hey guys i seem to be having some problems running this motor im not a begginer and i know how to tune a motor but this one has got me totally baffled let me tell you my setup...

Im running this motor on a gp cap 232 27% and its a new motor and im also running a bisson pitts muffler the fuel iam using is 15%nitro and 15%synthetic and 5% castor. The motor is on its side,,
ok now here is my problem, i tune the motor in at less than half a tank and make sure its all good, transition good and not lean, a nice smoke trail, now i fill it up and fly, first 3-4 mins is good then as the tank gets lower the motor goes slower then after a few more mintues it eventually stops now its seams to me that as the tank gets lower its leans out to the point it stops But there is still smoke comming out telling me its rich. and to me it had the smell of too lean but there is still the smoke comming out of the exhaust...
im running a 3 line pick up,
i dont have a problem with the idle or anything i just cant get it to run well in the air...
so when i gets it back to the pits i richen the high needle some more and it still seams the same....i totally lost...
all ur help would be appreciated
steve