Bing R100 /7 carbs on a 1971 BMW R75/5

[DonTom]

My 1971 BMW for now has Mikuni Carbs. The original 1971 Bings were pure junk, unlike the later Bings.

Today, I picked up a pair of R100/7 carbs (number on carbs is 64/32/ 357 & 64/32 358) for $150.00 for a pair. Kinda hard to refuse.

Anyway, to install these on my 71 R75/5, where do I start? Is there a jet kit to start with? Would it be the same jet sizes as in my old junky 1971 Bings?

Most of my riding is around 5,000 feet elevation, if that makes any difference.

I guess I also need throttle cables, choke cable.

If I go back to the stock air filter, what tubes do I use from the air filter casing to the carbs on a R75/5?

Anything else I should know before I try to install these on my R75/5?

-Don- Reno, NV

 

[mneblett]

Take the following with a huge grain of salt, because I haven't made this swap before.

In the garage there is currently a '73 R75/5 and a '94 R100 (later 32 mm Bing, not 40 mm).

My calipers say the length between the flanges is the same, as is the OD of the intake tube right before the carb.

I'm guessing that the R100 carbs will be a drop-in, *except* you'll have to swap over the choke/enricher levers -- the /5's used a solid wire that passes through a clamp screw, while the later R100's used a standard multi-strand cable with cylindrical end (like a throttle cable).

When I can find my Bing Agency book I can look up the initial jet and needle position recommendations.

 

[mneblett]

The 64-32-357/358 are listed as R100GS carbs:
Main jet 135
Needle jet 2.66
Jet needle 251
Atomizer 591
Idle jet 45

The R75/5 early Bings are listed as:
Main jet 140
Needle jet 2.73
Jet needle 241
Atomizer 590
Idle jet 45

The later R75/5s had a 2.70 needle jet, while the R75/6 had a 2.66 needle jet.

Seems you are already close (*assuming* the R100 carb jets have not been changed! check to be *sure* of what you have).

If it were me, I'd install the R100 Bings as-is and see whether you need to make any adjustments.

 

[DonTom]

The 64-32-357/358 are listed as R100GS carbs:If it were me, I'd install the R100 Bings as-is and see whether you need to make any adjustments.

That sounds simple enough. BTW, if I don't rejet will that make it a bit richer on my R75? I am not sure how these jet size work. But isn't a larger number richer and the main jet the most important of them all? And is that "5" point difference only one jet size? If so, it sounds like it will be close enough that I will never even notice the difference if I rejetted.

If I don't have to rejet, that will make things a lot easier.

-Don- Reno, NV

 

[mneblett]

That sounds simple enough. BTW, if I don't rejet will that make it a bit richer on my R75? I am not sure how these jet size work. But isn't a larger number richer and the main jet the most important of them all? And is that "5" point difference only one jet size? If so, it sounds like it will be close enough that I will never even notice the difference if I rejetted.

If I don't have to rejet, that will make things a lot easier.

-Don- Reno, NV

Yes, the larger the jet, the richer. You may want to go to a 140 and a 2.70 to replace the 135 and 2.66 because the '80s/'90s Bings were jetted lean for emissions purposes.

The main jets are only important for large flow demands. They are *not* not important, but the needle jet, needle position and extent of needle wear are more important because those cover 90%+ of part-throttle riding (unless you spend your life at 70+ mph on the interstate).

Your call as to whether to re-jet. You are indeed close with the R100 Bings, but you may still want to re-jet to take care of any flat or lean spots you encounter -- this would be tweaking, not massive corrections. I would install the R100 Bings and then see how the bike responds. If you have signs of leanness in a range covered by one of the jets, you can up the size.

Note: I do not have experience with jetting for 5000' altitude, so one of our Denver-level members will have to chime in.

BTW, the Bing reference book ($10? $15? can't remember) has an explanation of each of the carb fuel and air circuits -- invaluable if one is not familiar with the internals for understanding the "why" of the carbs, and hence understanding what the consequences of any particular change should be.

