Mercury 25 hp Bigfoot Fuel issue

[trout793]

Hi All, First post, but been lurker for a few months and found eveything I need with search function so far.

I'm having a hardtime with my engine, maybe somebody can help me figure out what I'm missing or didn't check. I just bought this boat last fall and only took it out once, it was staving for fuel.

The motor starts up just find and idles great. I give it some throttle and it starts to sputter and stall. I can feather the throttle, and it will almost get to 3/4 throttle before it stalls. If I WOT from idle it stalls. The motor will start right back up everytime after it stalls. If i put my hand over the air intake and give it throttle, it runs better and can almost get it to WOT.

Thanks to older posts here, I was able to check and replace if needed the following.

replace fuel tank (put fresh gas in)
replace fuel line / bulb from tank to motor.
replace fuel filter
replaced the fuel pump (brand new, not rebuilt)
replace spark plugs
REBUILD CARB 4 times with rebuild kit. The final time i rebuilt it, i let it soak in Sea foam overnight, sprayed it with carb cleaner and blew out with air. I made certain every hole and passage was free. I really, really took my time, the high speed jet is clear and so are the other jets. I also put a new boot over accelerator pump(old one had a tear) and new spring.

I did a compression test on both cylinders. 110-115 on both.

The motor specs are:
1999 Mercury 25 hp Big foot 4 -stroke Serial number #0G735589 model 1-f25311ud

I'm at a lost, I don't know what else to try. It seems like its still starving for fuel, since if I put my hand over intake it runs better for a bit and can rev it up (wot). I did not check electrical.
Thank you,

 

[trout793]

Re: Mercury 25 hp Bigfoot Fuel issue

I forgot to add that I tested the electric choke per the manual with a ohm meter. It was within the certain ohm range. I also hooked it up to a 12 volt battery and the needle moved after a few mins. It seems find.

 

[trout793]

Re: Mercury 25 hp Bigfoot Fuel issue

Does anybody think its still the carb? Somebody told me crud may still be in it behind the brass ports. But how do you remove those and be able to put it back together?

 

[Vehnis]

Re: Mercury 25 hp Bigfoot Fuel issue

I got same problems with my 1998 f 25 Bigfoot too...
Could it be CDI - box?? (cost a lot on money here in Finland)
Do you know is there oil pressure alarm system in this model that could put motor to idle automatically if there is not enough oil pressure???
I have fight with this problem since last summer......

 

[matd01]

Re: Mercury 25 hp Bigfoot Fuel issue

I also got similar problem with my 2006 four strokes 25 HP Bigfoot. It stalls after running for about 20 minutes. I replaced the spark plugs, fuel pump, fuel tank, fuel filter. Since the problem seemed to be linked to overheat, I replaced also cooling water impeller, tested the cooling water thermostat. Finally I replaced the propeller with one with greater pitch to reduce the max RPM of the engine from 6100 to 5800. Problem seems to be less frequent but occurs again this summer. I suspect it is a kind of fuel starvation due to heat causing fuel to vaporize.

"The pump creates negative pressure to draw the fuel through the lines. However, the low pressure between the pump and the fuel tank, in combination with heat from the engine and/or hot weather, can cause the fuel to vaporize in the supply line. This results in fuel starvation as the fuel pump, designed to pump liquid, not vapor, is unable to suck more fuel to the engine, causing the engine to stall. " http://en.wikipedia.org/wiki/Fuel_pump"

Since we frequently use ethanol fuel blend E10 in most states of USA and province of Canada, it does not help with the kind of fuel pump working with negative pressure.
"low-blend levels, ethanol increases the Reid vapor pressure (RVP) of gasoline (making it evaporate more easily)" http://www.afdc.energy.gov/afdc/vehicles/emissions_e10.html

It is only assumptions...

Also, I noticed that the fuel tank, when vent cap is closed, rarely pressurize contrary to my previous outboard (Johnson and Suzuki). I suspect that air could penetrate in the fuel line at the connection of fuel hose to the engine.

I will try to insulate fuel line with a kind of insulating tape sold for airplane engine. I plan to remove the fuel hose connectors. We will see what happen...

 

[Vehnis]

Re: Mercury 25 hp Bigfoot Fuel issue

Hey !

Mine F 25 Fourstroke Bigfoot is now like new one... Serviceman changed float valve and float, and attach one of the jets that was loosen. (I think that loosen small jet was attached by pressing straight to the carburator column)
This cause casoline surface to drop about 4 mm, wich cause problems in full throttle.

 

[floatmiboat]

Re: Mercury 25 hp Bigfoot Fuel issue

My advice is, if you have been throwing that much time and money into it and still having problems, find a mechanic who really knows that motor and pay them to have a look at it. My experience has been that a good mechanic will go through it methodically and find the problem quickly, which often turns out to be something simple. Not to discourage DIY repairs but, sometimes there's a limit. Good luck

 

[jayislandR]

Re: Mercury 25 hp Bigfoot Fuel issue

I have a 99 25 Bigfoot. Problem I'm experiencing is at 3/4 open throttle it will run for about a minute and suddenly cut out. No alarm. Just shuts down quickly. Able to restart it no problem. Tell tale is strong and water is running through it cool. Just had the carb cleaned by mechanic. Starts with no hesitation as always, idles smooth and accelerates flawlessly UNTIL that 3/4 wide open mark. Before I take it in to the mechanic again anyone have any idea what I'm up against here? I have my own ideas but lets face it, i'm not mechanic either.

 

[1bemlr]

Re: Mercury 25 hp Bigfoot Fuel issue

I have a 99 25 Bigfoot. Problem I'm experiencing is at 3/4 open throttle it will run for about a minute and suddenly cut out. No alarm. Just shuts down quickly. Able to restart it no problem. Tell tale is strong and water is running through it cool. Just had the carb cleaned by mechanic. Starts with no hesitation as always, idles smooth and accelerates flawlessly UNTIL that 3/4 wide open mark. Before I take it in to the mechanic again anyone have any idea what I'm up against here? I have my own ideas but lets face it, i'm not mechanic either.

I have a 2005 25hp 4-stroke Big Foot. Would run at WOT for about 20 seconds then cut out. If I primed the bulb before it died it would pick back up and run for another 20 seconds then cut out. At idle it would run much longer but eventually it would cut out as well. Plugs, fuel filter, and fuel were all replaced but changed nothing. Took it to my Marine shop. Pulling off the fuel pump assy they found the pin for plunger was missing causing the fuel pump to not work correctly. I am pretty sure this was the original pump as I purchased the boat and motor, both 2005, in 2009. Motor looked brand new and showed little wear. Mechanic indicated the new pump was re-designed. Wonder why ?? Anyway, they ran the motor for 30 minutes after installing the new pump with no problems. The pin was never found (A check under the valve cover and camera checked in cylinders) and mechanic thoughts are it was probably in the oil tank and may work its way out the drain. Just got it back today and most likely won't get it out for a few weeks. Will know more then....

 

[Ballymac]

Re: Mercury 25 hp Bigfoot Fuel issue

Lots on this one - and agree with floatmiboat - when in doubt, hire a good mechanic - they'll systematically go through the system. How are you setting idle? Need to be in the water & in forward gear. All adjustments should be in the water.
Has the motor been sync-n-linked?

 

[Shastaray]

Nobody has mentioned that the possible problem might be at the intake manifold o rings. I had the same symptoms and it wound up being a 3 dollar o ring.

 

[GA_Boater]

Nobody has mentioned that the possible problem might be at the intake manifold o rings. I had the same symptoms and it wound up being a 3 dollar o ring.

Nobody mentioned this thread is 9 years old.

Closed.