But!?!?…my bike ran fine when I parked it..
First, you can almost be guaranteed that if someone had a bike sitting in a garage for a period of time, that he/she did not start it on a regular basis to keep it in 
good running condition. Nor would the fuel tank have been completely filled or have a stabilizer added. The fact that the tank was not filled would allow for condensation to form in the tank. The inside of the tank would then begin to rust. Also, the fuel in the tank would tend to evaporate over a period of time and leave sludge on the bottom of the tank.
Likewise, fuel in the carbs and in the float bowls tend to evaporate and leave sludge as well as the carb having built up some varnish from whatever amount of time the bike had been stored.
Fuel in the fuel petcock will do about the same thing. Fuel in the fuel lines does so as well.
Some fuel may (at some time) have settled in the breather check valve, evaporated, and caused the ball in the check valve to stick.
So as the story goes…you buy this bike, start it up, it runs great and you ride off into the sunset.
The sludge in the fuel lines, float bowls and fuel tank now start to congregate in all of the orifices of the carbs. Rust starts to flake off the inside of the tank and add to the congestion. Varnish in the carbs begins to flake off and clogs the jets. The breather check valve won’t allow the tank to breath and causes a vacuum lock.
Add to this a couple of dried out vacuum lines which operate the diaphragm in the fuel petcock.
Then you have the do-it-yourself’r who thinks they can “just dig into that thing” and make some tweaks and turns and “fix er up!”
Whoops!! (It is not recommended that you learn the art of carburetors by trial and error) Some errors are VERY, VERY costly when it comes to Vintage Japanese Motorcycles.
The Pilot Adjusting Needle has an extremely fine, tapered point and if turned in beyond VERY LIGHTLY SEATED, will often result in a sheered off tip that is wedged in the metering hole.
This is not just a small problem (try about 0.78 mm small at the largest circumference of the metering hole)
In fact, it has the potential to render your carb body an expensive paperweight. It is not cheap or quick to repair this problem and you *will* pay dearly for it.
How bout the Sunday mechanic who decides he’s gonna rebuild those babies himself? Might work out if he is studious, meticulous and willing to embark on a steep learning curve.
Unfortunately, as mentioned earlier, carbs are not the best item to learn on by trial and error. There are many rubber parts that are destroyed by carb cleaner. Soaking your carbs or using the popular spray type cleaner can have PRICEY consequences for the unlearned.
Many carbs use a slide/diaphram combination that is a one-piece deal. Replacing them is not for the financially squeamish…. but it an almost certain proposition if you get carb cleaner on those rubber diaphrams.
Maybe we’ll continue later to add to the 101 ways that carburetors are messed up by the un-initiated. But for now, let’s just understand that unless you have deep pockets or a lot of time to study and carefully learn from those who really know, it’s best to utilize your precious time MAKING MONEY and letting a professional rebuild your carbs then it is to WASTE YOUR TIME AND MONEY trying to learn to do it right yourself. It certainly is possible, depending on the degree of your problems but you have the peace of mind and the assurance that if you send your carbs to a pro, the work is done right, its guaranteed and your transition back to riding will be as short and as smooth as possible.
