing was left alone. Any ideas?

 

[stevenrankin]

If it ran fine before you rebuilt the carbs, recheck your work. As to what it could be, anything from improperly adjusted cables to chokes on the wrong side, diaphragm's not installed correctly. Did you rebuild one at a time or both at the same time? It is best to do one at a time just in case there is a question of how things go back together. Good luck, St.

 

[jsey]

Interesting comment about the choke. I am familiar with ones ability to unknowingly install them incorrectly. Will have to double check that. The carbs were sync'd with a Carbmate and the diaphragms are in correctly I think. The little tab on them is positioned correctly and they seem to be tucked into their grooves properly. Can't remember but how would I tell if the chokes are installed incorrectly?

 

[omega man]

Added a bit to your title for clarity.
Good luck.
OM

 

[pauls1150]

See Snowbum's page, about halfway down:

Bing CV carburetors Part 1, basics, butterfly position, enrichener assembly, overhaul, hints,

overhauling Bing CV carburetors, HINTS; PART 1

bmwmotorcycletech.info

 

[stevenrankin]

I hate to be rude but have you checked your points and timing as well as valve clearances? Valves are the first thing to adjust then points and timing before addressing any issues with carburetors.

It has been awhile since I rebuilt 40mm carbs but I believe the choke parts are stamped left and right or L and R, LOL, I could be wrong, St.

 

[jsey]

So yesterday I did check the timing and the valve clearances. All good. I'm dual plugged with an electronic ignition so no points. Took the carbs off and went through everything. Cleaned everything thoroughly. I was wrong in my previous post the mains are 170. Checked the choke circuits. Everything was oriented and installed correctly. All cables were good and adjusted correctly. Needles are old style (not aluminum) and were set on the 2nd position. at 47.3 MM exposed. Diaphragms were installed correctly. This is what I did find. The butterfly's weren't even close to being properly positioned on the shaft. Not really sure how that happened as I thought I had positioned them correctly once I had tightened down those small screws some time ago. So obviously I re-positioned them and locktighted the screws. Need to start it today and re-sync the carbs and see how it runs. Got my rebuild kit from Euromotoelectric. It did occur to me... what if the diaphragms were just to stiff and not able to operate properly. When I changed them the old ones turned out to be seemingly ok but quite a bit more flexible. Anybody ever experience diaphragms that didn't work? The guys at Euromoto certainly know what they are doing so I sort of doubt that could be the problem.

 

[robsryder]

I have a 77 R100RS with the 40mm Bing CV carbs. I rebuilt this bike back in 2004, including the carbs (much as the OP describes). I rode the bike a good bit and went to the 40th anniversary gathering of the R100RS.

Not too long ago I was watching one of the excellent Boxer2Valve videos on re-building Bing CV carbs. In the middle of watching the video I got a sinking feeling and went into the garage. Sure enough, I'd got one of the carbs chokes reversed (didn't know about the indicator "dot" until watching the video). It did not take long to make that carb correct. Funny, the bike runs so much better now.

Here is the link to the video -

 

[GTRider]

Snowbum’s Bing pages will tell you more than you wanted to know about the Bings. Scroll down to the section on diaphragms and you’ll see that the thickness of diaphragms has changed over the years, and his admonishment not to use Stromberg or other than BMW diaphragms. On aligning the butterfly plates, that is usually best done with carbs off the bike so they can be held up to light and the plates adjusted by sighting through the bore and adjusting until no light can be seen with the plate closed.

Best,
DeVern

 

[stevenrankin]

Well, it may well be you found your problem. You would be surprised at the number of people who ask me for help with a problem after a repair or maintenance only to get my usual reply, "check your work".
I don't write it to be rude, LOL. St.

 

[FELAW]

So yesterday I did check the timing and the valve clearances. All good. I'm dual plugged with an electronic ignition so no points. Took the carbs off and went through everything. Cleaned everything thoroughly. I was wrong in my previous post the mains are 170. Checked the choke circuits. Everything was oriented and installed correctly. All cables were good and adjusted correctly. Needles are old style (not aluminum) and were set on the 2nd position. at 47.3 MM exposed. Diaphragms were installed correctly. This is what I did find. The butterfly's weren't even close to being properly positioned on the shaft. Not really sure how that happened as I thought I had positioned them correctly once I had tightened down those small screws some time ago. So obviously I re-positioned them and locktighted the screws. Need to start it today and re-sync the carbs and see how it runs. Got my rebuild kit from Euromotoelectric. It did occur to me... what if the diaphragms were just to stiff and not able to operate properly. When I changed them the old ones turned out to be seemingly ok but quite a bit more flexible. Anybody ever experience diaphragms that didn't work? The guys at Euromoto certainly know what they are doing so I sort of doubt that could be the problem.

