I just installed my rebuilt engine (which sat in my garage for too 6months) It starts and seems to rev ok but has a very rough idle and popping noise from the left carb. I have thoroughly check for vacuum leaks and there appears to be none. I have a new electric fuel pump filter and lines also. As I was to remove the left carb to have a look i noticed the top butterfly (sorry not sure whats its called) doesn't close completely like the right one? Could this be the issue or what else should i look for. Photos attached

What is this on? a kombi?

Have you tuned the carbs at all??
they need to be sucking at the same rate

What carbs are they?

That's the choke. If the carbs are that dirty then they obviously haven't been looked at when the motor was "rebuilt", so were they running okay before?

They were spotless before they got this dirty after running the engine for like 15mins in total. The engine ran fine on the stand (the guy that built it is overseas for another 4 weeks) nothing was touched or played with whilst it sat on my garage floor.

Solex 34 duel carb. in a kombi yes

That's a lot of oily crud for 15mins of running.

But aside from that the carbs need to be balanced for the engine to run smoothly.

Did the carbs get properly cleaned out? Rebuild kit isn't really needed unless a diaphragm has perforated or a gasket split but what is very important is that the jets are removed and blown thru or even soaked overnite in carb cleaner and the galleries and float bowl are all cleaned thru with carb cleaner.

Smallest bit of crud in a jet will have things running badly.