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US Flag Wednesday, March 22, 2023
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 gauges – if you want to, then do it after following my procedure to the letter. It's easy to miss-adjust things using gauges and make everything look correct. This is especially true is 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 twistgrip and inline adjusters.

Whenever reasonable, I do this on the bench with the carbs, cables, and twistgrip all connected normally – much easier, but it certainly can be done on the bike.
  1. 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.
  2. Adjust top adjusters or inline adjusters until slides reach the top at the same time (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!
  3. While feeling both slides, adjust one throttle stop until the slides start moving off the throttle stops at exactly the same time. This normally takes only a tiny tweak.
  4. Unscrew the carb adjusters until there is only a little play. This step is critical when a junction box is used!
  5. 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 out. 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's 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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