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Friday, April 25, 2025

Syncing Amal MK1 Concentric Carbs

Many think of syncing Amal carbs as some sort of difficult dark art. It's not actually hard if you do it my way! I never use vacuum gages - if you want to, then do it after following my procedure to the letter. It's easy to miss-adjust things using gages and make everything look correct. This is especially true if not mechanically synced first.

I know this is non-standard, but it works 100% the time for me on every bike I build or repair - I sync manually only. This is written for bikes with a junction box for the throttle and adjuster on top of each carb. It works even easier for bikes with separate cables to the twist grip and in-line adjusters.

Whenever reasonable, I do this on the bench with the carbs, cables, and twist grip all connected normally - much easier, but it certainly can be done on the bike.

Unscrew the throttle stops enough that they cannot touch the slides. Then while holding a finger on a slide turn the throttle stop in until it just touches - you'll be able to feel it. Then turn in 1 full turn. Do the same thing on the other carb.
Adjust top adjusters or in-line adjusters until slides reach the top at the same time when you twist the throttle (look in the carb and watch the slide edge reach the top). On some bikes a mirror will help and on some it's best to have a helper. This needs to be exact!
With the throttle closed, while feeling both slides, slightly turn the twist grip and see which slide moves first (if either). Whichever moves first needs its throttle stop turned in slightly. Keep trying until the slides start moving off the throttle stops at exactly the same time. This normally takes only a tiny tweak.
Unscrew the carb adjusters until there is only a little play. This step is critical when a junction box is used!
Go back to 2 until no adjustments are made in 2-4

If done carefully, the carbs are 100% mechanically synced and the cables are properly adjusted.

Adjust the air screws to 1-1/2 turns out from seated. Start the bike and once warmed set the idle to around 1100 making sure to turn the throttle stops exactly the same amount. Blip the throttle. If it stumbles, turn the air screws in slightly. Keep doing this until it doesn't stumble, or you are less than 1-1/4 out. If you are less than 1-1/4 out, you probably have some other problem like and air leak or worn slides.

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