Re: Mercury 25hp 2stroke
Yes, it can be many things in the fuel system leading to the carb.
The tank - vent could be closed/plugged, pick-up could be clogged, fitting could be faulty.
Hose to the motor - air leak, collapsed, poor fitting, bad primer bulb, bad connector at the motor.
At the motor - bad/collapsed fuel line, clogged fuel filter, clogged fuel pump, cracked/torn fuel pump diaphram, bad/collapsed line from fuel pump to the carb.
Carb - dirty or clogged, damaged jet, perforated float.
Start at the tank and move forward. When you have exhausted all other possibilities, pull, disassemble, clean and rebuild the carb...
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)