Constant-depression carburettors confuse people because nothing moves when you expect it to. Twist the throttle and the slide does not obey your wrist — it obeys the vacuum in the venturi, rising only as fast as the engine can actually use the air. That is why an airhead pulls so cleanly from low revs, and why the usual tuning instincts lead you astray.
The practical consequence is this: when the mid-range goes soft, rich or hesitant, almost nobody needs a bigger main jet. They need a needle and a needle jet, because those two wear together and take the whole middle of the throttle range with them.
And identify your carburettors by the number stamped on the flange. Not by what model the bike is. Bodies were shared across models; jetting was not.
Every figure on this page resolves to one model and year in the book. Pick your bike and the rows that apply to it come forward, with the rest marked.
Open this chapter in the book| Model | Carb # | Main jet | Needle jet | Jet needle / clip | Notes |
|---|---|---|---|---|---|
| R100S ('77) · R100T ('79) | 94/40/103–104 | 170 | 2.66 | 341 · pos 3 | BMW-WSM7 Slide 910 · atomizer 591 · idle jet 45 · idle mixture screw 1 to 1¼ turns · starting jet 80. The same carb set moves between models across years — it is the R100S set on the 12.77 sheet and the R100T set on the '79 sheet. Key jetting to model and year, never to the carb number alone. |
| R100RS | 94/40/105–106 | 170 | 2.68 | 341 · pos 2 | Pre-'81 RS with 40 mm exhausts. |
| R100S / R100RT | 94/40/107–108 | 160 | 2.64 | 341 · pos 2 | |
| R100 US | 94/40/109–110 | 165 | 2.66 | 341 · pos 2 | |
| 1981 — R100 · R100CS · R100RT · R100RS | 94/40/111–112 | 160 | 2.66 | 341 · pos 3 | BMW-WSM7 This guide had this set labelled “R100S”. It is the 1981 factory fitment for the whole 1000 cc range — R100, R100CS, R100RT and R100RS all share it on the 3.82 sheet. Idle jet 45, idle air jet 1.0 mm, idle mixture screw 1 to 1¼ turns out, starting jet 80. |
| R100 US / RS | 94/40/113–114 | 160 | 2.66 | 341 · pos 2 | |
| R100 ('81+ base) | 94/40/123–124 | 160 | 2.66 | 371 · pos 2 | Slide 913. Later than the 1981 model-year sheet, which specifies 94/40/111–112 with a 160 main for the whole range — see the row above. Treat 123/124 as a post-1981 or market variant and confirm the flange stamp. |
| R100 ('81+ base) | 94/40/125–126 | 150 | 2.66 | 341 · pos 3 | Slide 912. The leanest R100 jetting. Not the 1981 model-year specification — that is 94/40/111–112, main 160. Confirm the flange stamp before assuming this applies. |
BMW-TD77 Not every /7 got a CV carburettor. The R60/7 ran two inclined Bing slide carburettors with a needle jet and central lever float — a different instrument from the constant-depression units the rest of this chapter covers, and one with an accelerator pump.
| Model | Carb # | Main jet | Needle jet | Jet needle / position | Notes |
|---|---|---|---|---|---|
| R60/7 | 1/26/123 (L) · 1/26/124 (R) | 140 | 2.68 with accelerator pump | needle 4 · position 2 | Barrel 26 mm. Idle jet 40; no idle air jet — the only carburettor in the /7 range without one. Because the slide is cable-operated rather than vacuum-operated, the CV diagnostic logic in the rest of this chapter does not transfer: a slide carb obeys your wrist. |
| Model | Carb # | Main jet | Needle jet | Jet needle / clip | Notes |
|---|---|---|---|---|---|
| R75/7 | 64/32/13–14 | 145 | 2.66 | 241 · pos 3 | BMW-TD77 Idle jet 50 · idle air jet 1 Ø. Primary BMW data — this model was previously missing from the table. |
| R75/5 | 64/32/1–2 | 140 | 2.73 | 241 · pos 3 | Slide 950 · atomizer 590 · idle 45. |
| R90/6 | 64/32/11–12 | 150 | 2.68 | 241 · pos 2 | Slide 952 · atomizer 591. |
| R100/7 | 64/32/19–20 | 150 | 2.68 | 241 · pos 3 | Idle jet 50 · idle air jet 1 Ø. BMW-TD77 Confirmed against BMW's own technical data — main 150, needle jet 2.68, needle 46-241 at position 3, exactly as published here. |
| R80/7 | 64/32/201–202 | 145 | 2.66 | 241 · pos 3 | BMW-WSM7 Slide 911-24 · idle jet 50 · idle mixture screw ½ to 1 turn · starting jet 60 · bypass 0.7 / 0.65 mm. Confirmed against the factory sheet exactly as this guide already had it. |
| R100/7 | 64/32/219–220 | 150 | 2.68 | 241 · pos 3 | Slide 911-64 · idle 50 · bowl 70. |
| R90/6 | 64/32/311–312 | 150 | 2.68 | 251 · pos 2 | 64-3 series, slide 907-3, bowl 66. |
| R80GS / R100GS | 64/32/357–358 | 135 | 2.66 | 251 · pos 3 | Slide 907-3. |
BMW-WSM7 Beyond the jets. These are the passage and bore diameters from BMW's own fuel-preparation data, and they are what tells you whether a carburettor has been got at — a drilled bypass or a wrong float valve will not show up in a jet inventory.