Do I recall correctly and don't the butterflys have a specific orientation and shouldn't be flipped front to back or up and down because the edges are cut on and angle to accommodate opening and closing? Just a thought.

 

[jsey]

They have a dot punched in them that as I understand it go on top facing out

 

[AntonLargiader]

Do I recall correctly and don't the butterflys have a specific orientation and shouldn't be flipped front to back or up and down because the edges are cut on and angle to accommodate opening and closing? Just a thought.

Yes, that is correct. The dot goes up and out.

 

[jsey]

For those that may be interested. To reiterate, my problem was very poor acceleration after having rebuilt the carbs. After checking the timing, the valve clearances, vacuum leaks, float levels, cable adjustment, sync...everything, I came to the conclusion that the only variable had to be the diaphragms. My R100RS is a 1978 with 10,000 miles on it. I had decided to change them due to their age but kept the old ones which seemed to be in good condition. The old ones were clearly more flexible. This morning I changed back to the old ones and voila it runs better than it ever has. I read Snowbums page pn CV Bings and he does say never purchase diaphragms from anybody other than Genuine BMW. Here's proof of that. I did touch base with Euromotoelectric and they told me that the had sold a bunch of these with no reported problems. Snowbum says that the correct part number is 13 11 1 336 902 but now places like Max and Bob's list 13 11 1 335 322. I'll need to find out if they are one and the same. You live and you learn. In all of the back and forth over the last week I also learned that you should always hook up the choke cable before mounting the carbs. Ridiculous engineering if you don't haha.

 

[omega man]

Nice to have a follow-up.
OM

 

[jsey]

Replaced my old diaphragms with new BMW diaphragms and took the bike for a 400 mile run into the Adirondaks. Ran like a champ!
I noticed that when I start the bike it initially idles very nicely down around 500 RPM (which I am aware is low) after turning off the choke. 45 idle jet. After it warms up say 10 miles or so, it idles closer to 1200 RPM's. I understand that the piston expands and thus the idle increases. The only way to get it to idle under 1000 rpm when hot requires further backing off of the idle screw and keeping your hand on the throttle during cold start. Is this normal for a R100RS or should I be looking for a problem?

 

[20774]

I don't know what my /7 idles at initially off of choke. Within 15-30 seconds of starting on choke, I've begun to roll around the neighborhood as I pull the choke lever off in stages. Next time I might be at an idle situation, I'm several minutes later and not on choke, and the bike idles above 1000.

I don't think you want to strive for an idle below 1000 in any situation. You're compromising the engine oil flow to the lower end of the crank. Around 1100 or so is fine.

 

[jsey]

I don't know what my /7 idles at initially off of choke. Within 15-30 seconds of starting on choke, I've begun to roll around the neighborhood as I pull the choke lever off in stages. Next time I might be at an idle situation, I'm several minutes later and not on choke, and the bike idles above 1000.

I don't think you want to strive for an idle below 1000 in any situation. You're compromising the engine oil flow to the lower end of the crank. Around 1100 or so is fine.

Good point about the oil. Thanks

 

[melville]

Yeah, I'm off choke about 50 feet after start up and those 50 feet start about 3 seconds after start up. My bikes will idle off choke (700 to 800 rpm) at my first stop, which is about 1/4 mile from the house in any direction. Idle rpm is 1000 to 1100 once warm on the bikes with lightened flywheels and 800 on the heavy flywheel bikes.

 

[lkchris]

re: I had decided to change them due to their age

Just so you know, Bing recommends diaphragms be renewed yearly. (Not that anybody does this.)

Did you know that after some time and miles, the inrushing fuel wears the needles? It's not a friction free thing. Needle shape is important.

Because of those diaphragms these carbs are automatically altitude adjusting, so messing with jets is a bit futile. The factory setting is probably best. Of course when air is less dense because of higher altitude, the diaphragm will lift the needle jet less. Bing makes carbs for airplanes, so they know altitude compensation. Kind of like a "mass air sensor" which does the same thing for fuel injection.

Perhaps interesting ... when I changed compression on my '84 RS from USA standard 8.2 to rest of world standard the traditional 9.5, I looked up the carb difference in the Bing manual. Raise needle jets one notch, everything else, all jets the same.