My father's Ariens riding mower did similar to this , A pushrod would bend and start missing out. after two pushrods in one summer , the dealership told us to run premium fuel only . It has additives regular Jiffy Mart fuel doesn't . The valves will stick in the guides and this is the result . Bent pushrods and or bent valves .
You can add Marvel Mystery Oil to your fuel along with premium fuel for safety's sake too.