Diesel injectors cleaning

I disassembled and soaked my 306 dturbo injectors in Injector cleaner (the stuff your supposed to run through the engine) for 2 days once. Once reassembled and back in the car there was a little improvement, not massively better.

To clean them properly they're supposed to be ultrasonically cleaned and test by a bosch specialist.

Is it an older transit? maybe turn down the 'max load' screw on the fuel pump to lower the fuel getting into her, that'll cut the smoke and power down for the test, then turn it up again after.

It won't be a duff injector giving you black smoke, that would give blue/grey.
Black smoke is unburnt fuel from either lack of air, excess fuel or overload on engine.

You also get blue smoke from partially burnt fuel and white from vapourised but not burnt fuel ;-)