Mercury fuel issue (updated, with pics)

[fishdog4449]

First off, this is a great site and I have learned more about boats from the experts on this site than anyone else!


Ok, I am starting a new topic about this because it has me so baffled.
Engine is a 1980 merc 80
Memorial day weekend after about running EXCELLENT for 3 hours on lake anna I started noticing increasing hesitation when throttling up. after shutting the motor off so my kneeboarder could get back in the boat, it wouldn't start. tried various combinations of fast idle lever, choke, and pumping bulb. tried both gas tanks and reserve tank. engine would turn over, kick the bendix back down, and die. bulb was hard, but when pumped more, fuel was leaking from somewhere in the front of the engine.
got a tow in, about 15 minutes later, I gave it full fast idle, but no choke and it sputtered for a few seconds then revved up to normal fast idle speed. died when returned to normal idle.

Yesterday, I tried to start it on the muffs. Pumped the bulb HARD and no fuel leaks anywhere. Started within 1 second cold. Idled fine for 20 minutes before the neighbors got mad.

Here's what I know:
Compression 120PSI across all four.
Spark on all four cylinders.
Carbs rebuilt with new needles, seats, gaskets, and floats. floats set properly.
Water pump functioning well, engine running at normal temperature (checked as soon as motor started hesitating).
Gas was fresh.
Fuel pump rebuilt last fall.
Plugs are one year old, didn't look fouled.

thanks in advance for the help guys this one has me confuddled.

 

[DRIFTER_016]

Re: Mercury 80 phantom fuel issue

Re: Mercury 80 phantom fuel issue

Check to see that one of your floats isn't sticking. Sounds like one stuck and it flooded out.

 

[fishdog4449]

Re: Mercury 80 phantom fuel issue

Re: Mercury 80 phantom fuel issue

makes sense. how could that be fixed on the water if it should happen again? cuz it was fine in the driveway two days ago

 

[j_martin]

Re: Mercury 80 phantom fuel issue

Re: Mercury 80 phantom fuel issue

Best bet would be to take the carbs off and put a gasket kit in them. That way you get to take them all apart and clean and inspect everything. There are many items that could cause that, including a fuel logged foam float, junk in the needle valve, worn needle valve, etc.


hope it helps
john

 

[fishdog4449]

Re: Mercury 80 phantom fuel issue

Re: Mercury 80 phantom fuel issue

thanks john but thats exactly what i did a few days BEFORE this incident happened. disassembled carbs, soaked both carbs overnight and blew out jets, passages, etc with compressed air. Put in new gasket kits, new needles, new floats, adjusted the floats, new fuel lines. they were both very clean even before i soaked in cleaner. thats why it has me puzzled. the fact that the motor began hesitating a half hour or so before quitting on me makes me think it's an electrical problem despite the leaking fuel...but it did run for three or four hours before starting to screw up. all four plugs were wet with fuel which i have since been told indicates an overrich mixture which could explain why the motor never heats up..but my idle screws are set at 1-1/2 turns out i believe. i don't know where to go from here,....

 

[j_martin]

Re: Mercury 80 phantom fuel issue

Re: Mercury 80 phantom fuel issue

Wet plugs could be either over-rich or misfire. My next step would be to check the spark when the problem occurs.

Are you losing just a little gas out the front, or a lot? Can you get the cover off and find out where the gas is coming from?

It is possible for a float to stick if it touches the side of the tank.

hope it helps
John

 

[fishdog4449]

Re: Mercury 80 phantom fuel issue

Re: Mercury 80 phantom fuel issue

Well, mixture is set at 1-1/2 turns out. The engine runs great before said problem begins to happen 3-4 hours later, it does usually stall out when put in gear, but once in idle it runs fine. I will check the T-stat, someone else suggested that could be not letting the engine warm up so it won't burn all the fuel. I couldn't see any fuel coming out of the carbs anywhere when I ran it the other day on muffs, and I was squeezing the bulb pretty hard.

 

[j_martin]

Re: Mercury 80 phantom fuel issue

Re: Mercury 80 phantom fuel issue

Well, mixture is set at 1-1/2 turns out. The engine runs great before said problem begins to happen 3-4 hours later, it does usually stall out when put in gear, but once in idle it runs fine. I will check the T-stat, someone else suggested that could be not letting the engine warm up so it won't burn all the fuel. I couldn't see any fuel coming out of the carbs anywhere when I ran it the other day on muffs, and I was squeezing the bulb pretty hard.

You stated in your first post that one of the problems on the water was that fuel was leaking from the front of the engine. It would help to know the source of that leak.

A driveway test would be to pump it up pretty hard with the primer bulb, and while someone was holding that pressure, push the fuel lines under the cowl around to see if one of them leaks.

