knocker

I've had this problem on and off now for about 8 weeks, thought i'd sorted it over xmas but it seems to have come back. I rode the bike 2 days ago, about 15 miles to work and it was fine, when i came to the ride home about 4 miles into my journey it started to feel a bit down on power and the idle revs had dropped to the point where it was almost cutting out, it was also backfiring badly on the overun, at first i'd put it down to carb icing as it was quite cold and damp. Rode it to work the next day and as before the journey to work was without any problems but on the way home (in the rain) the back firing returned as did the really rough idle, the bike was fine when ridden at above 4kish revs and giving it a blast on a straight stretch of road seemed to cure the symptoms for a short time. Went out to have a look this morning and the idle is now erratic as soon as i start it, it sound very very lumpy and the revs jump up and down, if i give it some gas it seems to splutter a little before picking up but as before once the revs are up past about 4k it seems to run fine. I'm going to strip the carbs down again tomorrow and give them a proper clean as it feels like a fuelling issue but waht else could it be? Really doing my head in now as without the bike i'm having to use the car for work which averages a rather expensive 19MPG