xtz660 tenere inlet valve question

[bradman]

Hi, i bought a second hand tenere, and have had work done on it and it goes ok.
I was told that one of the inlet valves is blowing back, and blowing the engine out when it is idling.
It usually only idles for a few seconds before stopping when cold. After an hours run or so it idles better, but will still usually just stop with a cough after about 30 seconds. Sometimes it just goes from idling at just under 2 revs and blows down to idle at about 1.1 revs near stopping.
I've put some liqui moli in the fuel that supposedly helps clean the valves and plug etc.
Does this sound like replacing all 3 inlet valves will make it run properly? Or does anyone with experience with motors know something else? I mean, could the engine itself be fucked or is this symptoms other yamahas or bikes in general have when an inlet valve goes wonky?

Thanks in advance for any answers.