| Dimension | 26 mm slide (R60/7) | 32 mm CV | 40 mm CV | Sheet | Notes |
|---|---|---|---|---|---|
| Starting (enrichener) jet | 0.9 mm | 60 · 70 | 80 | 12.77 → 3.82 | 60 on R75/7 and R80/7, 70 on R100/7, 80 on the 40 mm bikes and everything from '79 on. |
| Starting air jet (starting chamber vent) | 2.0 mm | 2.0 mm | 2.0 mm | all | Unchanged throughout. |
| Mixture passage dia. | 7.5 mm | — | 6.0 mm | all | The 32 mm CVs have no separate passage. |
| Air passage dia. | 6.0 mm | — | 6.0 mm | all | |
| Mixture bores in the rotary slide | — | 2.0 / 1.2 / 0.7 mm | 2.0 / 1.2 / 0.7 mm | all CVs | The three enrichener bores. Identical across every CV airhead in these sheets. |
| Idle air jet | 0.8 mm | 1.0 mm | 1.0 mm | all | |
| Idle mixture screw — turns out | air screw ¼–1¼ | ½ to 1 | 1 to 1¼ | 12.77 → 3.82 | The 32 mm and 40 mm bikes are not the same, and the R100/7 sits at exactly 1 turn. Note the R60/7 slide carb has an idle air screw, which works the opposite way round from a mixture screw. 1981 range: 1 to 1¼. |
| Bypass bore 1 / bore 2 | 0.8 mm / — | 0.7 / 0.65 mm | 0.8 / 0.65 mm | all | Progression bores. If the transition off idle is wrong and the jets check out, these are what to measure. |
| Float valve diameter | 2.2 mm | 2.5 mm | 2.5 mm | all | 2.5 mm on every CV airhead in these sheets. |
| Float chamber vent bore | — | — / 2.0 mm | 2.0 mm | 12.77 → 3.82 | Absent on R60/7–R80/7 in 12.77; 2.0 mm on the 1000s, and on everything from '79. |
| Idle outlet bore | 0.8 mm | 1.0 mm | 1.0 mm | all | |
| Air filter | one “Micro Star” dry element | serves both carburettors | — | all | BMW's own description. A single common element, not one per carburettor. |
| Item | Value / part | Applies | Notes | ||
|---|---|---|---|---|---|
| Lean emissions jetting | ping at highway speed | = too lean | late R100 carbs | Bing's own chart warns: lean-jetted late carbs can ping at cruise and damage pistons & valves. Persistent ping → richen (main +1 step or needle up). | |
| Carb-to-head hose | use 13-72-1-254-654 | replaces -360 | late R100 esp. | SI 13-020-90: wrong hose lets the carb vibrate → float needle over-admits fuel → poor mileage, top stains. Note the direction: -654 is the older, stiffer compound; BMW later softened it to -360, and the softer hose makes late slides wear their needle slots faster. | |
| Wear service interval | ~60,000 mi | needle worn stepless = long overdue | All CV | Replace jet needle + needle jet + diaphragm as a set. Aluminium late needles wear faster than steel. | |
| Jet sizing logic | flow-rated, not Ø | 160 = 1.6 mm ref | — | A 110 flows 10% more than a 100 — not 10% bigger hole. Mid-range mixture lives in needle/needle-jet wear, not the main. |