yzf 400 carb problems

[SkAgS]

a guy from work is having problem with his bike ,
only had it for about 3 months and in that time about the first 6 hours of riding the bike it started to idle high and seem to be leaning out , after taking the bike into the shop they found a piece missing off the slide , ordering a new one from yamaha 3 weeks and 250 dollars later for what looks like a 5 dollar part its back in and going great , untill about 6 hours later same thing happens,
he's to scared to buy another one and put it in as he thinks it's just going to happen again. and could possibly get sucked into the engine ...
looking at the carb there seems to be quite a bit of slack on the slide witch i think could be vibrating ? causing it to eventually give ?
anybody had any problems with anything like this before.
we have looked at another mates yzf 450 but an 03 model , the carbs slide is different but the carb is roughly the same size, does any body know how much would be involved in putting a newer carb on ? and anyone got one for sale
any options or recommendations of where to take the carb for someone that might be able to shed some light on the subject would be appreciated thanks

bike is a 99 yzf400 its a Keihin carb and he is in christchurch

thanks

[00yzf400]

Hi I was out riding today on my yzf400 and all of a sudden my bike started revving really high like the throttle was stuck 3 quarters open or something is that what happened to yours cause mines got me stumped thanx