2005 2 stroke Mercury Saltwater 150hp hard starting makes humming sound

[Proline05]

I am original owner of 2005 2 stroke Mercury Saltwater 150hp that has 300 hours on it now. Ever since I got it, it's been extremely hard to start. I make sure the primer bulb is firm, turn the key, choke, put throttle forward half way and the engine cranks fine. After several attempts to start, the engine will stop cranking while the key is turned and it makes a humming sound until I turn the key back to off. This will happen a few times and then the engine will start. I can immediately turn the the key off and the engine will start with no problem. Anyone have this problem and what is the solution? Thanks.

[Silvertip]

1) Weak battery
2) Loose or corroded battery cables (disconnect, clean and reconnect cables -- both ends.)
3) Solenoid going bad
4) Starter beginning to fail (but listed last because of low probability)

[Proline05]

1) Weak battery
2) Loose or corroded battery cables (disconnect, clean and reconnect cables -- both ends.)
3) Solenoid going bad
4) Starter beginning to fail (but listed last because of low probability)

Thanks Silvertip. That's a good starting place. I replaced one of the deep cycle batteries a few months ago with a cranking battery and the flywheel spins great so I don't think it's the battery. I'll check the cables and solenoid soon as work doesn't interfere with my boating. Hopefully it's just corroded and I won't need to get new solenoid or starter, hard to get parts here in Hawaii.

[Silvertip]

But you said it stops cranking after a bit and begins to hum. That says weak battery. After a couple minutes rest, a battery can recover to the point where it is again able start the engine. You also said you replace one of the deep cycle bateries with starting battery. Did you mean you had a deep cycle battery that was used to crank the engine and you replaced that with a starting battery?Approach this logically and don't start throwing parts at the motor hoping to get lucky. Have the starting battery load tested at any auto store (charge it before you do this). If that checks ok, then go for the cable cleaning. Then try jumping around the solenoid using a a jumper cable. If none of this works, the starter is suspect.

[Proline05]

But you said it stops cranking after a bit and begins to hum. That says weak battery. After a couple minutes rest, a battery can recover to the point where it is again able start the engine. You also said you replace one of the deep cycle bateries with starting battery. Did you mean you had a deep cycle battery that was used to crank the engine and you replaced that with a starting battery?Approach this logically and don't start throwing parts at the motor hoping to get lucky. Have the starting battery load tested at any auto store (charge it before you do this). If that checks ok, then go for the cable cleaning. Then try jumping around the solenoid using a a jumper cable. If none of this works, the starter is suspect.

That's correct, I changed out one of the deep cycles for the starting battery but I still turn the battery selector to all when I try to start it. Had the same problem before I got the starting battery. What I find most odd is once the engine starts I can turn the engine off and it will immediately start, or I can anchor for hours and it will start. Usually the boat is used on the weekends