MK3 Carbs

Hi I had the same problem it turned out to be the pilot jets and the bodys needed ultrasonic cleaning that done the tick on mine.

Mo

Replace both needle jets ,which are the first thing to wear out in these carbs. 

If you can't get them sonic cleaned remove the air screws ( first note the number of turns that they are set at) then squirt carb cleaner directly into the air passage and through the jet. You'll probably have to reposition the needles that you reset......If that doesn't cure things you may need to pass a strand of fine cable wire thro' the jet, then give it a good squirt of cleaner.  Premier concentrics hve a a separate removeable pilot jet.   Good luck, Howard

Try a single 32mm Amal. Good advice above. A big increase in MPG without much performance loss at the lower end.Smoother throttle and simplicity of cable routing.

Works for me..

Your problem appears to be at tick over and the smaller throttle opening and I don't think moving the needles will solve it. However the needles and needle jets do wear out and sometimes at an alarming rate, it wouldn't be a waste of money replacing them.

Have you checked;

The choke sides haven't dropped down, or float levels too high, are the air screws less than 1.5 turns in, burning oil, the wrong grade of plugs ie too cold? All these will give the symptoms your motor is experiencing.

To remedy this effectively, did the carbs work well in the past on these or other settings, if so then suspect worn parts or debris in pilot jets. 

You may even find that even with new jets needles etc the carbs will still not set up in which case they are worn out. Amals in the 70's were not made that well and the quality varies massively, some looked unusable even though showing hardly any wear.

If you go down the single carb route don't hack at or sell your airbox, keep it with the bike. The price of second hand ones is eye watering.