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Author Topic: 91 merc problem  (Read 9149 times)

Mac Attack

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Re: 91 merc problem
« Reply #15 on: Jul 25, 2018, 06:21 AM »
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

What you posted here (the link) is correct.
Except, as I posted below, we are not speaking about diesel fuel (or alcohol).
Stoichiometry for natural gas and gasoline is closer to 10:1, not 14:1.

Jethro

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Re: 91 merc problem
« Reply #16 on: Jul 25, 2018, 08:12 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??

While I agree it sounds like it could very well be an electrical gremlin, it would be rare to have a coil with an intermittent problem like that. They tend to either work or not. Anything is possible though.

stguy

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Re: 91 merc problem
« Reply #17 on: Jul 25, 2018, 08:23 AM »
This one worked great until I hit 4100 rpm, and then not so much, sometimes

You are correct, usually they either work or they don't.


Roccus

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Re: 91 merc problem
« Reply #18 on: Jul 25, 2018, 08:43 AM »
the 2+2 engine is a simple but at the same time complex engine... the actual "kick in" of these is caused by fuel being metered by the second progression hole in the lower 2 carburetors.. controlled by the carburetor linkage.. this is why set up is so crucial on these motors... an accelerator pump  with the aid of 2 check valves helps  that resemble needle and seats more than check valves, Controls acceleration. during idle.. only the top 2 carburetors have air/fuel mixture screws... the lower 2 receive a token amount of fuel but lacking air cannot fire , this lubricates the lower cylinders and the fuel is scavenged and burned by the upper 2 cylinders via a complex network of check valves and bleed hoses ... once the throttle plates are opened the accelerator pump takes over for the temporary lean condition and the carburators then meter fuel via  the main jet like a normal carb....

Now for your problem...most likely you have a bad coil... it's very common on those engines... a stator or trigger is also a strong possibility but I'd place money on the coil.

stguy

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Re: 91 merc problem
« Reply #19 on: Jul 25, 2018, 09:01 AM »
the 2+2 engine is a simple but at the same time complex engine... the actual "kick in" of these is caused by fuel being metered by the second progression hole in the lower 2 carburetors.. controlled by the carburetor linkage.. this is why set up is so crucial on these motors... an accelerator pump  with the aid of 2 check valves helps  that resemble needle and seats more than check valves, Controls acceleration. during idle.. only the top 2 carburetors have air/fuel mixture screws... the lower 2 receive a token amount of fuel but lacking air cannot fire , this lubricates the lower cylinders and the fuel is scavenged and burned by the upper 2 cylinders via a complex network of check valves and bleed hoses ... once the throttle plates are opened the accelerator pump takes over for the temporary lean condition and the carburators then meter fuel via  the main jet like a normal carb....

Now for your problem...most likely you have a bad coil... it's very common on those engines... a stator or trigger is also a strong possibility but I'd place money on the coil.

Do you know anything about props?

Turnbuckle

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Re: 91 merc problem
« Reply #20 on: Jul 25, 2018, 09:38 AM »
Do you know anything about props?

Don’t hit a rock with them.
At the mouth of the river at Sebago in the Spring, it's East to West unless you want a beating from the rest! ;D ;D

stguy

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Re: 91 merc problem
« Reply #21 on: Jul 25, 2018, 09:48 AM »
Don’t hit a rock with them.

Only done that once, talking on the phone while leaving the basin and kept the red markers on my right, whoops!

SHaRPS

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Re: 91 merc problem
« Reply #22 on: Jul 25, 2018, 09:53 AM »
Only done that once, talking on the phone while leaving the basin and kept the red markers on my right, whoops!

Red right RETURN. lol

No more talking on the phone while driving a vehicle or boat here in RI.
Wicked Wec

Roccus

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Re: 91 merc problem
« Reply #23 on: Jul 25, 2018, 11:10 AM »
Do you know anything about props?
.. yes..more than the average boat owner..and then some...

