OK, this is going to sound really, really strange. . . but here I go. . .

Have you checked the rear bearings?

My OS 1.40 would load up excessively 3 out of 4 times on the ground so bad that fuel would run out of the carb. I tried everything until it finally deadsticked one flight. The rear bearings were rusted to the point they were making noise. I replaced them and it runs like a top ever since.

Don't ask me to explain what the rear bearings have to do with the mixture. . . . I don't know. . .it is just one of those mysteries of life. . .