Carburetor Synchronization

Parts: none

Tools:  vacuum gauge (4 column), hose clamp, Phillips screwdriver,  flat blade screwdriver, thin gloves, needle nose pliers.                                                                                   

1. Be sure engine is hot (go for a short ride)
2. Remove the long chrome carburetor linkage covers – 3 Phillips screws each side

Note:    If the radiator fan comes on while you are syncing the carbs, it will change your reference point. Some people disconnect the fan and point a house box fan at the front of the engine/radiator to keep things cool during synchronization. Others leave the radiator fan connected and either use a new reference point when it comes on or they wait for it to shut off and settle back to the original reference point.

Note:    The No.3 carb is the reference carb and cannot be adjusted – all 5 of the other carbs are adjusted to read the same as No.3 carb.

Note:    Carb Location (the spark plug wires are also numbered as given below)

Right side, front to rear = 1, 3, 5

Left side, front to rear   = 2, 4, 6

Note:    Thin gloves and angled needle nose pliers make getting the hoses and caps back on much easier around the hot engine.

Note:    It is a good idea to check your vacuum gauge to be sure that all four columns read identically when connected to the same vacuum source. The line restrictors have been known to cause incorrect readings. You can get 3 T’s or Y’s from a pet (aquarium) supply store and connect all four vacuum gauge columns at one time to the No.3 intake manifold – they should all read the same.

3. Start the engine and clamp the vacuum hose that runs from the No.6 intake manifold to the fuel petcock. You need to keep a vacuum on the fuel petcock for fuel to flow – the vacuum would be lost when you removed this hose to sync carb No.6. After clamping the hose shut the engine off.

Hint:     If you do not have a hose clamp, you could use a hemostat or visegrips.

4. Use a flat blade screwdriver to push/pry the No. 3 and No.4 vacuum hoses off the intake manifold – you can leave the little hose clamp in place
5. As in step 4, remove the small caps from No.1 and No.5 – there are no vacuum hoses on these two
6. Connect the four column vacuum gauge to No.1, 3, and 5 intake manifolds on the right side and No. 4 on the left side where you removed the vacuum hose or cap

Note:    Be sure that the gauge and its hoses do not touch the hot exhaust pipes!!!

7. Start the engine and be sure that it is idling at ~900 rpm. The idle adjust is a black knob located between the No.1 and 3 intake manifolds on the right side of the bike - it is ~2 inches farther in from the backside of the intakes. It is easy to turn with one finger.

Note:    After starting the engine, be sure not to blip the throttle rapidly when using mercury gauges as you can suck the mercury into the engine!!! Let the throttle speed down slowly!

Note: When using the screwdriver to turn the adjustment screw, be sure to use light pressure on the screw. Heavy pressure may affect the actual setting.

8. Adjust carbs No.1 and 5 to read the same as No.3. The adjustment screw is the blue painted Phillips screw next to each carb. The No.3 carb has no adjustment screw. (No.4 is connected to the vacuum gauge but will be adjusted in a later step)

Note:    Honda specification calls for the carbs to be within 40mm (1.6 inches) of mercury, but you can do better – probably within 5 – 10mm.

9. Slowly rev the engine a couple of times and recheck the balance

Note:    Some people prefer to balance the carbs at ~3000 rpm (cruising speed)

10. Shut the engine off and remove the vacuum gauge hoses from No.1 and 5 intake manifolds
11. Reinstall the caps on No.1 and No. 5 intake manifolds
12. Remove the clamped vacuum hose from No.6 (leave the clamp on) and the cap from No.2
13. Move the vacuum gauge to the left side of the bike and connect the two free gauge hoses to No.2 and 6 intake manifolds (No.4 was connected in step 6)

Note:    If mercury got stuck in the vacuum gauge columns when you shut the engine off, you can blow into the end of the vacuum gauge hose to force the mercury back into the reservoir.

14. Repeat steps 7, 8, and 9 to balance carbs No.2, 4 and 6 on the left side
15. Shut the engine off and remove all four vacuum gauge hoses
16. Install the vacuum hoses and caps to all intake manifolds:

Hose =No.3, 6, 4                     Cap = No.1, 5, 2

      17. Remove the clamp from No.6

      18. Install the chrome linkage covers – 3 Phillips screws each. Be sure to get the       cable hooked on the front inside of the left linkage cover when installing. 

      19. Take bike out for a test drive