My riding lawn mower has a Briggs & Stratton 18.5HP Intek motor (Model number: 31P777-0602-B2). The other day it began bellowing dark smoke, then sputtered and died. It would not start following.
At the time, it smelled strongly of gas, and there appeared to be gas in the engine oil.
I thought maybe the float was gunked up and it was letting too much gas through, so I pulled the carb, cleaned it up (did not appear to be gunked up at all), changed the oil and put it back together. (Note: this carb does not have a main jet adjustment, only idle speed and idle mix)
The engine then ran, but very poorly (especially at full throttle). It stumbled and appeared to be backfiring out the intake. So I pulled the head found the intake push rod was bent (exhaust rod looks fine). I didn't think the head gasket looked obviously blown, but I'm not really sure what to look for there.
I can replace the rod and check clearances, but I don't want to put a new one in and have it fail immediately. What could have caused the rod to bend the first time? What would be my next step to try and diagnose the original failure?
The intake valve sticks slightly. The sticking can be overcome with a small amount of force. For example, I can hold the head and the intake valve will stay compared to the exhaust valve which slides out on its own. The intake takes a few light taps to slide out. Is a slight sticking enough to bind during operation?
I've read that the valve guides on these engines can move which can lead to bent rods, but I'm not really sure what they're supposed to look like before they've moved. Do these look OK?