Engine Starved for Gas

[Chartman]

I need help diagnosing a problem with a Poulan lawn tractor that I bought used last summer:
Poulan Pro 300EX; 42 inch cut; Product Model 960420088; Maintenance Level 00; Model: PB18542LT; Serial Number: 020209D002294
Briggs and Stratton Engine; 18.5 Hp; Model: 31C707; Type 0603 B2; Code: 090904ZA
It starts and runs fine for about 20 minutes. Then it starts to sputter, as if the engine is starved for gas, on one pass. It will recover a bit on the return pass. My front lawn has, what I would describe as, a slope that may be significant. The problem gets progressively worse, and the engine will stall shortly after that without intervention.
When the sputtering problem appears, I can disengage the lower drive, (required to) lift the hood and observe the clear in-line fuel filter. It will be about empty of fuel. I can open the fuel tank cap and I observe that the filter will rapidly fill with fuel and the engine recovers.
It appears to be a simple case of lack of venting by the fuel tank cap is preventing fuel flow.
I tried the “dump test” to see if the cap allows air in the tank as fuel flowed out the open tube at the bottom. The cap passed the test. Even so, it seemed too coincidental that the fuel filter filled up when I unscrewed the gas cap, so I bought a new cap. I was not too surprised when the problem persisted today, even with the new cap.
It occurred to me that I didn’t know if this is a “hot running engine” problem or a “fuel tank half full” problem; as these conditions seem to occur simultaneously.
Today I got through almost all of the front lawn when the problem became extreme. I could see the fuel filter was pretty much empty when the engine was about to stall, but with the lower unit disengaged and the engine idling, it would suddenly fill up rapidly; even with the cap on tight. I don’t know what caused that change.
I stopped the engine and emptied the grass, then decided to fill the tank (about half full at the time) completely to the top.
I then went on to mow the back lawn, which has a steeper incline than the front, and had no problem for about 20 minutes. It did start to sputter at one point, but I was almost done and it recovered on its own. The tank was about 85% full.
The engine sounds great while it is running, but perhaps there is a vacuum leak to the gas tank?
Perhaps a cylinder head gasket leak?
I am very sure this is a “starved for gas” problem, but other than that, I’m pretty much stumped!
Help!?

 

[bertsmobile1]

In order for fuel to get out of the tank. air has to get in.
That air will be dusty.
Add some grass clippings and you get a plug that forms in the fuel outlet of the tank.
When the tank is full you have a bigger pressure head to force the fuel through the blockage.

IF it is a Cal spec mower there will be a filtered air vent into the tank that can get blocked.
Try pulling the fuel line off at the pump and blowing back through it ( gas cap removed ) when the spluttering starts.
If it goes away for a few minutes , time to clean out the fuel tank.
If there is black debris in the fuel filter then change the fuel lines as well.

 

[Chartman]

In order for fuel to get out of the tank. air has to get in.
That air will be dusty.
Add some grass clippings and you get a plug that forms in the fuel outlet of the tank.
When the tank is full you have a bigger pressure head to force the fuel through the blockage.

IF it is a Cal spec mower there will be a filtered air vent into the tank that can get blocked.
Try pulling the fuel line off at the pump and blowing back through it ( gas cap removed ) when the spluttering starts.
If it goes away for a few minutes , time to clean out the fuel tank.
If there is black debris in the fuel filter then change the fuel lines as well.

I REALLY appreciate your input, bertsmobile1.
This machine was most likely purchased in NY or somewhere in the northeast. I saw no evidence of a fuel tank filtered air vent; just an ordinary fuel tank cap.
I don't think there is a fuel pump either. The fuel flows:
Bottom of tank -> in-line shutoff valve -> fuel filter -> plastic elbow on carburetor above the cup.
There is a solenoid shutoff on the bottom of the carb cup, that I think prevents back-fire when the engine is shuts down; but I don't see how this could be the problem.
Based on your suggestions, I think I will empty and clean all of the fuel lines, valve openings and exit port on the bottom of the fuel tank.
I probably won't mow again until next week, so we'll see if it helps at that time.
SL

 

[bertsmobile1]

I REALLY appreciate your input, bertsmobile1.
This machine was most likely purchased in NY or somewhere in the northeast. I saw no evidence of a fuel tank filtered air vent; just an ordinary fuel tank cap.
I don't think there is a fuel pump either. The fuel flows:
Bottom of tank -> in-line shutoff valve -> fuel filter -> plastic elbow on carburetor above the cup.
There is a solenoid shutoff on the bottom of the carb cup, that I think prevents back-fire when the engine is shuts down; but I don't see how this could be the problem.
Based on your suggestions, I think I will empty and clean all of the fuel lines, valve openings and exit port on the bottom of the fuel tank.
I probably won't mow again until next week, so we'll see if it helps at that time.
SL

A gravity tank will be an easier cleaning.

