Re: Remove Exhaust Manifold, or Rydlyme flush?
I'd try the acid flush first. A common maintenance measure on Atomic 4s, most of which are raw water cooled. If the passages are not completely blocked, it may help enough to eliminate the overheating problem. I did it several times on my previous boat which had a raw water cooled Universal 5411. I used oxalic acid from the hardware store.