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I have a 1987 kawasaki ninja. I've had it for two years and it ran good until this year. When I started it this week, gas started to flow back into air filter box. Has anyone ever encountered this?
If the float valve in one of the carburetors sticks it can force fuel up into the throttle body. Sometimes tapping on the side of the float bowl(s) will un-stick it. If the carbs have never been cleaned it may be worth doing this.
Best to check the intake valve seals. If the shim is too large it won't allow the valve to seat properly, allowing pressure to be pushed back into the carb.