Starting problems

My bike started having starting problems from what seems out of the blue, about a week ago I come out of work and the bike had trouble starting. It usually fires up straight away then stalls immediatly, then will do this a few more times for a while making alot of awful noise inbetween but eventually starts after about 20ish seconds.

After the first time the engine runs it'll start up again fine with no problems within the next hour or so. Since this started I replaced the spark plugs, the oil and filter and the air filter (I didn't expect that to help but I couldn't help myself ) and I've charged the battery but nothing helped. It's on full choke, and when it eventually runs it'll idle very happily and will ride for hours problem free.

About 3000 miles previous to the problem the only modifications I had was a new exhaust and removing those pesky washers under the carbs. It's a K1 650s and I haven't had a single problem in it's 15k miles of life.

Thanks for any help you guys can give me