Hi i have a nimbus 36er oracle. The Foss tube that I have in it, seems to go down over a month or so on its own, yet has no leaks ( when checked under water)
I tried putting a normal inner tube in, but valve won’t fit through wheel valve hole.
Do I drill or file this hole to accept this, or buy new Foss tube?? Is this a common fault on foss tubes to gradually go flat on there own?
Mine does. It looses maybe 2-5 psi in a week.
I’ve seen others that have simply drilled the hole a little larger to fit the tube then rode thousands of miles with no problem. Ed Pratt did that on his round the world ride. I’d drill it a little larger and make sure to clean up any sharp edges.
Jim
Yeah I also have that on my 29" Nimbus. Always asks for air. The 32" actually was punctured already when it came out of the box, right at the seams, but I fixed that one and it has held ever since.
My 29" Oracle has never lost air.
Neither. Yes, they are a little more porous than butyl tubes. Just put a little air in once a month and you’ll be fine. Or you can drill it out to fit a Schraeder valve, but as mentioned above, make sure you file the edges nice and smooth. I have to add some air to my 36" tire every few weeks. But I’ve had that tube in there for several years now. It’s been patched once or twice, but still going strong.
{DISCLAIMER: I am not connected to the makers of the Foss tubes, and still hoping to learn, some day, how they chose that name…}
My Foss tube (36er) slowly loses air. I ride it nearly every day and top the pressure off every two weeks or so, at which point it’s usually lost about 5 PSI. This is my second Foss tube and the first one behaved much the same.
My road bike (700x28) gets ridden once a week and I top off the tyres every time it gets ridden. It’s usually dropped by about 10 PSI.
My muni (26x2.35, standard tube) never seems to lose air. I ride it at most once a month (sad, I know) and top the tyre off maybe once every six months.
So, my anecdata says that Foss tubes leak more than standard (heavy?) mountain bike tubes but less than standard road bike tubes. I’m happy with that tradeoff for the weight improvement they bring.
Every time I pump up my tyre I think of you 
Ha! My last name is the Scandanavian word for waterfall, or “falls”. My great-grandfather on my father’s side came from Norway to the United States in 1890, before Ellis Island was in use. His last name was Gundersen. He and his brothers were going to change their last name to Foss, as an indication of the region of Norway they came from. But at the time, he was the only one that went through with it. So I’m not related to most people named Foss. They moved to Wisconsin, where loads of Scandinavians also emigrated. My dad is from LaCrosse, WI but I was born in Michigan. NAUCC 2018 will be held in my hometown, Livonia!
Livonia was named after an area of Northern Europe from the 13th century that is now basically the area occupied by Lithuania, Latvia and Estonia. I did not learn this in my childhood growing up in Livonia, Michigan, but from a circus performer that came from Latvia! My Livonia may not have been named directly after the Baltic Livonia, but after Livonia, New York, which has less than 1/10th the population of the Livonia in Michigan. I know, none of you asked.
The creators of the Foss tubes, as far as I can find out, seem to be from Taiwan. Maybe it means something in Chinese? Or the technology maybe came from someone from Scandanavia, or they just like the sound of it. It doesn’t quite sound like air escaping slowly… 
In defense of Foss tubes, don’t forget that as well as being light weight they are supposed to offer some level of puncture protection.