oilly deposit in inlet port

[bolla]

Hi
My Bike yamaha TTR 600 (same engine as XT 600) started cutting out usually while at lower revs,at first it would kick start stright away but got worse and now if it will start it will either die stright away or after a few hundred yards and wont start again.
So far I have changed spark plug,HT cap,petrol,cleaned carb there was quite a bit of yellow stuff in carb a bit like rust ( I have a plastic tank)and i assumed this was the problem but still no good I have disconected and tapped up wires to side stand switch and had a look for any loose wires,also checked tappets.
Spark plug is rather black but when the bike has been runng it usually has not warmed up and choke is still on before it cuts out.
When I took carbs off I noticed there was some black oily sludge deposits in one of the inlet manifold I dont think this should be there any suggestions? and could this be my problem