If your BMW R1100S has developed an uneven idle, hesitation at small throttle openings or the classic Boxer “surging”, the Big Brass Screws (BBS) are one of the first things you should inspect.
These simple air bypass screws gradually collect carbon, oil vapour and dirt. Fortunately, cleaning them is one of the easiest maintenance jobs on the motorcycle—and it can make a surprisingly big difference.
On my own R1100S the engine had begun surging noticeably. After removing and cleaning both screws, the improvement was immediate. The surging was almost gone and the engine felt smoother throughout the lower rev range.
What Are the Big Brass Screws?
The Big Brass Screws are located in the throttle bodies—one on each cylinder.
Their job is to regulate the amount of bypass air at idle, allowing the engine to idle correctly and making throttle body synchronization possible.
Because they are constantly exposed to intake air and crankcase vapours, they gradually become coated with deposits.
Common Symptoms
Dirty Big Brass Screws may cause:
- Rough or unstable idle
- Boxer surging at low throttle openings
- Hesitation when pulling away
- Poor throttle response
- Difficulty synchronizing the throttle bodies
These symptoms can also have other causes, but cleaning the BBS is a quick and sensible first step before chasing more complex faults.
What You’ll Need
- Medium flat-blade screwdriver
- Throttle body cleaner or carburettor cleaner
- Soft cloth
- Cotton swabs (optional)
Step 1 – Count the Turns
Before removing each screw, count exactly how many turns it takes to lightly seat it.
Write the number down.
This allows you to reinstall the screw in exactly the same position and preserves your idle adjustment.
Never tighten the screw hard against its seat. Lightly seated is all that’s required.
Step 2 – Remove the Screw
Remove one screw at a time.
You’ll normally find the tapered tip covered in black carbon deposits.
Also inspect the O-ring for cracks, flattening or damage.
Step 3 – Clean Thoroughly
Spray throttle body cleaner onto a cloth and clean:
- The tapered needle
- The threaded section
- The area behind the threads
- The air passage inside the throttle body if accessible
Avoid damaging the O-ring.
Step 4 – Reinstall
Screw it back in until it just seats.
Then back it out exactly the same number of turns you noted before removal.
Repeat the process on the other throttle body.
The Result
On my motorcycle the improvement was dramatic.
The engine idled more smoothly, throttle response improved and the familiar Boxer surging was almost eliminated.
It’s one of those maintenance jobs that costs nothing but a few minutes of your time.
Frequently Asked Questions
Should I adjust the screws while cleaning them?
No.
Unless you’re performing a complete throttle body synchronization, simply reinstall each screw to its original position.
Should I replace the O-rings?
If they’re cracked, flattened or hardened, yes. Otherwise, they can usually be reused.
Can dirty Big Brass Screws cause surging?
Yes. They are a common contributor to poor idle quality and low-speed surging, although ignition, valve adjustment and throttle body synchronization should also be considered.
How often should they be cleaned?
Inspect them every 10,000–20,000 km, or whenever the engine begins to idle poorly.
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