What would make a Case SC tractor suck air in through the exhaust and push it out through the carb?

My grandpa has been working on tractors his whole life and recently him and I started working on a little Case SC tractor that has him stumped. The tractor is sucking in through the exhaust and blowing out through the carb. We took the head off and cleaned it out and made sure everything looked good. The valves are all sitting good and look just fine. We put the head back on and adjusted the valves and turned the engine over. The tractor is still sucking in through the exhaust and blowing out through the carb. It’s not a little blow through the carb. either, it’s a full puff like what should come out of the exhaust. Any ideas what could all be causing this?