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Author Topic: 91 merc problem  (Read 9148 times)

fishlessman

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91 merc problem
« on: Jul 24, 2018, 07:52 AM »
1991 115 merc, i believe its a 2+2.  i get it up on plane at 4000 rpm, then it cuts out to about 2400 rpm, then slams back to 4000 again every minute or two.   what operates the 2+2 function on the carbs. some times i have full power for a half hour then this starts, sometimes it starts this way first thing.  tough to bring it to a shop where you cant show them whats happening

stguy

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Re: 91 merc problem
« Reply #1 on: Jul 24, 2018, 08:03 AM »
Start with your anti- siphon valve coming out of your fuel tank then check all of the fuel lines all the way to the carbs. The ethonal gas rots your fuel lines from the inside and sometimes a piece will act like a valve or if a line is making a  bend when your asking for more fuel the suction will collapse the line.

I was out in your area in a friends boat a few weeks ago and he started having the same issue, changed out his fuel lines and the problem went away.

SHaRPS

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Re: 91 merc problem
« Reply #2 on: Jul 24, 2018, 08:14 AM »
Start with your anti- siphon valve coming out of your fuel tank then check all of the fuel lines all the way to the carbs. The ethonal gas rots your fuel lines from the inside and sometimes a piece will act like a valve or if a line is making a  bend when your asking for more fuel the suction will collapse the line.

I was out in your area in a friends boat a few weeks ago and he started having the same issue, changed out his fuel lines and the problem went away.

I am with stguy.

I had a very similar issue and come to find out my fuel line rotted and it was clogging my fuel from getting to my engine. They were tiny little clear plastic particles.
Wicked Wec

fishlessman

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Re: 91 merc problem
« Reply #3 on: Jul 24, 2018, 08:19 AM »
ive had that problem two years ago, it was between the tank and ball and would collapse the ball. i did not change the line between the ball and carbs though, easy to change that section.  this seems more like the power transition between running on 2 cylinders to when it switches to 4 as the rpms seem to drop and raise consistantly.  if changing the line doesnt work, i have a second tank to try and run it with.

fishlessman

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Re: 91 merc problem
« Reply #4 on: Jul 24, 2018, 08:24 AM »
I am with stguy.

I had a very similar issue and come to find out my fuel line rotted and it was clogging my fuel from getting to my engine. They were tiny little clear plastic particles.

theres a very thin clear liner in the newer hoses that can de-laminate inside the hose, really hate ethynol.  going to try the other tank i run the kicker on with the bigger motor, that should give me a better idea with regard to the lines

stguy

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Re: 91 merc problem
« Reply #5 on: Jul 24, 2018, 08:30 AM »

If it's not a fuel issue, try moving all the wires around and see if you can make it happen, could be a loose connection??

It also sounds like it could be a coil breaking down??

boondox

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Re: 91 merc problem
« Reply #6 on: Jul 24, 2018, 03:41 PM »
If it's not fuel or plug wire it may be carbon fouled??  As in the passages to the other cylinders on the down stroke it pulls fuel in  on up stroke it fires . Like a two stroke.. try running a good fuel treat ment like sea foam if this is the case.. try last if not fuel hose or filter?  or wires like stguy was saying..

boondox

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Re: 91 merc problem
« Reply #7 on: Jul 24, 2018, 06:17 PM »
If you get as far as the sea foam and it works maybe replacing the spark plugs and wires on all cylinders.. this the cause of carbon fouling usally.. incomplete combustion not stoich as in stokyo metric efficiency  like a 14:1 ratio of combustion.. even ignition timing being retarded can cause fouling... or over oiling ?? As in to rich of fuel.. that's if the sea foam works?? Always look at the spark plugs and look at charts and compare what's going...if wondering....

Mac Attack

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Re: 91 merc problem
« Reply #8 on: Jul 24, 2018, 06:27 PM »
Start with your anti- siphon valve coming out of your fuel tank then check all of the fuel lines all the way to the carbs. The ethonal gas rots your fuel lines from the inside and sometimes a piece will act like a valve or if a line is making a  bend when your asking for more fuel the suction will collapse the line.

I was out in your area in a friends boat a few weeks ago and he started having the same issue, changed out his fuel lines and the problem went away.

Did you own an ST1100
Or ST1300

Asking because of your name here.

Thanks

Mac Attack

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Re: 91 merc problem
« Reply #9 on: Jul 24, 2018, 06:31 PM »
If you get as far as the sea foam and it works maybe replacing the spark plugs and wires on all cylinders.. this the cause of carbon fouling usally.. incomplete combustion not stoich as in stokyo metric efficiency  like a 14:1 ratio of combustion.. even ignition timing being retarded can cause fouling... or over oiling ?? As in to rich of fuel.. that's if the sea foam works?? Always look at the spark plugs and look at charts and compare what's going...if wondering....

Stoichiometry is 10:1 for naturally aspirated combustion.
14:1 is excess air and you’ll burn up pistons with the flame temps being too hot.

stguy

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Re: 91 merc problem
« Reply #10 on: Jul 24, 2018, 06:50 PM »
Did you own an ST1100
Or ST1300

Asking because of your name here.

Nope...had cb, kz, fj, gl, yz, rm, it, and a few others but never an st, they are cool looking bikes though.

Thanks

Mac Attack

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Re: 91 merc problem
« Reply #11 on: Jul 24, 2018, 07:32 PM »
Nope...had cb, kz, fj, gl, yz, rm, it, and a few others but never an st, they are cool looking bikes though.


mine on the right in 1996
My dad on the yellow BMW.
He died on it 3 yrs later.
My sone Jeff on the left.
He's 33 now.
My son chris on the right.
He's 31 now.




boondox

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Re: 91 merc problem
« Reply #12 on: Jul 24, 2018, 09:27 PM »
Stoichiometry is 10:1 for naturally aspirated combustion.
14:1 is excess air and you’ll burn up pistons with the flame temps being too hot.

Rethink that it's really 14.7:1

As seen here..

https://www.google.com/search?ie=UTF-8&client=ms-android-att-us&source=android-browser&q=air+fuel+ratio




boondox

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Re: 91 merc problem
« Reply #13 on: Jul 24, 2018, 09:40 PM »
Stoichiometry is 10:1 for naturally aspirated combustion.
14:1 is excess air and you’ll burn up pistons with the flame temps being too hot.


Compression ratio is 10:1 on gas motor but can be as high as 14:1 for diesel motors

Mac Attack

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Re: 91 merc problem
« Reply #14 on: Jul 25, 2018, 06:19 AM »

Compression ratio is 10:1 on gas motor but can be as high as 14:1 for diesel motors


1st - you were speaking about gas to fuel ratio, or Stoichiometry.  Stoichiometry for a naturally aspirated combustion engine is 10:1.  Not 14:1
2nd - it's been a long time since I've seen a diesel outboard
3rd, and lastly, I said that a 14:1 air/fuel ratio would be running excess air, or lean.  A lean burn produces a hotter flame and CAN burn up pistons.  Further, that is, unless things have been designed to handle a lean fuel ratio you don't want mess with what the motor was designed for unless you know what you are doing.  Things like running higher octane fuel to slow down the burn rate and/or using higher compression ratios, and advancing/retarding the ignition timing to avoid detonation are all options when playing with air/fuel ratios.  But you had better know what you are doing.


But, again, you were attempting to speak about Stoichiometry in your original post Boon and I simply corrected and/or clarified what you posted.

 



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