Synchronize Carburetors on Small Honda Fours


Carburetor synchronizing is something many owners prefer to avoid. Chalk it up to a fear of undue complexity or just plain getting it wrong, it’s a job otherwise capable weekend mechanics typically decide to turn over to more experienced hands. That’s certainly a wise move if you’re mechanically challenged. But if you can confidently tune your bike — adjusting the valves and setting points and timing — then you can probably handle a carb synch.

The point of synchronizing is to make sure each carburetor is pulling exactly the same amount of air, so that each cylinder is filling the same. Importantly, tuning your bike is the first step in a carb synch, because if the valves aren’t set correctly and the ignition timing is off, trying to synchronize carburetors usually makes a bad situation worse by masking other problems with carb adjustments.

BikeMasterSynching a twin by ear can be done with experience, but a four is a different matter as three properly set carbs can cover up one poorly tuned unit. And if you’re synching the carbs on a multi-cylinder like our subject 1973 Honda CB350 Four, you have to have the right tools. In the old days it was common to use a mercury stick — a roughly 30-inch tall set of four clear plastic tubes in a common reservoir filled with mercury — to measure engine vacuum. Mercury was used because its high density made measuring easier and more accurate on a smaller scale.

Mercury sticks are no longer available, but you can buy synch sticks that use a non-toxic fluid and electronic synchronizers are also available, although 4-cylinder versions are pretty pricey. In between are vacuum gauges, with a good set like the one we got from BikeMaster running in the $225 range.

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