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Flat Spot

Started by Les T, 28 Apr 2013 at 10:52

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Les T


Having recently jouned the club and bought a Dragonfly that has been converted to twin carburettors, after taking it out for a test ride I am very pleased with it as it went very well except that there seemed to be a flat spot while accelerating, this occured in 4th gear at about 40mph. above that speed it revved very freely.

The bike has been fitted with 2 x 375/7 amal monoblocs 15/16" bore, the throttle needles are in position 2 , I haven't yet checked to see what size main jets are fitted. It starts well and runs very well except for the flat spot in acceleration at about 40 mph, Anyone any ideas on what may be wrong or how I can cure the problem?

Many thanks

Les T


       Dragonflies often have a flat spot or period of rougher running around the 40 mph mark. Most owners take it up to about 50 before changing into top gear. Having said that, since we have had to use the dreaded unleaded, I have found most postwar Dougs respond better with a slightly smaller cutaway and an increase of 10 on the main jet. To compensate for this you may need to drop the needle one notch.


Les T


Many thanks for the information, should now be able to sort the problem.

Les T