Suzuki DR125SEY Running Problems

[PeterH]

I have a DR that I bought with a running problem for my son to ride (fed up with his twist and go!).
Carb is clean, spark good, compression good. Starts with a bit of fiddling but runs but can't get the carb adjusted to get a tickover and the plug is showing rich mixture.
I gave up and sent it to the local bike shop who also cant figure it out. Carb appears to be OK - Diaphragm good, needle set lowest knotch, pilot set correctly etc.
It appears to be running rich - they don;t know why but are suggesting I replace the carb although they can't find anything wrong with it. I don't feel happy about paying them for their time so far (money gone, bike still not fixed!!) so don't know about spending more on a carb that might work but will cost me more to buy. So back to my workshop.

On the lhs of the carb on the intake side (not the filter side) there is a drilling which is open.
I have blocked this off but it makes no difference.
My logic says that if it runs, it can't be far off. If it's rich, the needle is lifting too soon. Have there been any issues with the diaphragm spring. I am not farmilliar with diaphragm carbs. Where does the vacuum get picked up to suck the slide up?
Anyone have any ideas.
I have an impatient teenager desperate to get on his first real bike.
____________________
Trying to get it right