I had the same problem, mine had a tuliped exhaust valve. Same, no compression and burnt the electrode off the plug.

By chance were the sparkplugs left loose, like finger tight. I had a person install a fresh engine and never tightened the plugs, just left them finger tight. It burnt the electrodes of the sparkplug out just like yours. It's shown as cause and effect in the Champion sparkplug catalog. This was a stock 260 HP Mercruiser small block, ran about two hours to destroy the plugs, timing was correct.

timing on my engine is 36*, abd yesm they were loose when we pulled them out. Boat1 what was the damage to the engine due to the loose plugs?

Btw, I Am More Than Eager To Sell The Boat. Its A Cigarette Cafe Racer, I Will Consider Any Reasonable Offers. Shoot Me A Pm And Lets Make A Deal.

thats a lot of timing for a marine engine....bet it killed a exhaust valve(s)...

Need to do a leak test

Taking the engine down on monday to have a look at the heads. Most propably a valve seal or a tuliped valve. These plugs were loose but the ones that are not seem to be ok, I was thinking of going with a bit colder ones but these ones seem fine.

I forgot to mention in my post, the cylinder with the tuliped valve also had a loose plug and the electrode was gone. I never found the root of the problem, I'd be curious to know what caused it. Anyone? I have a hard time believing the plug was loose the entire time.
BC

Detonation causes loose plugs. How old was the gas?

timing can do it in a flash...literlly !!