Ok, Z425 with Briggs 23HP OHV engine.
Checked keyway. ok
Replaced carb. ok
Checked coil gap. ok
Spark plugs. ok
Adjusted valves, 1/4 inch past TDC ok (twice)
Runs like a top when you can get it to run. pops and backfires through carb trying to start.
Checked compression in both cylinders today. 180 on one and 120 on the other.
I haven't been able to get it to run since the last time I mowed. Took an charger with lots of power and still the engine popped and backfires through carb until it finally started. Ran for an hour and never has started again.
I did all the things above and it still backfires through the carb when trying to start.
Before I rip into the low compression cylinder, is maybe the valves still out on the low compression cylinder?
Forgive me but it has been 50 years since I worked on these small engines at this level. HELP?