OT Briggs driving me Batty!

So I inherited an old Simplicity from a friend (1973 Soveriegn 3614H) and it's powered by a big old cast iron B&S 16hp horizontal shaft single cylinder. The stupid thing runs fine for about 28 minutes then sputters and you can only keep it running if you throttle down to 1/4 or less. Park it for the day and next time you come out it runs fine......for about 28 minutes!

Every time I work on it, it acts like it's fixed, for about 28 minutes.

1. We thought it was the coil. Have to pull the motor to get the airshroud off to change the coil so I bypassed it and have the factory points firing a battery ignition. Ran great for 28 minutes.
Dies off and still has nice blue spark when it dies.

2. Carb had a cobbled gasket, rebuilt it with new oem parts. 28 minutes.

3. Removed the PO's inline fuel filter thinking it was plugging up. 28 minutes.

4. Flushed fuel tank, thinking sediment plugging fuel outlet. 28

5. It's got to be the valves, they heat up, lash is too tight, after running long enough to come up to temp it has to be held open.... Nope, pulled the head anyhow to decarbon. Black and sooty but no huge deposits, valves seal, cyl looks good-no ridge / scoring, nice and clean, no bent valve. 28 minutes.......

What am I missing?! Do I need to yank the sucker and drop a magnetron coil into it? Thought I'd read that the magnets on the flywheel need re-polarized opposite for magnetron compared to the points ignition.

Final thought, ignition switch crapping out?

Thoughts, advice, commiseration?

Just for fun if not too much trouble take a compression check after 2-3 minutes and again at 29 minutes immediately after it stops. I f you have very nearly the same compression both times it would rule out the "easy spin" feature.

Try removing the gas cap and let it run.

Dad had 1 that would do that. He had it to a shop many times. Mechanic finally caught it just as it died,
pulled the head and found a valve seat would float after it got so warm. As it cooled off, the valve spring
apparently pulled the seat back into the block. Mechanic peened the valve seat down and the darn thing ran
good for another many years.

That's what I'm thinking, a loose seat, a loose guide riding up and holding a valve open, or a gummy guide, valve getting hot and sticking.

Have you checked under the shroud for grass packed fins?

That valve seat floating seems favorite to me, just long enough to bring everything up to temp and then it cuts out.

I should be able to yank the head and stake that sucker in with no issues.

Thanks for the guidance guys!

Steve, to answer your question, yes, the cooling fins are clean.

I had an Onan two cylinder doing the same thing. Ran fine until a half hour. Then it went nuts. Turned out to be a bad intake manifold
gasket. Might be worth checking out.

I think your butt is shutting down after riding it 28 minutes with no seat. lol! Any merit to M-man
suggestion about the gas cap?

condensor

thanks Used Red...

I did try opening the cap, no change.

Condenser is new but I know that doesn't mean it's good. Swapping it with the old one gives me the same 28

Real bummer is with this mower I can get the yard done in 40 minutes...

I feel your pain. Mom's lawnboy was doing something like this and I never really did find anything I could see as the problem ,but it has been working knock on wood. Some of the simple things if ever found can drive one Batty ! I don't know what else to tell you to do except find someone with a yard that only takes 20 minutes to mow and sell it to them ! lol.