putting too much oil through the engine will burn it up, even if it doesnt go through the jets?
you're running things together again charlie.
first off, you stated to leave the oil injection hooked up while running premix to make sure it gets to the engine. that will feed too much oil to the engine vs. fuel.
secondly, when you switch to premix, you have to rejet the carbs to larger jets in order to compensate for the oil added going through the jets, to allow the right amount of fuel to continue flowing through them.
when making the switch, if you flush the lines and the carb with premix with a pump bulb, and you monitor the mixture flowing through the carb via/ the return line going back into the tank, there's no need to leave the oil injection system connected, and you get it done all at one time.
yes, too much oil vs. fuel, going to the engine will burn it up. not changing the jets to larger orifices when adding oil to fuel will cause a lean condition with the fuel. you're getting way too much oil, on top of not enough fuel.
you couple that with leaving the oil injection pumping oil to the engine, and yeah, you will cook the top end.