 

[DonTom]

because the '80s/'90s Bings were jetted lean for emissions purposes.

A bit lean at sea level should work out well since most of my riding is around 5,000 feet elevation.

Thanks for the info.

-Don- Cold Springs Valley, NV

 

[snowbum1]

A bit lean at sea level should work out well since most of my riding is around 5,000 feet elevation.

Thanks for the info.

-Don- Cold Springs Valley, NV

......................................
There is a ton I could answer about, but, perhaps later.
The /357 and /358 carby's were used on several models, including the R80 series from 1985, and contrary to information, even in Bings book, they came with a few differences in jetting. They are quite decent carbs, and the slide version (cutaway) is quite usable on earlier models. They may have aluminum needles, a sore point...more on that later, herein.

These carbs were also used on the USA-shipped R100GS, R100RT/RS, and even the Mystic....all from the later eighties.

Check the carbs out carefully: These later carburetors typically came with damnable ALUMINUM NEEDLES. If yours are at all worn, they will not allow good performance, they can vary on any ride.....things will not be stable. The wear that is the most annoying is the GROOVE on the NEEDLES. That allows the needle to move up and down IN THE SLIDE! Be sure the needles are OK. If worn, also replace the needle jets...they do wear. The needles do not remove like the old /5 carbie needles do. BTW...check the diaphragms carefully.
BE SURE the fuel level is proper. Floats deteriorate. Usual tests and adjustments for them. After the carbs are installed (and be SURE the enricheners are ACTUALLY installed correctly, some SHAFTS were MISMARKED BY BING!!), then you can do some adjusting.

Check floats by removal, unless they are new or nearly. When removed, put them in a container of fuel. ~~1/3 should be ABOVE the fuel. If the floats sink, replace them. Don't get the Bing dual independent float kits unless you have $$$, and are willing to follow my adjustment methods JUST FOR THOSE KITS...it's on my website.

Assuming the floats test OK in fuel, and are re-assembled into the carbs (knurled pin in proper way, carefully...):::
The floats setting should be the normal method, on-bike: lift the floats slowly and carefully. These carbs use a sprung floats needle, unlike early /5 carbies....so...., fuel is ON, lift float with fingertip very gently....until fuel JUST stops flowing. Allow the float to come back down a tiny bit at a time. At the point the fuel flows, the top of the float must be parallel to the base of the carb. Adjust the tang for that condition.

I suggest you start with 135 or even larger main jet and a 2.68 needle jet, and set the clip at the third position from the top. BE SURE the O-ring on the central jet assembly is OK....be sure all that central jet stuff is clean. NOTE that the MAIN JET controls things from ~~3/4 throttle to WOT....has no effect at lower throttle amounts.

Use 91 octane or so fuel, unless you are going to test at over 5000 feet, then you can use 89 octane. I'd not use 87 or lower for these tests on that particular model bike. By the way, be sure the enrichener gasket is OK, 4 screws tight (1 droplet Loctite blue), then idle the engine.
Spray brake cleaner or some other stuff, at the hose adapters from carbs to cylinder heads. NO IDLE CHANGE MUST BE NOTED.
Do same for throttle shafts.
After a careful synch and full balance job, and you must have valves and ignition set properly first; then do some hills or mountain pass tests. Go up a pass that is steep and long enough that you can maintain various throttle settings, gearing, rpm's, for AT LEAST 1/2 mile.
At the end of the half mile, shut the engine off by backing the throttle FULLY OFF and pulling in the clutch all at the same time, and then killing the ignition, and coast/brake to a stop. DO NOT let the motor be turning, which will change plug color.
Remove the spark plugs (which I assume are proper heat range!!) and look at the color of the central insulator inside the plug (at the inside tip), AND, look at the ~~1/8" wide metal at the inside of the spark plug that surrounds the central insulator. The color and over-all look of these will tell a LOT about how the engine is running.

