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Hard or stiff diaphragm can cause issues and hard to find. I always put in a rebuild kit if I'm going to pull it apart. It can have a pin hole that you can't see unless you're in a dark room and use a flashlight from the back .

Well I figured out the issue…installed new pump today and had the same problem..got it to fire up and noticed fuel and smoke coming from the middle cylinder…the reed valves in middle cylinder are MIA 

That's not good!

So I have  05 merc 90hrs... I ran into a similar issue with mine... not sure if it was lund or an aftermarket hose but the lining deteriorated and fell apart Internally and plugged the fuel filter and pump..  I replaced the hose and cleaned out everything and haven't had an issue since.. the hose was Grey not black like actual fuel hose.. looks like u may have found ur problem tho, good luck!

check to see if the oil injection is working !  not going get many chances to fix before motor is toast