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Old 10-10-2014, 03:46   #1
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Mercury 15hp 2 cycle

Today I am going to replace the prime pump diaphragm, again. I can't figure out why this is failing as often as it is....every 2 months, or less. I guess it could be the Ethanol in EU gasoline???

I use starbrite's enzyme fuel treatment for gasoline, but this started before using it and has continued after use.

Anyone else experienced a problem with these priming diaphragms?

Bill
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Old 10-10-2014, 05:00   #2
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Re: Mercury 15hp 2 cycle

I have a Mariner 25, had to replace the pump diaphragm once, but nothing like the problem your having.
Is it tearing?
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Old 10-10-2014, 05:13   #3
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Re: Mercury 15hp 2 cycle

No, not tearing, but once I noticed that the neoprene coating on the diaphragm seemed to be disolved...but the other times, it looked OK.

Bill
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Old 10-10-2014, 05:15   #4
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Re: Mercury 15hp 2 cycle

I have a 1997 merc 2 stroke. Used it extensively. I've put in 2 carb rebuild kits since new. Been a great motor. All I could think of would be the fuel, unless you are not using genuine parts?

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Old 10-10-2014, 05:37   #5
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Re: Mercury 15hp 2 cycle

I have a friend over in Hampton Va. that that used to run a little crab boat with the Merc 15. He was at wits end with the same problem.

I suggested that he thoroughly clean or replace the check valves because if they are sticking .........the diaphragm will either lock up and tear, or it will be actuating fine with no pressure.

Check........... the check valves thoroughly.
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Old 10-10-2014, 06:39   #6
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Re: Mercury 15hp 2 cycle

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Originally Posted by svBeBe View Post
No, not tearing, but once I noticed that the neoprene coating on the diaphragm seemed to be disolved...but the other times, it looked OK.

Bill
What is the failure mode? I assume fuel leaking under the engine cowl?
OldSnipe has an excellent point if a valve sticks the pump is just moving fuel back and forth, but no pumping
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Old 10-10-2014, 08:27   #7
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Re: Mercury 15hp 2 cycle

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What is the failure mode? I assume fuel leaking under the engine cowl?
OldSnipe has an excellent point if a valve sticks the pump is just moving fuel back and forth, but no pumping
Thanks OldSnipe and a64pilot.

The failure mode is that it will not start unless I use a syringe to manually squirt some fuel into the throat of the carb.

I have cleaned and/or replaced the primer check valve every time I have cleaned or put a carb kit in the carb. The primer check valve in this case is a tiny steel ball which is held in place with a spring. I am not sure, but possibly some debris was blocking the ball from closing. I have not been in the investigation mode while cleaning the carb...only in cleaning mode.

I will check it closely this time before I start cleaning and replacing. At least one time, the problem was a partially dissolved diaphragm...that was easy to see when replacing.

Bill
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Old 10-10-2014, 12:02   #8
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Re: Mercury 15hp 2 cycle

i had so many problems with my 98, 15hp 2 cycle merc, i sold it for a good price, and bought an 04, 9.9 yamerha 2 cycle off craigs! starts every time, runs like a swiss watch, and the oil mixes at 100/1...very little smoke! my merc had the damn shift on the tiller, and they can put that where the sun don`t shine!...clyde
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Old 13-10-2014, 01:36   #9
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Re: Mercury 15hp 2 cycle

The check valves were fine. The port for prime fuel that goes through the bowel assembly was clear and clean, but the port that it connects to that goes through the carburetor housing was clogged with a one-way clog. This was the darnedest thing I have ever experienced. Compressed air would blow through one way, but not the other. It took time and lots of pressure to get that ethanol reverted ball of corn out. It works great now. I cleaned and rebuilt my spare. I found the exact same issue with it, but could not free the clog. I changed the carb body with a used one...it is fine now.

Bill
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Old 13-10-2014, 02:46   #10
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Re: Mercury 15hp 2 cycle

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The check valves were fine. The port for prime fuel that goes through the bowel assembly was clear and clean, but the port that it connects to that goes through the carburetor housing was clogged with a one-way clog. This was the darnedest thing I have ever experienced. Compressed air would blow through one way, but not the other. It took time and lots of pressure to get that ethanol reverted ball of corn out. It works great now. I cleaned and rebuilt my spare. I found the exact same issue with it, but could not free the clog. I changed the carb body with a used one...it is fine now.

Bill
Glad you found the problem. When you get some free time, soak the other body again and work a smokers pipe cleaner into the passage/s.

Here is a link to a great forum. I use it quite often when I am tinkering around with outboards. There are a lot of guys on there that are a walking library of knowledge.

Boat Repair Forum

V/R...... John
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Old 13-10-2014, 03:36   #11
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Re: Mercury 15hp 2 cycle

Thanks, and thanks to everyone who responded.

Bill
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