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So it's not a lawn mower but hopefully someone here will have some advice. I purchased a one ton mini excavator with a B&S XR 13.5 gas engine. The issue I'm running into is bubbles in the fuel tank and the tank pressurizing to the point that it blows fuel out if I take the cap off after running it for about 20 minutes. The unit does get fairly hot and of course we've had a string of 100+ degree days (It was 104 outside when I noticed this problem). I don't necessarily feel like the fuel cap is venting properly but I assumed the fuel system should be under vacuum on a gravity system and not under any positive pressure. I'm going to try another fuel cap but want to have some input if anyone has any other ideas on things to check especially since it continued bubbling once the cap was removed.

I think there may be a fuel return on this machine but either way pretty sure it needs a vent on the fuel tank

Please post all the engine numbers so we have an idea what you are looking at

Rivets said:

Please post all the engine numbers so we have an idea what you are looking at

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Model # - 25T237-0085-F1 (according to B&S website, I can't see a tag with model number or serial number on the engine itself)

JF 420-209-23033102-Y43 is stamped on the inside of the cooling fins on the cylinder.

Engine Family: PJDGS.4202UA according to Emission Control Information decal on fuel tank.

Was the engine running when you took the video? I assume it was but I couldn't tell for sure. If so then the bubbles are normal and caused by engine vibration. Years ago manufacturers would put baffles in the fuel tanks to help limit that but then the baffles could deteriorate and plug the fuel outlet. If that video is with the engine not running, that fuel is getting a lot hotter than it ever should IMO.

As far as pressure in the fuel tank, that should never happen and enough pressure could cause carb flooding. That would make me think the fuel tank is not venting properly. From the parts breakdown it looks like the engine has a fuel cap with a carbon filter in it so as to not vent fuel vapors into the atmosphere. That's pretty common these days on engines with fuel tanks mounted on them. Typically those types of caps have a tether on them. If that's the case then replacing the fuel cap should eliminate the pressure you're finding when you remove the cap.

The fuel tank should be designed to keep the fuel level low enough so it doesn't contaminate the fuel cap but with a little effort you can fill the tank all the way up into the filler neck. If those carbon filter fuel caps get fuel in them that can deteriorate the carbon and plug the vent.

VegetiveSteam said:

Was the engine running when you took the video? I assume it was but I couldn't tell for sure. If so then the bubbles are normal and caused by engine vibration. Years ago manufacturers would put baffles in the fuel tanks to help limit that but then the baffles could deteriorate and plug the fuel outlet. If that video is with the engine not running, that fuel is getting a lot hotter than it ever should IMO.

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The engine was NOT running when I shot that video and the machine was definitely hot. I've been contemplating installing a fan in the rear of the excavator to help draw heat away from the engine. It's super compact and I'm not sure that there's enough air circulation for this machine to properly cool in this deep south heat. It'll be nice in the winter but it's rough right now. I included a picture of the machine just for reference of available air flow. The engine, tank, and hydraulic system are all under the seat and floorboard.

VegetiveSteam said:

As far as pressure in the fuel tank, that should never happen and enough pressure could cause carb flooding. That would make me think the fuel tank is not venting properly. From the parts breakdown it looks like the engine has a fuel cap with a carbon filter in it so as to not vent fuel vapors into the atmosphere. That's pretty common these days on engines with fuel tanks mounted on them. Typically those types of caps have a tether on them. If that's the case then replacing the fuel cap should eliminate the pressure you're finding when you remove the cap.

The fuel tank should be designed to keep the fuel level low enough so it doesn't contaminate the fuel cap but with a little effort you can fill the tank all the way up into the filler neck. If those carbon filter fuel caps get fuel in them that can deteriorate the carbon and plug the vent.

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I ordered a replacement cap yesterday using the B&S part number because I had the same thought of flooding due to pressure. It looks slightly different than the one that is currently on the engine but I'm hoping that is the difference between one sold in China and one sold in the US. The machine only has 3.9 hours on it so it's possible the cap was just faulty from the factory. It ran fine through the first couple tanks of fuel and after the last fill-up is when it began stalling and shutting down after about 20-30 minutes of run time and usually only when under a load and hot. It's just a strange scenario that I've never encountered. Hopefully I can find some resolution fairly quickly. This is a handy little machine on our land.

