Overfilled with what? Oil? Nothing to effect starting like a sensor but you don't want it overfilled anyway.
Still no reason not to start.. but do know that overfilling the tank leads to the tank vent check valve getting wet and when it dries out the rubber flap inside tends to stick down and not vent anymore which lets vacuum build up as the pump removes the fuel. But has nothing to do with starting or the EFI system on the 750s or the carbs on the 650s. Look to the roll-over sensor or kill switch for a problem there. If it's ever been in water look at the connection(s) under the fuse box as well.Sorry, not oil. Fuel. I had just a bit left in the jug and figured I'd just squeeze it in. First attempt to start it sputtered but did not run. After that there has been no fire.
You will...someday. I did.Hell I always fill mine till its almost overflowing and never had issues.