On an engine with one carburettor per cylinder, the slides must lift at the same moment and by the same amount, and both idle mixtures must match. If one cylinder does more work than the other, the bike vibrates, idles badly, uses more fuel and wears unevenly. This page gives the general method from the Amal tuning guide linked in the course. Many multi-cylinder bikes with linked carburettors on one shaft need a vacuum gauge set and the steps in the manual instead.
Safety
- Work with the engine cold for the cable steps. The idle steps need the engine running and hot: keep hands, sleeves and tools clear of the exhaust and fan.
- Pulling a plug lead off a running engine can give a shock. Use insulated pliers and never hold the bare terminal.
- Run the engine outdoors or with a fan blowing fumes away.
You need
- The idle speed and any vacuum gauge procedure from your bike's manual.
- Good cables with free inners. A sticky cable makes balancing impossible: Service the throttle cable.
Before you start
- Each carburettor must already be clean and set up the same (same jets, needle position and float height): Clean the carburettor.
- Check for air leaks at both manifolds. A leak on one side makes that side impossible to match: Find an intake air leak.
- Ignition timing and plugs correct on both cylinders.
Steps
- Back out both throttle stop screws so the slides rest fully closed.
- With the handlebars straight and the grip closed, set a small amount of free play in each cable at its adjuster on the carburettor top. Tighten the locknuts.
- Remove the air filters or intake rubbers so you can feel the slides. Slip a finger into each intake, or watch the slides with a torch.
- Open the grip very slowly. Adjust the cable adjusters until both slides begin to lift at exactly the same moment. Check again at about one quarter throttle that both are open the same amount.
- Refit the filters. Start the engine and warm it fully.
- Screw in both stop screws equally until the engine idles a little fast.
- Pull the plug lead off one cylinder. The engine runs on the other. Set that carburettor's pilot screw for the fastest, smoothest running, then lower its stop screw until it just keeps running. Refit the lead.
- Do the same on the other cylinder.
- With both running, the idle will now be high. Lower both stop screws by the same amount to reach the idle speed in the manual.
- Blip the throttle and let it close. Feel that both slides close down onto their stop screws, not onto the cable.
With a vacuum gauge set
If the manual gives a vacuum balance, fit the gauges to the ports on each manifold, run the engine at the speed given and turn the balance screw or cable adjuster until the readings match. Use the tolerance in the manual.
Check your work
- Smooth idle with less vibration than before.
- When you open the grip, both slides start together and the engine picks up cleanly.
- Both plugs show the same colour after a ride.
If it did not fix the problem
- One cylinder much colder at the exhaust: check its spark and compression: Do a spark test, Compression test.
- Slides worn loose in their bodies give lumpy running that cannot be balanced. A workshop can check slide fit.