My son has a used Toro 8hp (Tecumseh) rear-engine riding mower. After a bit of cleaning, repairing and replacing it runs good except -- it won't START (when cold) unless he sprays Starting Fluid in the Air Filter. It'll restart when hot and runs fine. (He's been thru several cans of gas and cleaned out the carb bowl, maybe even had a friend rebuild the carb, not sure.) Any thoughts?