ORIGINAL: Gray Beard
I don't think it is the valves, it sounds like just a tune problem but I have found engines new, right out of the box with the valves out of adjustment. On a new engine I adjust the valves or check them after it's first gallon or so of fuel. It's worth a look and not a very hard thing to do.
I'm running into a problem that sort of sounds like what you are going through but with a two stroke and it craps out on me at about 1/4 of a tank draw down. In my case it is the tanks location, it is too far away from the engine and I loose pressure. I'm working on installing a pump right now. Just something else to look at in your search.
the valves may not be your problem, but I found that I needed to reset the valves on all my saitos twice, half way through break in and then after break in. Plus I always check them before even starting a new one ( or used ).
Are you useing a tech. I generaly tach. my saito's to 10K and they seem to run very well. If I tach. them to 10 K on the ground I should be under 11K in the air.
good luck
sticks