Having a problem with an MC21 SE, someone may have some ideas?

The bike will not idle on 2 cylinders below 2000rpm, below 2000 the front cylinder just cuts out. If the idle screw is turned up so the bike idles at 2000rpm or above it stays there fine.

The idle is solid on 1 cylinder and does not hunt around etc and the bike will hold a constant rpm on the throttle. Above 2000rpm the bike runs fine and pulls all the way through the range as expected.

There is a noticeable 'chuffing' noise from the right hand pipe when the bike is being revved in neutral once the throttle is released whilst the revs drop back down.

The bike does not smoke more the usual.

The bike has around 10,000km and we believe it to be on the original engine.

We have already checked for air leaks on the intake, balanced the carb slides, replaced both plugs, set the oil pump, cleaned the carbs + jets, replaced a missing reed gasket but the bike still wont idle under 2000.

It may be useful to add that the bike can be hard to start when hot.

The bike is totally standard apart from a wiresplice.

Any advice would be appreciated