of two cents worth in.
I suspect your problem could be a combination of a few different things.
1. You very likely have an air leak somewhere between the impeller and the strainer. The "self priming" works by the impeller moving air out of the line between the impeller blades and the water (sea-level) in the suction hose. As the air is moved out of the hose the normal atmospheric pressure on the water is greater than the reduced air pressure in the hose thereby "pushing" the water toward the pump. Once (or if) the air pressure is reduced adequately to have the water pushed to the impellers then the dynamics change significantly as now the impeller is dealing with a significantly more dense element, water.
As air is far less dense than water, any minor air leakage is adequate to disturb the dynamics of the impeller's attempt to reduce the air pressure and thereby induce water flow. Common sites for leakage include the paper gasket on the seawater pump cover, hose clamps (especially above the waterline), and the seals
on the seawater strainer (especially the cover to service
the strainer basket). Other less common leaks can include heavily scored surfaces inside the sea water pump, cracked pipe fittings, bad seal in the seawater pump shaft, and an intake hose that is internally collapsing.
2. Any significant obstruction in the suction side of the system makes air leaks (discussed above) more significant. You indicated the motor had not been started in 12 months. You didn't say if those 12 months were spent on the hard
or in the water. If it was in the water I would suspect the inlet could be partially clogged with growth. The thru-hull and seacock needs to be scraped clean along with the hull
. There could be growth well up inside the seacock and hose, so a long narrow scraper would be best. Complicating this is whether or not you have a strainer grate over the through hull
or not and whether or not you have one with a removable grate. AFTER this scraping, if there is a convenient place above the waterline to remove the hose that comes from the seacock you can back flush the line, seacock, and thru-hull with a garden hose and nozzle. You should be able to tell rather easily if it is clean or still blocked.
3. When an impeller spins inside the pump it is synthetic rubber being dragged at a high speed over stainless steel
and bronze. This generates heat rather quickly. Fortunately, if everything has been maintained well and is in serviceable condition, water is pushed to the impeller within a few seconds and cools the impeller. Once primed (again the other components in the system are in serviceable condition) the hoses and impeller is sitting in water ready for the next engine start, be it minutes, hours, days, weeks, or months.
Once you destroy an impeller by overheating it, you need to be careful when inserting a new impeller. Frequently there is residue from the impeller melted onto the inside of the pump housing. If you don't remove this residue it can damage the new impeller in a short amount of time. There is good literature available on the web on the rebuild
process that discusses this and other issues.
Bottom line is this is a system with many components that all have to be working well for the system to work well. I think if you should diligently check each component and service/repair/replace each that is not performing at or very near optimal. Don't stop when you find the first problem as you may have multiple items slightly askew generating your issue.
Good luck my friend.