My 1956 Sea King (12D13 or 12D14) is having water pump issues and I can not figure it out. A little challenging to explain but here it goes and hope someone can help me out! Engine was overheating while on plane (on a small aluminum boat). Confused me and my son (father son project and great learning experience for us both) because when we put it in the water and started it at idle it pumped very well. (This engine has been on this boat for many years, mounting height is at the same place it has always been when it pumped good on plane so it is not a mounting height issue with cavitation plate well below bottom of hull) So we have pulled the impellor many times (using a verity of high grade sealant and no sealant options when installing the pump housing and plate), changed the impellor/new and always have the same result. Next step was to bucket test it. If the water in the bucket is above the height of the pump height the engine can low idle neutral and fast idle neutral and it pumps well. As soon as the water splashes out of the bucket to the point that the water is only a couple inches above the cavitation plate and below the pump height in the housing the pumping stops at high neutral idle. It will reprime itself when you slow the idle back to low idle neutral (which is why I think the pump is working correctly since it pumps at low idle). There is a shift linkage port on the side of the housing and we accidently figured out that if we removed the port cover we did not lose water pumping as quickly as we raised the neutral idle rpm but we would still lose prime when the water level went below the pump height at high idle. I think removing the shift linkage port lowered the exhaust pressure in the housing as exhaust came out the side port. Here is my theory 1, if the engine is way down in the water the lower housing is filled with water and this water keeps the exhaust gas from entering the water pump from the top between the pump housing and shaft. In addition, when we removed the shift rod disconnect port this lowered the exhaust pressure in the housing which also help eliminate exhaust gas getting in the pump from the top due to the exhaust pressure in the housing. The pump housing/engine schematic(part number 303070) does not show any kind of shaft seal for the housing and the housing fits pretty snugly to the shaft. Theory 2: I had the impellor replaced a while back (long story)and maybe me buying a impellor that matched the previous is not correct and causing the issue. (exhaust passages are clear, removed lower water intake screen to make sure bent up screen was not the problem. priming pin hole on pump housing cover is not blocked, impellor is in the correct direction) Does anyone have any ideas? I am willing to buy a new housing and impellor and plate to try if anyone knows the part numbers and where to buy? I have service manual but there are no part numbers.