What does the spark plug look like? Does it show signs of the engine running hot?

I'm going to try to get out there after work and pull it to check. I'll report back on what I find.

You are boiling the fuel in either the carb bowl or fuel line and it bubbles back into the tank on a gravity system. It is way too hot.

Hammermechanicman said:

You are boiling the fuel in either the carb bowl or fuel line and it bubbles back into the tank on a gravity system. It is way too hot.

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Sounds like a fan to pull some air through the confined space the engine is mounted in is probably my solution at this point. I don't know any other way to dissipate the heat from around the engine. I'll measure the area I've got to work with while I'm checking that spark plug this evening and see what I can come up with to make it work.

It ain't the heat , it's hot everywhere - I've had over 100 degrees for 20+ days here and not seen anything like what you're describing and no one else is having this issue, it's something else in my opinion - Neve have I heard of gasoline boiling in a fuel tank

Wrap the fuel line in Tin Foil if you can. Be sure to route fuel lines as far as possible from Exhaust pipes. You may want to make a heat shield between the heat source and the tank. Sounds like old fashioned vapour lock. Drill a small hole in the cap before you become a fireball.

VegetiveSteam said:

Was the engine running when you took the video? I assume it was but I couldn't tell for sure. If so then the bubbles are normal and caused by engine vibration. Years ago manufacturers would put baffles in the fuel tanks to help limit that but then the baffles could deteriorate and plug the fuel outlet. If that video is with the engine not running, that fuel is getting a lot hotter than it ever should IMO.

As far as pressure in the fuel tank, that should never happen and enough pressure could cause carb flooding. That would make me think the fuel tank is not venting properly. From the parts breakdown it looks like the engine has a fuel cap with a carbon filter in it so as to not vent fuel vapors into the atmosphere. That's pretty common these days on engines with fuel tanks mounted on them. Typically those types of caps have a tether on them. If that's the case then replacing the fuel cap should eliminate the pressure you're finding when you remove the cap.

The fuel tank should be designed to keep the fuel level low enough so it doesn't contaminate the fuel cap but with a little effort you can fill the tank all the way up into the filler neck. If those carbon filter fuel caps get fuel in them that can deteriorate the carbon and plug the vent.

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Most of the above is a good round up, but I have to add my most significant belief. Assuming the engine was running or recently shut down hot in your video, we have hot fuel boiling, or more commonly called vapor lock in a pressurized system like on cars. Since a gravity feed, two causes of vaporized fuel. One less impact on running is vibration excitation of fuel molecules in the tank crossing energy level to vapor and bubbling up and causing pressure in the tank which may overcome the venting capable. The hotter the fuel in the tank, the greater the problem or likely hood of this. The second bubbling is similar but does cause running problems. The same heat and vibrations of the fuel in the carb or fuel lines. Less likely is it occuring in the carb bowl or carb fuel passages. More likely is in the fuel lines. When in the lines, since we are talking gravity feed, fuel must flow down by gravity but bubbles of fuel vapor must rise. This causes fuel flow down to be greatly resisted by bubbles rising. That can stop flow into carb or not have enough pressure on inlet needle to overcome the float needle and limit fuel to the carb bowl or effectively change the design of the fuel level in the bowl which is to carb design and cause lean running or misfires. These bubbles will rise to the tank and will add to any bubbles created by the tank itself. To test this, a few things can help. Run with best cooling. Replace rubber lines as a temporary test with clear HIGH TEMP lines and insure they do not get hot and leak and catch fire! This allows the observation of bubbles in the line. Best first test is to add fuel that will not vaporize as easily with additives to help. Non ethanol fuel will help but may have as mush as 5% ethanol in it. Treat fuel with Lucas Fuel treatment in a green tall neck bottle. Add a fair amount of Marvels Mystery oil to the gas not quite as much as a 50 to one ratio but not less than 100 to 1. If these treatments lengthen the time it takes to re-create or eliminates the problem with nearly the same ambient temperature, it is vaporizing fuel problem. These fuel treatments can be used continuously if it solves your issue.

If you have a fuel pump on that machine! Check the fuel pump diafrage.