Re: mercruiser 5.7 spark plug reading result
Check the timing in neutral.
Compression is held by the pistons and rings and the valves. The rings have to seal and the valves have to seal, pretty simple.
Adding some oil to the cylinder thru the sparkplug hole and trying the compression test again is a good way to test ring sealing. This is called a wet test. You may have done this already, pretty standard compression test item. If the numbers go up then it means the rings may need some help, but also gives a better indication of what is happening during actual running of the engine as the rings will be grabbing oil and holding better.
The valves have to seal well. The valves themselves need to be "lapped" meaning their sealing surfaces must be smooth and perfectly shaped to the opening in the head, called the valve seat. The seat also must be perfectly shaped and smooth. Lapping involves spinning the valve in the seat with some lapping compound, this is done with considerable "up" force on the valve. The machine shop should have done this or similar during the build process. If this was not done well then the valves may not hold compression well at higher cylinder pressures (higher throttle settings and rpm).
A leakdown test can help determine where compression might be leaking. This test is done with the cylinder at the top of a compression stroke, compressed air is sent into the cylinder thru a fitting in the spark plug hole. Then you listen to the intake manifold, if hissing is heard the intake valve is leaking. Listen to the exhaust, if hissing then exhaust valve leaks. Listen to the crankcase, if hissing then the rings are not sealing or a piston is holed. (I do not think any of your pistons have a hole, that would show almost zero psi on the compression test.)
There are also bench test for heads that can show improperly seated valves.
Again, the valves must be lashed properly or they may not have a chance to close and seal well at higher rpms.
Weak valve springs can also cause a higher rpm valve sealing or bouncing problem.
Check the timing in neutral.
Compression is held by the pistons and rings and the valves. The rings have to seal and the valves have to seal, pretty simple.
Adding some oil to the cylinder thru the sparkplug hole and trying the compression test again is a good way to test ring sealing. This is called a wet test. You may have done this already, pretty standard compression test item. If the numbers go up then it means the rings may need some help, but also gives a better indication of what is happening during actual running of the engine as the rings will be grabbing oil and holding better.
The valves have to seal well. The valves themselves need to be "lapped" meaning their sealing surfaces must be smooth and perfectly shaped to the opening in the head, called the valve seat. The seat also must be perfectly shaped and smooth. Lapping involves spinning the valve in the seat with some lapping compound, this is done with considerable "up" force on the valve. The machine shop should have done this or similar during the build process. If this was not done well then the valves may not hold compression well at higher cylinder pressures (higher throttle settings and rpm).
A leakdown test can help determine where compression might be leaking. This test is done with the cylinder at the top of a compression stroke, compressed air is sent into the cylinder thru a fitting in the spark plug hole. Then you listen to the intake manifold, if hissing is heard the intake valve is leaking. Listen to the exhaust, if hissing then exhaust valve leaks. Listen to the crankcase, if hissing then the rings are not sealing or a piston is holed. (I do not think any of your pistons have a hole, that would show almost zero psi on the compression test.)
There are also bench test for heads that can show improperly seated valves.
Again, the valves must be lashed properly or they may not have a chance to close and seal well at higher rpms.
Weak valve springs can also cause a higher rpm valve sealing or bouncing problem.