Northwest Fishing Reports

I have a 12’ Jon boat with a 4 hp Merc 4 stroke for fishing-great little boat.
I have replaced the following:
Carb.-Brand new
Fuel Pump-Brand new
Gas tank-Brand new
Gas lines-Brand new
Plug-Brand new
Fuel filter-Brand new
Fuel squeeze bulb-Brand new
The problem is it runs like a champ for an hour plus, then for no reason, it starts to sputter.
Squeezing the fuel bulb will keep it running. Looking at the fuel filter, it is all but empty when this happens.

Note: This is the second Fuel pump, no change.
Any suggestions would be appreciated.
Rick

Welcome to the site! I'll take a stab at it. 3 possibilities come to mind. Sounds like this motor has an external fuel tank? Does this tank have a vent? If its a modern tank, it's likely in the fuel cap. It may be restricted and not letting air into the tank.

The fuel pump will be sucking fuel from the tank, so everything between the tank and fuel pump is under vacuum and any poor connection or air leak could draw air instead of fuel. Check quick connectors, fuel lines under the engine cover etc... Also check fuel pump to crankcase signal hose well.

3rd one is my favorite....Fuel boiling / vapor lock. Fuel pumps are no good at pumping vaporized fuel, fuel lines under the cover could be getting too hot from misrouting, fuel pump mislocated or the engine could simply be overheating. Thermostat, water pump, combustion to cooling water leak etc...

Good luck!

Will echo remarks above though I doubt it's warm enough to cause fuel boiling.

The most likely issue is tank not venting correctly and the very first thing I'd check.

The fuel pump on this engine is a simple diaphragm pump that needs liquid fuel to work, that's why when the problem exhibits and you squeeze the bulb the engine picks back up. A second indicator is the near empty fuel filter. Dollar to donuts you've got a tank venting issue, pinched line, or some other restriction between engine and tank. Don't overlook the fuel line connector either!

Were all parts, in particular, fuel line, bulb, fuel pump, fuel filter, and carb Mercury brand or aftermarket? Matters greatly on some.

Thanks for the suggestions.
I agree it may be a fuel delivery issue. The new gas tank is self-venting. I did try opening the cap with no change. Also the lines are new-no crimps.
Also, I replaced the connections at the motor and tank-thinking air getting sucked in-no dice.
My next step is to isolate/insulate the fuel lines under the cowling-if this doesn’t work, it may come down to an electrical issue?
Again-thanks

While Sideburns covered it pretty well, I will second his third ( ) and favorite part about fuel vaporizing. Could be a thermostat that is sticking or, if the engine was run in salt water, the cooling passages getting plugged up. That would all come down to heat. If it gets too hot the fuel system is susceptible to vibration causing bubbling in lines, filters. I would go more with a fuel vapor issue as the filter looks empty.
Electrical could be a problem. If you have a meter that can check inductance you can check the coils, which would be my 'go to', then the trigger coil. Numbers for those can usually be found on 'The Googles'.
What color was the spark plug porcelain tip when you removed it after running? White? Tan? And look down deep into the plug also and tell us if the color is different?
Try changing out the fuel to non ethanol if you don't already run it to see if there is a difference.
Best of luck!

^^^^ yep. Ethanol boils at a much lower temp than gasoline alone. Fuel boiling is probably a less likely scenario than many of the things youve already checked, but because of this, is something that and intelligent person could easily miss. Good call on questioning the ignition system. Ignition coils often fail only when hot.

I should also mention that air in a fuel filter can at times be misleading. A good portion of the filter may have air during normal operation, depending on the angle of the filter. Also, that air bubble will expand normally when the fuel pump pulls a vacuum on the air trapped inside. -But that being said, if there is no air in the filter at the start, and air seems to appear even with a full tank you obviously have an air leak in the fuel line down stream of the filter.

One more note, as hewesfisher was hinting above earlier, just because the part was replaced doesnt mean its good. Some of my biggest blunders in diagnostics have come from bad parts!

Sideburns wrote: ↑

Fri Apr 12, 2019 2:19 pm

One more note, as hewesfisher was hinting above earlier, just because the part was replaced doesnt mean its good. Some of my biggest blunders in diagnostics have come from bad parts!

Yessir.

What year is this Mercury 4hp 4 stroke?

Sorry-just saw this.
2006

Your engine is fully compatible for ethanol fuel up to 10%, and JHMO, that is not a contributing factor to your issues. It's my opinion your problem is the new self-venting tank. I'd borrow a different, known good tank and run off it to isolate to the engine or fuel supply.

Will try-Thanks