I wouldn't run it with an exhaust leak.....
I'm pretty sure running lean will cause the exhaust to get hotter and glow cherry red, not rich.
I had a 220 bayou one time that, when running, the exhaust would get red hot. Took the carb apart and ethanol gas had eaten away the aluminum housing around the main jet allowing more fuel to flow.
What happens is that there is more fuel than can be burned in the amount of time it has in the cylinder, so it is still trying to burn as it is going out of the exhaust valve. Then you have fire in your exhaust that should normally only have hot gasses.
Lean condition will make the fire in the cylinder burn hotter.