Do the test such that you can HOLD a certain amount of throttle; AND, at a goodly rpm. I suggest around 5000 rpm.
Do the tests at ~1/3rd throttle; 1/2 throttle; 3/4 throttle, and FULL throttle.
The results will tell you about the needle, needle jet, main jet.
If OK, then you are good to play with, if you need-to, the idle mixture a wee bit, so coming off, gently, from idle, is very smooth.

When setting up those carbs use a target idle rpm of 1000. DO NOT go below 850, nor above 1150. I use 1025 myself.

For the unknowing: The needle jets for Bings come in such as 2.64; 2.66; 2.68. Each change in a needle jet, per step up or down, is closely the same as THREE grooves changes on the needle itself. Thus, a 2.66 and needle on groove 3, is somewhat comparable to a 2.68 and needle on groove 1.
The needle and its jet have no real effect at WOT, assuming a reasonable decent RPM. They also have hardly any effect at all at idle and just coming off idle. The carbs have an idle mixture adjustment, and the adjustment is in the FUEL, not AIR, system...so turn that screw SLOWLY.
All the adjustments/tuning, is in articles on the bings on my site. There is even an in-depth article on the original troublesome R75/5 CV carbs.

There is lots more, but this should suffice.

snowbum
http://bmwmotorcycletech.info

 

[DonTom]

......................................
There is a ton I could answer about, but, perhaps later.
The /357 and /358 carby's were used on several models, including the R80 series from 1985, and contrary to information, even in Bings book, they came with a few differences in jetting. They are quite decent carbs, and the slide version (cutaway) is quite usable on earlier models. They may have aluminum needles, a sore point...more on that later, herein.

Thanks for all the info.

I just found both of my original 1971 Bing Carbs here. 64/32/3 & 64/32/4, the very worse carbs I have ever experienced with any motorcycle, at least then they got warm and would not idle below around 3,000 RPM's (if we can call than an idle). When going slow, I had to ride the clutch because of that, if I didn't want the engine to die. That's why I went with the Mikunis.

But what I need to know now is, if I can take all the Jets and such out of these old Bings and put them in my R100 Bings and then be jetted correctly.

The other thing I need to know is if there is any advantage to the Bings over my Mikunis other than elevation compensation. I mainly only use my 1971 R75/5 on hot summer days (has no fairing). Seems I am always between 4,000' and 7,500' elevation with an average of perhaps around 5K where my two Reno homes are (both homes up higher than most of Reno). Here, in the SF Bay Area I am just a few feet above sea level, but I never ride back to here anyway. I have the main jet in my Mikunis set for 5,000' feet elevation(jet size 175). But since this is a hot weather bike, I have wondered if I should go even leaner. Jets in these can be changed quickly while on the road and I carry sea level jets (200) with me anyway.

Check the carbs out carefully: These later carburetors typically came with damnable ALUMINUM NEEDLES. If yours are at all worn, they will not allow good performance, they can vary on any ride.....things will not be stable. The wear that is the most annoying is the GROOVE on the NEEDLES. That allows the needle to move up and down IN THE SLIDE! Be sure the needles are OK. If worn, also replace the needle jets...they do wear. The needles do not remove like the old /5 carbie needles do. BTW...check the diaphragms carefully.
BE SURE the fuel level is proper. Floats deteriorate. Usual tests and adjustments for them. After the carbs are installed (and be SURE the enricheners are ACTUALLY installed correctly, some SHAFTS were MISMARKED BY BING!!), then you can do some adjusting.

Seems Bing carbs had problems for even longer than I thought!

Use 91 octane or so fuel, unless you are going to test at over 5000 feet, then you can use 89 octane. I'd not use 87 or lower for these tests on that particular model bike.

Even with a dual plugged bike at 5,000'?

When setting up those carbs use a target idle rpm of 1000. DO NOT go below 850, nor above 1150. I use 1025 myself.

