Black residue in inlet port

[Gawain]

As the title suggests. Fairly thin and solid, wont come off in any quantity with a cloth but a bit of carb cleaner and a wipe and its gone. Its only in the secondary inlet port. Iv given it a clean and Il check again fairly soon but in the mean time any suggestions what the residue could be or the cause?
Thanks guys!

[bolla]

I also had this on my TTR600
I think it may be normal,bike runs ok
Will post a pic if I can find it

[Pigford]

If its only the secondary, must be when its on small throttle opening & the secondary is still shut, therefore has low pressure in the port, so any blow back (at valve overlap) entres & leaves a deposit?
Interesting............ never thought much about this one!

[Gawain]

Thats an interesting idea. The set of carbs I got, not from this engine, also had residue on the secondary butterfly. Problem is my Srx400 engine has clean inlet ports

[oldbmw]

Could it possibly be oil from the engine breather? No idea of teh plumbing on your bike.