Corza, the problem with those butterfly-valve equipped chokes, is that vacuum is meant to OPEN the butterfly as the revs and throttle settings increase. the problems i have noted with them, particularly on smaller engines (such as your 140) is that once the valve clearances open or close up a bit, the blow back, which Cactus has mentioned, creates a small drop in vacuum, which appears to be enough to allow the butterfly to close again... basically it has the effect of closing the throttle when you are trying to hold it open...
a really quick check to see if it is working properly, is to remove the air filter, start the bike, and while watching the back of the carby, rev the engine and watch the effect on the butterfly, as the throttle opens and closes... then hold the throttle wide open... the butterfly should open, and remain that way... if it flutters, or closes back up, you have a problem... ie there is not enough suction/vacuum to hold it open... to test it further, physically hold the butterfly open with your finger, then rev the engine. if the engine revs a lot cleaner, particularly up top, then yes, you have a problem, and it's most likely valve clearance related...
oh, and even though yours, and your missuses bike may have done the same hours running, it doesnt mean the valves will wear and seat, at the same rate.. If you weigh a bit more than missus, then your's will probably seat quicker, as there is a higher load on the engine during the running in period...
in case we havent made our point yet, check your valve clearances, lol, then do the carby thing i have just suggested, then pull the carby off, put it in a bucket of petrol, take three steps back, throw match in bucket. Whooshka. best thing for those carbies.
Then go on ebay, and buy an OKO 26mm flatslide. DHZ has them with everything you need, and ken can supply the jets you'll need to at least get you IN THE BALLPARK.
good luck mate, keep us updated