Thank you to everyone that answered this thread.
When I first removed the needle valve holder, oras some call it “the valve seat” (I call it the needle valveholder because that is what towerhobbies calls it) I noticed that avery small piece of the O-Ring looked torn off. But dad and I failedto remove the nipples. Of course we he had to remove the nippleslater, but we didn't remove them the first time we tried to clean thecarb. Instead He just blew through them and felt they were clearenough. We tried swapping the needle valve holder with that of theother FX engine. After that didn't seem to help, we swapped engines,and put the valve holder back in the FX engine it came out of. Thistime the other engine wouldn't start at all. As we began to take thecarb on the second engine apart, we found that a small chunk of theneedle valve holder's O-Ring had lodged It's self into a nipple. Mycurrent conclusion is when I was flying the airplane, a small chunkof the faulty O-Ring let loose and lodged it's self into the nipplecausing the engine to stall. Hindsight we should have removed theNipples the first time. Then I would be a 100 percent sure that iswhat happened. The problem is, if my hypothesis is correct, thenthere is no way of detecting this before it actually happens, unlessof course you remove the needle valve assembly and look at theO-Ring. For, as long as the O-Ring is still sealing the needle valveadjustments will stay the same. After reading my thread to Bax, oneindividual posted that O.S. Indeed had some faulty O-Rings. I wishthat response was still there, but due to the rules in that thread itwas removed by the mod.