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alistair still has parts available - barrels, carburettor, castings - see all listings.


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Douglas Dragonfly carburettor fuelling

Started by jixxer400, 06 Feb 2023 at 19:28

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Hi all,
I have a Dragonfly which has the original carburettor, and I have replaced all of the correct jetting and needle slider with new replacement parts, and the bike still continues to run lean.
I have previously read a few posts on here suggesting the difference with todays fuel as opposed to the fuel of the 50's might call for slightly different jetting and slider.
The post that I read suggests running with a No3 cutaway slider and a 150 main jet, then lower the needle to its lowest point, therefore making it richer.
I have tried this, and the bike does run richer, but it also idles far to quickly to the point you cannot alter the idling speed low enough with the screw.

Now, before I start messing around with too many different things, does anyone have any advice to offer or things to try.

Thanks in advance, Stuart.


           It sounds as if you have a worn carburettor body. The bore for the slide is probably worn so that with the engine running, the slide gets sucked back against the wear and wont drop fully onto the throttle stop screw. Try starting the engine and allow it to warm up, then stop the engine and restart without giving it any throttle - if it now ticks over properly, but then keeps running faster after the throttle has been blipped, it is a worn body.



Hi Eddie,

Thanks for the reply,

Yes that makes sense, I will have a little tinker tonight and let you know.

By the way, before I changed the slider to a 3 cutaway and the jet to a 150, the carburettor behaved normally albeit running lean.

Thanks for the reply. 👍🏻