Stumbling Slant --what is left??

my68barracuda said

I have a Briggs and Stratton 3.5 hp lawn mower that was not within striking distance of 90 psi when new, is now 50 years old and still runs. When new it took a pull on a rope wound around the flywheel to start it. Now I can almost spin it by hand. Not much power left but still starts and runs.

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Yes, lawn mowers run in low compression, a little more for riders.. but they spin a blade...and dont have to propel a 3000+ lb car to 70 mph...the crank shaft isnt near as big n heavy either. The compression is relative to application and rotating mass.
One of the advantages I have in this hobby is that I was an actual line/diagnostic mechanic for years, Machinist Apprentice as well.. i started out working at little blue hole in wall shop next to savinos market in fresno starting out with a swedish merchant marine mechanic that owned it... I've wasted so much time and had return visits all for the sake of helping a poor womans car limp along a little more. They never ran good... they ran somewhat better...but only graduated to worse over short time. Like plugs... you can change them every week and feel a tiny difference/improvement....but Low cylinder pressure is like not being able to breath...like lung cancer.
As soon as he said 90 lb we pretty much established that the cylinder head at the minimum would have to come off and be inspected. I'm not gonna feed this person false hopes aka piss down his leg and tell'em its raining.
Pull the valve cover to check the stem heights ...then pull the head...its predictable after years of seeing it.