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I have a craftsman 25 hp lawn tractor. Found 2 bent push rods in one of the cylinders. Replaced pushrods, adjusted valves (took a few tries), mowed grass for bout half an hour. Shut off engine for a couple minutes, tried to restart, engine backfired and hard to start. Removed valve cover and found exhaust valve pushrod clear off of rocker arm and just laying there. Adjusting nut is tight. Screw that holds rocker arm in place is tight. What would cause this? Intake push rod is ok. Neither rod is bent.

Mitsu said:

I have a craftsman 25 hp lawn tractor. Found 2 bent push rods in one of the cylinders. Replaced pushrods, adjusted valves (took a few tries), mowed grass for bout half an hour. Shut off engine for a couple minutes, tried to restart, engine backfired and hard to start. Removed valve cover and found exhaust valve pushrod clear off of rocker arm and just laying there. Adjusting nut is tight. Screw that holds rocker arm in place is tight. What would cause this? Intake push rod is ok. Neither rod is bent.

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Could be a bad rocker arm or a valve guide

Are you adjusting them properly.
You need to be 1/4 inch past tdc on compression stroke.
Try https://www.briggsandstratton.com/us/en/support/faqs/servicing-the-valves

You may also have a valve seat problem.

I thought I seen on a earlier thread about the rocker arm studs coming out or loosening?? Might just want to check them.

Reassemble and then crank with the plug out and the valve cover off and watch the movement, check for sticking valves and
like said before, may have a loose seat.

reynoldston said:

I thought I seen on a earlier thread about the rocker arm studs coming out or loosening?? Might just want to check them.

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I'm the guy who had the 19.5 B&S single cylinder that had the rocker arm bolt / tower backed off and the push rod loose and the rocker arm dangling. The original problem in that case was a stuck lifter on the intake side hung up in the block and wouldn't move. As far as a rocker arm just jumping off the push rod, there would have to be something causing play to do it. Could be wear where the rocker arm meets the tower bolt. I know I had some trouble adjusting the valves on that 19.5 B&S because when I'd set them and then pull the engine through a few turns by hand, the valves would increase their gap! No reason for it, but they would. Finally I adjusted one thousandth less than what I was using and then everything seemed to work fine. Try adjusting them on the minimum specs for each valve and see if that helps. But stay within the specs as you don't want a burned valve on account of not enough clearance!