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Bertrand has written a small tutorial on balancing the carburetors of old English cars using the TwinMax depressiometer... Thanks to him !
Preamble to the synchronization of a Norton
We assume that your carburetors are in good condition, (needle, ball without abnormal wear, etc.), and that the float and richness settings are correct. The ignition has been correctly adjusted before.
In these conditions, for the synchronization adjustment itself, 2 parameters remain to be fine-tuned:
- the adjustment of the idle screws which, as its name indicates, allow to adjust the opening of the bushel at idle, control cable released.
- the adjustment of the cable pulling (lifting of the bushels or opening of the butterflies) between 2000 and 3000 rpm for example.
Balancing the carburetors of a Norton Commando
The engine is stopped after having been warmed up beforehand. You just have to connect your TwinMax to the intake pipes:
On the Italian bikes (Guzzi, Laverda), there are holes provided on each intake pipe. You just have to buy the small fittings proposed with the Twinmax...or make them by drilling screws and soldering a small tube of the right diameter.
On the Commando... easy! Disconnect the small pipe between the pipes and connect the Twinmax pipes instead. Even the diameter is right. One more reason to ride Norton!
On the Bonneville and Trident, the noble solution is to fit small tubes on each intake pipe and to plug them after use with a small rubber stopper. Important ! The length of the 2 rubber tubes must remain the same, otherwise it distorts the adjustment! The vacuum taps must be at the same level on each intake pipe.
- Once the TwinMax has been connected, it must be powered up by pressing the red button.
- With the motor still stopped, turn the "Sensitivity" knob fully towards "Maxi".
- Bring the dial pointer to zero by means of the "Zero" button.
- Decrease the sensitivity a little.
- Start the engine and let it idle.
- Adjust the idle speed by adjusting the idle screws of the 2 carburettors so that the Twinmax needle remains at zero.
Adjusting the Cable Pull
Once your idle screws are properly set, you need to adjust the balanced cable pull.
- Bring the speed up to 3000 rpm and lock the throttle.
- By acting on only one of the 2 carbuses, play on the adjustment of the throttle cable after having loosened its lock nut, so that the needle of the twinMax remains at zero.
- Re-tighten the lock nut (be careful, it can make the adjustment vary a little)
- Release the throttle
- That's it! And sufficient most of the time.
You can start again with the TwinMax set to its maximum sensitivity, but it can be impossible because our old carburetors are sometimes a bit unstable, especially at idle. It works well like that... But I know you will try!
Think of it this way, the engine gets hot, it doesn't like it, and if it gets too hot, the idle won't be good at its normal temperature.
With a little practice, it only takes a few minutes.