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I just acquired a Skag SW48-14KA walk behind. We will say to resurrect from a neighbor’s rear shed. After replacing the pull start assembly and getting it started when I flip the switch to turn the mower deck on it starts to spin and the motor starts to die. If I flip the deck switch off the engine starts running again.
I do not hear anything weird when the deck starts trying to spin.
Engine is a Kawasaki FC420V and appears to run well. Good compression started easily especially after sitting for a couple of years.
Any ideas?

Thanks in advance.

The carb is not clean.
When you flip the blades on the throttle opens wide to cope with the extra load and it can not get enough fuel