I may have a set of coils and a switch box for that 91 merc... I had a 95-115 2+2 that lost a fly wheel and ate the stator and trigger...it was cheaper to buy a used engine than buy a flywheel and ignition system because the flywheel was obsolete and it would require the entire ignition system to be changed.. I stripped the carbs and electrical system off of the motor thinking it was going to fit my 2004 115- 2+2( spare parts not knowing they would not work)... but they changed the electrical system a bit by eliminating the switch box and went to straight CD cols power packs...I'll look and see if I still have the coils and switch box...I also may have a 17 and 19P aluminum prop...I've given some stuff away ( helped a few others in your predicament)..not sure whats left from that motor.. what I do have can be had for the cost of shipping or you can come and get it for free.

stguy

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Re: 91 merc problem
« Reply #24 on: Jul 25, 2018, 11:22 AM »
.. yes..more than the average boat owner..and then some...

I may have a set of coils and a switch box for that 91 merc... I had a 95-115 2+2 that lost a fly wheel and ate the stator and trigger...it was cheaper to buy a used engine than buy a flywheel and ignition system because the flywheel was obsolete and it would require the entire ignition system to be changed.. I stripped the carbs and electrical system off of the motor thinking it was going to fit my 2004 115- 2+2( spare parts not knowing they would not work)... but they changed the electrical system a bit by eliminating the switch box and went to straight CD cols power packs...I'll look and see if I still have the coils and switch box...I also may have a 17 and 19P aluminum prop...I've given some stuff away ( helped a few others in your predicament)..not sure whats left from that motor.. what I do have can be had for the cost of shipping or you can come and get it for free.


I'll shoot you a pm about my prop question if you don't mind??

Turnbuckle

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Re: 91 merc problem
« Reply #25 on: Jul 25, 2018, 11:36 AM »
Only done that once, talking on the phone while leaving the basin and kept the red markers on my right, whoops!

Did it once myself. First year I had my boat down here about 10 years ago. First time going out on Sebago through the gut after ice out from Jordan’s. One of the don’t go between here and shore before the ferry cove was gone. Maybe ice took it. I found the rock at about 25 mph.
At the mouth of the river at Sebago in the Spring, it's East to West unless you want a beating from the rest! ;D ;D

Roccus

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Re: 91 merc problem
« Reply #26 on: Jul 25, 2018, 11:54 AM »

I'll shoot you a pm about my prop question if you don't mind??
works for me..

fishlessman

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Re: 91 merc problem
« Reply #27 on: Jul 25, 2018, 12:18 PM »
the 2+2 engine is a simple but at the same time complex engine... the actual "kick in" of these is caused by fuel being metered by the second progression hole in the lower 2 carburetors.. controlled by the carburetor linkage.. this is why set up is so crucial on these motors... an accelerator pump  with the aid of 2 check valves helps  that resemble needle and seats more than check valves, Controls acceleration. during idle.. only the top 2 carburetors have air/fuel mixture screws... the lower 2 receive a token amount of fuel but lacking air cannot fire , this lubricates the lower cylinders and the fuel is scavenged and burned by the upper 2 cylinders via a complex network of check valves and bleed hoses ... once the throttle plates are opened the accelerator pump takes over for the temporary lean condition and the carburators then meter fuel via  the main jet like a normal carb....

Now for your problem...most likely you have a bad coil... it's very common on those engines... a stator or trigger is also a strong possibility but I'd place money on the coil.

i replaced the stator a few years ago. its acting differently then before when it went.  now with the coils, if i flip the 2 uppers with the lower im thinking it would act differently than it is now if one of the lower coils is bad with this motor?  going to run down the gas lines first for problems but that seems to be the next logical step.  probably a job for the late fall as i dont see taking it out of the water til then.  if its a coil it has to be one of the lower two the way its running

Jethro

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Re: 91 merc problem
« Reply #28 on: Jul 25, 2018, 12:51 PM »
No more talking on the phone while driving a vehicle or boat here in RI.

Got spoken to last year here in NH by Marine Patrol while on Winnisquam travelling at headway speed. Very lucky I avoided a ticket.

boondox

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Re: 91 merc problem
« Reply #29 on: Jul 25, 2018, 07:38 PM »

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.


Seriously mac you better do more research..

 



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