How ‘modern’ is the Amal info that you are referring to ? I ask because the rubbish containing ethanol, etc., etc., is nothing like the petrol of yesterday, and your info may now be inappropriate. If your info mentions a 170 main jet then you will need a 180 or 190 in a 498cc Flyer. Needle position is a matter for experiment, and I would try the third notch from the blunt end !
Thank you Brian, I have today purchased both 180 @ 190 main jets to replace the original 170 so as to experiment. Yes, I understand & agree with what you are saying but am curious as to how the notches are counted