Well leaking fuel pump diaphragm is a logical idea except not likely two engines would have this same problem but possible. Knowing if this is happening when the engine is running OR when it is shut off would help tell the problem.
For your future info - That carb solenoid has nothing what ever to do with stopping gas flow thru the carb, only mostly shuts off flow thru metering jet to help prevent "Äfter Fire."
It is ridiculous to think it takes 8 oil changes to get gas contamination out. One good drain, two at the very most, and the very slight remaining gas will quickly evaporate. I only drain once.
Walt Conner