Even with my Mikunis, the idle will increase a little when the bike gets very warm, but nothing like my old Bings. But if I set it for 1,150 RPM's before a ride, then after an hour hard ride I will probably be closer to 2,000 RPM's for my idle. Should I expect a more stable idle with the R100 Bings?


-Don- SSF, CA

 

[DonTom]

Bing R100RT carbs on a 1971 BMW R75/5

I just realized I made a mistake.

Is there any difference between the R100/7 carbs and the R100RT carbs?

The used Bing carbs I recently purchased was from a R100RT, not a R100/7.


-Don- SSF, CA

 

[mneblett]

I just realized I made a mistake.

Is there any difference between the R100/7 carbs and the R100RT carbs?

The used Bing carbs I recently purchased was from a R100RT, not a R100/7.


-Don- SSF, CA

What year R100RT? The '79-'84 (US) models had 40 mm carbs, not the 32's of the R100/7.

Do the part numbers on the sides of the Bings say 64/32/xxx or 94/40/xxx?

The Bing book does not cover the 32 mm R100RT ('88-'95) carbs -- I'd use the R100/7 (non-flat top) settings as a starting point.

 

[DonTom]

What year R100RT? The '79-'84 (US) models had 40 mm carbs, not the 32's of the R100/7.

Do the part numbers on the sides of the Bings say 64/32/xxx or 94/40/xxx?

The Bing book does not cover the 32 mm R100RT ('88-'95) carbs -- I'd use the R100/7 (non-flat top) settings as a starting point.

Heck, I will have to get back to Reno to get the numbers from these RT carbs. I am not sure when that will be. Came here to escape the +100° F Reno heat which seems to have no end in sight.

-Don- (in 65° F SSF, CA)

 

[DonTom]

What year R100RT? The '79-'84 (US) models had 40 mm carbs, not the 32's of the R100/7.

Do the part numbers on the sides of the Bings say 64/32/xxx or 94/40/xxx?

The Bing book does not cover the 32 mm R100RT ('88-'95) carbs -- I'd use the R100/7 (non-flat top) settings as a starting point.

I just noticed my first message in this thread has the numbers of these R100 RT Bings: "number on carbs is 64/32/357 & 64/32 358".

-Don- SSF, CA

 

[mneblett]

I just noticed my first message in this thread has the numbers of these R100 RT Bings: "number on carbs is 64/32/357 & 64/32 358".

-Don- SSF, CA

Missed that. 357/358 are listed in the Bing book as R80GS and R100GS carbs. I'd use them as-is (after confirming they are still jetted as original).

 

[DonTom]

Missed that. 357/358 are listed in the Bing book as R80GS and R100GS carbs. I'd use them as-is (after confirming they are still jetted as original).

They were in a sack that said they were from a R100RT, IIRC, but that could be wrong. I bought these from a guy who had around ten Bing pairs (20 Bing carbs) in a larger sack from various models.

I guess I will have to try them and see what happens.

But I cannot decide if it's worth the trouble since I always ride this airhead at ~5000' feet elevation and my Mikunis do fine when jetted for such. But I decided to buy these Bings since they were cheap, when I had the chance.

-Don- SSF, CA

 

[88bmwjeff]

What year R100RT? The '79-'84 (US) models had 40 mm carbs, not the 32's of the R100/7.

Do the part numbers on the sides of the Bings say 64/32/xxx or 94/40/xxx?

The Bing book does not cover the 32 mm R100RT ('88-'95) carbs -- I'd use the R100/7 (non-flat top) settings as a starting point.

While the Bing book doesn't cover the 32 mm R100RT (88-95) carbs, they have part numbers of 64/32/357 & 358. At least my 88 RT came with those carbs. I would assume that you could set the carbs to the same starting point as the GS models, since the motors were identical--at least that's my understanding (granted that understanding could be incorrect). If anyone has information to the contrary, let me know.