I have the same carb on my Comet. I run a 220 main with the needle in the middle position. Idle mix screw open 1 1/4 turns. Stable and reliable start and runner.I have just purchased a series C Comet and having issues tuning it to run correctly. It came with a 930/71 Mk1 concentric carb, which was worn, so I replaced it with a new premium one from Burlen services. I used the same settings as the one that I took off but it ran very rich. Wet sooty plug and white smoke from exhaust. I've reduced the main jet from the 270 that was fitted to the previous one and gone down to a 250. I've replaced the throttle valve to one with a higher cutaway and moved the clip on the needle up to the top notch. None of this seems to have made a difference. The bike runs ok for short runs but the plug and clouds of white clearly indicate it's far from right. Anyone using the same carb and can let me know their settings or suggest other changes I can try. Many thanks.
Not needed with the new "premium" carbs from Amal as the float has a metal (not plastic) tab that you can gently bend to adjust the fuel level in the bowli ran a 220 main Jet in my comet.but if you think your fuel Level is too high ,you can drift out the yalve seat out of the float bowl. then it is possible to fit a washer under it and this means the float Closes earlier.best is to use a fibre washer.but copper will do it ,too.Erik