Bad Gas or What

OK, I am not sure if this is the best forum for this question, but it seems to be a fuel issue. I am looking for some ideas:

I have a 92 Yamaha XT225 (my wife's technically) and an 00 XR650R. I normally ride the Yamaha to work, but on Monday, the carburetor started overflowing out the drain tube, as though the float needle is jammed open. So I rode the XR to work on Tuesday and Wednesday, and on my way home last night, the Carb on the XR started overflowing, and the float/needle is clearly jammed open as well.

I plan to pull them each apart on Saturday to try and fix them, but seriously, WTF? I just moved to Mexico about 4 months ago, so I have been running nothing but PeMex Roja in each one, mostly from the same filling station, but I am thinking if there was a contaminant in the fuel, the screen in the tanks should have caught that. Could the different fuel have deteriorated the face of the needle on both bikes? Could a bad batch of fuel have gummed up the faces? If it is a bad batch of fuel, could these two bikes be more sensitive to that than all the other cars on the road (I haven't noticed more broken down cars on the sides of the road than normal, but it is Mexico, so it is hard to tell)

Any thoughts?

Fuel filters , they work  

Well, yes, there's that, but both bikes have some sort of screen inside the tank at the petcock, as far as I remember, so it is not as though there is a lot of sediment flowing into the carburetors. It still seems a bit suspicious that both bikes went from working just fine to dumping fuel everywhere very quickly and in the same week. I bought some new fuel line and a new filter (they only had one in stock) today, along with some carb cleaner, so barring any major issues, they should be back on the road this weekend, it just seems to be a major coincidence...

The screen filter in the tank only stops huge dirt/debriss ( othervise it will get clogged up all the time and i have never seen a bike with clogged up tank filter , but i know that good filters will get blocked eventually as i was using them for years ) , i will give you a example , i have rinsed and cleaned my tank , filled it full of brand new fuell ( the tank was spottless ) , i have installed new filter , now the funny bit , i have used about 2 litress of fuell when i have noticed shite in the filter and i am not talking one grain of dust , it was loads of little particles , total size was about small grain of rice , not much ? Considering the amount of fuel filterred it was a shocking amount of dirt !

I am not forcing noone to use fuel filter , i am just talking from my own experience , as i hate to take the carb out and try to clean blocked jets , or leaking valve , but thats just me . 

I guess in my dusty environment a fuel filter is probably a necessary bit. I was picturing a large particle/grain of sand causing the float needle to be stuck open, but it could be an aggregate of many small particles too. It is strange that both bikes had the same issue within a week, but in retrospect, both bikes have seen about the same amount of use over the past few months, so if it is an aggregating amount, it isn't so unusual that it would happen at about the same time. I've got a fresh can of carb cleaner, a half meter of fuel hose, the only fuel filter in town and an 18 pack of beer, so I am sure one bike or the other will be running by the end of tomorrow.

".. so I am sure one bike or the other will be running by the end of tomorrow"

I'm not, I know they don't run on beer  

Well, I go them both running. Incidentally, the Float Needle on the XT had some bits of rubber blocking it, presumably from the dried up petcock gasket I changed recently, and the XR had a bunch of grit in the float valve. Rode the XR in today, still need to find another filter for the XT, but it seems as though it was a random coincidence that they both failed in the same manner the same week.