Twin-cylinder bike idles rough and vibrates (carburettors out of balance)

On a twin or multi with one carburettor per cylinder, the cylinders must share the work. If one slide opens earlier or further than the other, or one idle mixture is different, that cylinder pulls the other along. The bike vibrates more than it should, idles roughly or unevenly, picks up badly from idle and uses more fuel. It often starts after a cable change, a carburettor clean, or a fall.

Is it safe to ride?

Yes, gently. Check first that both cylinders are firing, because a dead cylinder feels the same and can wash oil off its bore.

Quick checks first

• Feel both exhaust pipes just after a cold start, carefully and without touching. A pipe that stays cold means that cylinder is not firing.
• Look at both throttle cables at the carburettor tops. Is one slack and the other tight?
• Look for a split intake rubber on one side.
• Did anyone work on the cables or carburettors recently?

Diagnose it step by step

1Are both cylinders firing?

• No, one pipe stays cold: check that cylinder's spark and plug cap: Do a spark test. Then compression: Compression test.
• Yes: go to step 2.

2Do both cables move freely, with the same small free play?

• No: free or replace the cable: Service the throttle cable.
• Yes: go to step 3.

3Is there an air leak on one intake?

• Yes: fix it first, or that side will never match: Find an intake air leak.
• No: go to step 4.

4Remove the plugs after a ride. Are they the same colour?

• No: the cylinders run different mixtures. Check both carburettors have the same jets, needle position and float height, then balance them.
• Yes, but still rough: balance them anyway: Synchronise twin carburettors.

5After balancing, is the idle still uneven?

• Yes: a worn slide, a blocked pilot jet on one side, or a valve or ignition fault. Have a workshop check with a vacuum gauge set.
• No: done.

Causes, most likely first