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I have an issue with erratic idling on my old Velo - which has a slide carb. The cause here seems to be worn slide or throtlle body. There is a small amount of play if you wiggle the slide which then effectively acts as an air leak. Not sure if yours has slide carbs...Another possibility is the old chestnut of a more obvious air leak somewhere. The classic is the small nozzles for the carb synchronization/balancing on the carb outlets - if you have them - are letting in air. The sealing caps can come off. Another is the flange faces, another is the vac take off etc etc.
Thought it may be worth adding something about carb setup and how they derive the "correct" idle jet size.The manual has routine for starting with airscrew at a baseline then running it and shifting the airscrew further outward to make it leaner, this until it's heared that the cylinder you are working on starts to falter as it goes too lean. Then bringing the idle speed back downward by 100 rpm when turning the airscrew inwards to make it run slightly richer than that peak rpm just as it falters. Then you reset the main idle speed control (master adjust for carb rack) to bring rpm in reasonable range overall. Repeated for each cylinder, you'll then have set the mixture for those idle jets in use.After that, if you put vacuum guages on and run at the above setting it should now target 20/22 cmHg as ideal tickover vacuum. If this reading is too high, that indicates that you need the throttle slides too low (causing too much resistance) to control the engine with the jets you've used. It follows that these are too small a jet, giving lean mixture and needing extra air control to stop idle running away.Equally, if Hg reading is too low it would mean the idle jets are too large and causing rich mixture, then too high a slide position to compensate with additional air to prevent stalling. It's a circular loop, but should give you the correct fuel metering to run competently.