goirish, I had an older K&B .61 giving me fits once. I would set the needles, it would run fine and then quit a few minutes after taking off. I ended up replacing the engine with a new K&B .61 Twister. I was fooling around with the old engine on the test stand when I found the problem. The o-ring on the low speed needle had aged and shrunk, allowing engine vibration to turn the needle, gradually leaning the mixture. Since the low speed needle effects the mixture well up into the throttle range, and I would always throttle back by half after gaining altitude, this bad o-ring caused me a lot of dead sticks. You might check this by running the engine at different rpm's and watching for the low speed needle turning. Edited to add: You might just try turning the low speed needle with a small screw driver to see how easily it will turn. There should be some resistance.