115 Mercury

[TLMoore]

I have a 1978 6 cyl 115 merc that has started giving me problems at top end. It starts, idels and comes out of the hole great. The promlem is after you plane out, after about 15 seconds, it starts to loose power. If you pump the bulb at this point, it will recover a bit but not completely.
I replaced the pump diaphram but it did not help.

 

[Inline6]

Re: 115 Mercury

When you get up on a plane the fuel demand is greater and that is where your problem is. Since you replaced the fuel pump diaphram look elsewhere.

Look for some kind of restriction in the fuel supply - or it may be the vent to the fuel tank is closed off or clogged shut depending on what you have. It may be the pick-up tube in the tank is clogged. Make sure there are no leaks in the fuel line - it may be sucking air.

When "all else fails" look back over the work you have already done (the fuel pump) and make sure that work was done correctly.

Good Luck,
Inline6

 

[cell]

Re: 115 Mercury

pump diaphram

I know...but I got to ask.


Did you replace both of em?

 

[Chris1956]

Re: 115 Mercury

Cell, the later 70s inline sixes have a single fuel pump.

 

[cell]

Re: 115 Mercury

Cell, the later 70s inline sixes have a single fuel pump.

really?well thats the problem..............lol


Maybe go through the carbs.They still have three of em right?Check the compression.

 

[hkeiner]

Re: 115 Mercury

The Mercury Service Advisory in the below linked thread may help you find the problem. All you need is a vacuum gauge, a T connector, and some clear hose. It may seem like unnecessary effort to use a vacuum gauge to troubleshoot, but it is a pretty sure way to confirm what is the the problem and elimate what is not the problem. Just my experience...


http://forums.iboats.com/attachment.php?attachmentid=26607&d=1240244928

http://forums.iboats.com/attachment.php?attachmentid=26608&d=1240244937

http://forums.iboats.com/attachment.php?attachmentid=26609&d=1240244945

http://forums.iboats.com/attachment.php?attachmentid=26610&d=1240245079

 

[TLMoore]

Re: 115 Mercury

Thanks for all the reply, I have pulled the fill hose to the tank and inserted the fuel line and had the same results. THis should eliminate the tank vent and potential cloggged tube.
I'll try the vacuum test.
Have thought about bypassing the fuel pump with a 12VDC fuel pump and see if I experience the same problem. Anyone know what the max pressure i should be concerned with?

 

[TLMoore]

Re: 115 Mercury

I don't see the link, Please resend.
This sounds like a test that I should perform.
Thanks!

 

[cell]

Re: 115 Mercury

http://forums.iboats.com/showthread.php?t=283363



Whens the last time you decarbed the engine?

 

[hkeiner]

Re: 115 Mercury

I don't see the link, Please resend.
This sounds like a test that I should perform.
Thanks!

I updated my above post to include the links.

Have thought about bypassing the fuel pump with a 12VDC fuel pump and see if I experience the same problem. Anyone know what the max pressure i should be concerned with?

You should be able to troubleshoot the problem using the Service Advisory vacuum gauge tests without having to hook up an electric fuel pump. If your want to directly test the PSI output of the mechanical fuel pump, you can hook up a pressure gauge between the fuel pump and the carbs to see if the pressure reaches specs. The shop manual for your motor should provide the PSI specs at idle and WOT. Below is a link to the specs for my 1996 150hp motor, to give you an idea.

If you do use an electric fuel pump to troubleshoot, the PSI output should not exceed the specs for your motor.

http://forums.iboats.com/attachment.php?attachmentid=29405&d=1243750241

 

[cell]

Re: 115 Mercury

Just wondering........will that service bulletin apply to a '78 merc?

 

[hkeiner]

Re: 115 Mercury

I don't see why not, as long as the fuel pressure specs for the '78 are not significantly different than those of the later model motors. For reference, the fuel pressure specs for my 1996 150hp motor are 3 1/2 psi at idle and 6 psi at WOT. However, someone with direct experience working with fuel pumps from that era would be a more reliable source for an answer to your question.

Testing the PSI on the downstream side of the fuel pump with a pressure gauge would certainly be a valid test regardless of the motor's year. You only need to obtain the specs for that motor.