I have a 16.5 briggs ridinglawn mower when you start it the motor lopes at idle... I have cleaned the carb 3 times now... still no luck... it has the fuel shut off valve in the bottom of the bowl... could this be closing and opening causeing my problem... I have checked it and it getting 12 volts at the hot wire and the other I have re grounded it... have change fuel line and filter... no luck... ti stops lopeing when you engage the blades... but soon as you take them back out it lopes... make it for a loping ride... help??