I used to buy O-ring material from Ga Rubber and cut to size
You cut it on the bias, about 60 degrees to the axis, and use super glue. They kit companies are selling O-rings made of the wrong material if it's expanding. I've seen the same thing with float valve O-rings if I don't specify the right ind.
I'm gonna be doing some CB650 carbs early next month and appresiate the heads up on the problem. Will probably lok on www for source of fuels resistant material and make my own rings after reading this thread. Unless, the bowl edgesare falt enough, in wjich cace, I'll just whittle something out of the nitrile rubber gasket sheet material from the autoparts store.
Have a mess of it from other projects
Anybody got a decent CB650 gas tank cheap?
Mine is full of rust and giant flakes of some sort of misapplied polymer tank coating crap.
Hope to get CB650 in decent shape for winter solstice gift to my brother.
-Duck