I have a JD JX75 that I have been using for years with no problems. The last couple times I have finished mowing, I pull the throttle control back to slow and the engine continues to run. I have tried to use the gas shutoff switch, but that does not seem to work. The engine runs until it has run out of gas. Is there an adjustment to make to the throttle to shut down the engine? Any help would be greatly appreciated.