Carburetor Synchronization Wrench | Honda CB360 / CB500K / CB550 / CB750
Your carbs are clean, rebuilt, and back on the bike — but the engine still idles rough, hangs at high RPM, and fights you on cold starts. The problem is carburetor synchronization, and this 8mm wrench is the tool that fixes it.
Why You Need This
Even with freshly rebuilt carburetors, new diaphragms, and good intake manifolds, your Honda twin or four will run rough if the carbs are not synchronized. Symptoms include a high idle that won't settle, jerky acceleration, difficulty starting, and an engine that shakes at idle. Synchronization evens the airflow through each carburetor so every cylinder pulls the same vacuum — and this wrench is how you make the adjustment.
The wrench has two working ends: an 8mm socket that fits over the lock nut, and an internal flat head driver that reaches through to turn the synchronization screw. You loosen the lock nut with the socket end, make your adjustment with the driver, then tighten the lock nut back down while holding the screw in place so it doesn't shift.
This wrench comes with a Lifetime Warranty from Common Motor.
Specifications
| Spec | Detail |
|---|
| Size | 8mm socket with internal flat head driver |
| Fits Models | CB360G/T (1974–1976), CL360 (1974–1975), CJ360T (1976–1977), CB500K (1971–1973), CB550K (1974–1978), CB550F (1975–1977), CB750KA (1977–1978), CB750FA (1977–1978), CB750A (1976–1978) |
| Also Fits | CB350F, CB400F, and many Yamaha, Suzuki, and Kawasaki models with vacuum-gauge synchronization |
| Sold As | Single wrench |
| Warranty | Lifetime |
Note: This wrench is included in both the CB360 Carburetor Synchronization Tool Kit and the CB550 / CB750 Carburetor Synchronization Tool Kit. If you already have the full kit, you do not need this wrench separately.
How to Use This Wrench
Difficulty: Intermediate — requires a running engine and a vacuum gauge
- Warm the engine to operating temperature. The Honda CB360 factory manual specifies an idle speed of 1,200 RPM; the CB550 factory manual specifies 950–1,050 RPM.
- Remove the brass screws from the synchronization ports on each carburetor and connect your vacuum gauge lines.
- Read the vacuum on each cylinder one at a time using a single gauge and gang valve — this gives a consistent reading across all cylinders.
- Insert the wrench into the sync screw access hole. Use the 8mm socket end to loosen the lock nut, then turn the internal flat head driver to adjust the synchronization screw.
- After each adjustment, recheck idle RPM and your reference carburetor reading. Each change affects the other cylinders, so work back and forth until all readings match.
- When all cylinders read the same vacuum, tighten the lock nut while holding the driver steady so the screw does not shift.
- Reinstall the brass port screws and take the bike for a test ride.
On the CB360 twin: One carburetor is static (the reference) and the other is adjusted to match it. On the CB550 four, early carbs (1975 and earlier) have all four individually adjustable — find the two closest readings and bring the others in line. Late carbs (1976 and later) have a fixed reference carburetor.
Our Honda CB360 carburetor synchronization video walks through the full sync process on a CB360T using this wrench and the complete tool kit.
Frequently Asked Questions
Does this wrench work on all Honda CB360 and CB550 models?
Yes. It fits every CB360G, CB360T, CL360, and CJ360 (1974–1977) as well as every CB500K (1971–1973), CB550K (1974–1978), and CB550F (1975–1977). It also fits the CB750KA, CB750FA, and CB750A (1976–1978), plus the CB350F and CB400F small fours.
Can I use this on non-Honda motorcycles?
Yes — the 8mm socket and flat head driver combination works with many Yamaha, Suzuki, and Kawasaki models that use vacuum-gauge carburetor synchronization with the same 8mm lock nut and slotted sync screw.
Do I need the full sync tool kit, or just the wrench?
If you already have a vacuum gauge, hoses, and a gang valve, the wrench alone is all you need to make the adjustment. If you are starting from scratch, the complete CB360 Sync Tool Kit or CB550 / CB750 Sync Tool Kit includes everything — wrench, vacuum gauge, hoses, adapters, and gang valve.
How do I know if my carbs need synchronizing?
Common signs include a high idle that won't come down, an idle that hangs when you come to a stop, rough or jerky acceleration, difficulty starting (especially cold starts), and engine vibration or shaking at idle. If you have recently rebuilt your carburetors or removed them from the bike for any reason, a vacuum sync should be part of your reassembly process.
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Last updated: March 2026