Even if a t-stat was stuck open, it would start as easily as in the morning.

hope it helps
John

 

[fishdog4449]

Re: Mercury 80 phantom fuel issue

Re: Mercury 80 phantom fuel issue

ok will try that soon as it stops storming.

 

[fishdog4449]

Re: Mercury 80 phantom fuel issue

Re: Mercury 80 phantom fuel issue

ok. tested it today. bulb held firm under pressure for about 5 pumps, then bottom carb started leaking gas out of the little tube that sticks up through the body into the throat AND the little jet in the front of the carb. sounds like float isn't set right but I set the ruddy thing last weekend?? only thing i don't understand is how would that make the engine start hesitating and get worse and worse and finally stop, wouldn't it flood out immediatly?
top carby was fine.
still wondering about engine temp too

 

[fishdog4449]

Re: Mercury 80 phantom fuel issue

Re: Mercury 80 phantom fuel issue

well i adjusted the float and carb no longer leaks. pressurized bulb and moved hoses around, no leaks whatsoever. tested spark and spark on 1 and 2 jump little less than 1/2 inch intermittently, number 4 jumps 1/2inch at least, 3 intermittently makes a little less than 1/2. i put new wires on number four a while ago and i will repeat that for 1,2,3 and see if that makes a difference, also will run at night to watch for arcs. started up the motor and ran it for a bout 10 minutes. took some pics of the plugs if one of you experts can read something out of these that woudl be gReat! obviously number 1 looks bad (cylinder numbers down from right to left)

 

[j_martin]

Re: Mercury fuel issue (updated, with pics)

Where is the fuel pump on that engine? If it gets its crankcase pulse from #1, likely the diaphragm in the fuel pump is bad, flooding out #1.

hope it helps
John

 

[fishdog4449]

Re: Mercury fuel issue (updated, with pics)

thanks john
it is situated in between 1 and 2, closer to 2. I will disassemble and inspect diaphragm tomorrow

 

[Toon]

Re: Mercury fuel issue (updated, with pics)

plug 3 has water on it where did that come from

 

[j_martin]

Re: Mercury fuel issue (updated, with pics)

plug 3 has water on it where did that come from

It seems too clean also. Could be a little water getting into the cylinder.

John

 

[fishdog4449]

Re: Mercury fuel issue (updated, with pics)

where would water be getting in with 120psi across all four? there should be atleast a little difference if theres a weak gasket area right?

 

[j_martin]

Re: Mercury fuel issue (updated, with pics)

where would water be getting in with 120psi across all four? there should be atleast a little difference if theres a weak gasket area right?

Could be getting into the exhaust chest and spraying into the cylinder through the exhaust port. Look in the hole. If the piston is squeaky clean, you have a problem.

It's not the main issue, and it usually has no symptoms, but can cause harm over time.

If you just fired up the engine, and shut it down quickly, it could be just combustion gasses (mostly water) condensed.

hope it helps
John

 

[fishdog4449]

Re: Mercury fuel issue (updated, with pics)

Could be getting into the exhaust chest and spraying into the cylinder through the exhaust port. Look in the hole. If the piston is squeaky clean, you have a problem.

It's not the main issue, and it usually has no symptoms, but can cause harm over time.

If you just fired up the engine, and shut it down quickly, it could be just combustion gasses (mostly water) condensed.

hope it helps
John

I will check that john thanks!
Had another thought, when I re-adjusted the float on the bottom carb, the little piece of metal that is pushed up by the float and in turn pushes the needle up, seemed to bend VERY easily, seemed almost brittle. Is it possible that after I run the motor for 3 or 4 hours, the constant upward pressure from the float causes that piece of metal to "set", so it no longer puts pressure on the needle?
Thanks!!

 

[j_martin]

Re: Mercury fuel issue (updated, with pics)

I will check that john thanks!
Had another thought, when I re-adjusted the float on the bottom carb, the little piece of metal that is pushed up by the float and in turn pushes the needle up, seemed to bend VERY easily, seemed almost brittle. Is it possible that after I run the motor for 3 or 4 hours, the constant upward pressure from the float causes that piece of metal to "set", so it no longer puts pressure on the needle?
Thanks!!

It could be fatigued. If it seems noticably weaker than the other one, I sure would replace the float assy.

The flooding problem, however, appears to be occuring in the top cylinder, not the bottom one.

hope it helps
John

 

[fishdog4449]

Re: Mercury fuel issue (updated, with pics)

Ok I will replace that before I go out next time.
Right about the top cylinder, only reason i focused on the bottom is becuase thats where the leaking problem was occuring. Also, these are year old plugs and the motor was lugged all last year because we had the wrong prop on she probably toped out at 4500 maybe. Depending on how the fuel diaprhagm looks, I will put new plugs in and go for a run then look at the new plugs.