"I just went through a similar
"I just went through a similar scenario also! I changed props from 19 x 18 to 19 x 17 and now 19 x 16 to get the WOT up. It just kept getting worse on the starboard engine. Finally went to a professional shop and they found water coming out of one of the cylinders when doing the compression check! Turned out the riser had rusted through and was letting water drain into the cylinder resulting in rusted valves as well. After replacing all 4 risers on both engines and getting new reconditioned heads on the one engine, we had the pressure up to 150 psi on all 8 cylinders but still had low RPM (3500). So all the spark plugs, wires, distributor and coil was changed. This brought the speed up to 3800 RPM. Still not 4400, then we found out the new Mallory distributor had a cracked stator ring. So found an old GMC distributor and a new ECM and it went up to 4000. Finally, found out the tachometer was off by 400 RPM. Put a digital tach on and voila, 4400 RPM with 1/2 full fuel tanks (300 gal full) and empty water tank (100 gal.) and 19 x 16 props (3 blade Nibral cupped)
With full tanks, only 4000 RPM can be obtained at 30 MPH. Hope this helps?"
"I just went through a similar scenario also! I changed props from 19 x 18 to 19 x 17 and now 19 x 16 to get the WOT up. It just kept getting worse on the starboard engine. Finally went to a professional shop and they found water coming out of one of the cylinders when doing the compression check! Turned out the riser had rusted through and was letting water drain into the cylinder resulting in rusted valves as well. After replacing all 4 risers on both engines and getting new reconditioned heads on the one engine, we had the pressure up to 150 psi on all 8 cylinders but still had low RPM (3500). So all the spark plugs, wires, distributor and coil was changed. This brought the speed up to 3800 RPM. Still not 4400, then we found out the new Mallory distributor had a cracked stator ring. So found an old GMC distributor and a new ECM and it went up to 4000. Finally, found out the tachometer was off by 400 RPM. Put a digital tach on and voila, 4400 RPM with 1/2 full fuel tanks (300 gal full) and empty water tank (100 gal.) and 19 x 16 props (3 blade Nibral cupped)
With full tanks, only 4000 RPM can be obtained at 30 MPH. Hope this helps?"