 

[Chartman]

Okay; So today I pulled the fuel line from the carburetor and put the end in an empty container. I opened the fuel valve and let the fuel drain from the mostly full fuel tank with the fuel cap firmly in place. It drained out with no problem.
I removed the tank/shutoff valve/in-line filter/hoses assembly.
I cleaned everything thoroughly; inside and out. I blew out all of the passageways with compressed air.
The shutoff valve is a straight in-line device. I thought I should be able to see straight through it when the knob is in the “open” position. I could not; nor could I pass a wire through it. I could blow through it in the open position and not in the closed position. I was surprised that it had a flow direction imprinted on it. I decided to replace it.
I checked the Parts Manual that I downloaded from Briggs and got the fuel filter part number (691035) to replace that, too.
At Ace hardware I found the same Briggs shutoff valve (only “new”) in a blister pack with the knob in the “off” position; so I could not tell if it would be a clear shot through when it is in the “on” position. I prevailed on the clerk to open it, so we could check. It turns out that it is NOT a clear through passage, even in the full on position. I found another shutoff valve (at 50% higher price) that DID have a clear shot through in the open position, so I decided to buy that. I also bought a new Briggs fuel filter with the exact replacement part number I found in the Parts Manual. I was about to open it and install it when I read the package carefully. It clearly identifies this filter as “Fits engines with fuel pumps”. I have no fuel pump. This filter is a lot smaller than the one on the engine. I decided that it may not be free flowing enough for a gravity feed engine, so I stuck with the original fuel filter, which seemed easy enough to blow through.
After reassembling everything, I filled the fuel tank almost to the cap and started up the engine. It started immediately, as always.
I completed the front yard with no problems and went straight on to the back without stopping.
About half way through, the same fuel starvation problem began. The engine sputtered every time the slope of the terrain tilted the fuel away from the engine. Disengaging the lower unit and loosening the gas cap would help the engine recover. I could see the gas flow into the clear fuel filter as the engine recovered. The tank was down maybe only a quarter of a tank (or less). There was plenty of fuel in the tank and the fuel level was well above the carburetor inlet. It was a struggle to complete the mowing. I think it would have worked to fill the tank to the top again; but this ain’t right!

I am running out of ideas for what to try next. I might just eliminate the fuel filter and see how it runs. I am thinking that the problem may actually be down stream of the tank, valve, filter, hoses system. Perhaps the engine is not providing sufficient vacuum to pull in the gas it needs when hot? Poor valve seating or piston rings? Could there be “back pressure” from somewhere in the engine that keeps the fuel from flowing out of the tank?
Anyone have any ideas?

 

[ILENGINE]

First order of business, I ran the 691035 filter on gravity flow engines for years without issues, so don't worry about the fuel pump rating. I went back to the 394358S filter after 2012 because it was causing issues with the filtration level not allowing enough flow with the new run under slight tank vacuum EPA vent on the tank.

You said the slope may be significant in your yard. Do you have any way to measure the slope. I am wondering, and this is just a wild though. The angle is causing the fuel to run to one side of the fuel bowl causing it to have a high enough level to burn the fuel out of the carb enough to get the main jet out of the fuel in the carb.

The other thought is the opposite of fuel starvation, and the slope is causing the fuel bowl vent to get flooded with fuel causing a rich run issue.

Just throwing some random thoughts out there for consideration.

 

[bertsmobile1]

If it works every time from full to 3/4 then it is definately a fuel supply problem.
The pressure head is not enough to unseat the float needle.
Is this the same carb that the mower came with.
Fuel pumps run about 3 to 7 psi which while not much, is more than the pressure from a gravity head.
Gravity feeds use coarser filters than pressure fed ones.

Have a close look at the needle in the float and the seat.
Chances it is sticking a little.

 

[Boudreaux In Eunice La.]

Ok in between my wife talking to me and trying to read all these post's...... I will comment .....

I agree with IL Engine about the filters...... I use the 691035 filters all the time on regular 8 HP Briggs and Techs the same size ......

The 691035 is a 40 Micron filter which is smaller than the 394358S filter which is 75 Microns. Not smaller in size, but smaller in the fact that it catches smaller particles of debris........

The 394358S is a small white filter and the other is a clear see thru filter.......And much bigger......

I did see see where you blew thru all the passages and then let the fuel flow from the tank into a container with no issue........

BUT even with NO fuel pump no matter what filter you have on there, the engine still works the same way with or without a fuel pump.... The engine sucks air into the carb via the intake stroke on a regular gravity feed tank..... With a fuel pump the engines are bigger and usually have bigger carbs and fuel usage / consumption.... So there is 2 ways to get more fuel to the carb and the piston / pistons.......

The only thing I can suggest is that your fuel lines are [flapping] on the inside...... Ethanol has took it's toll on them, and it don't take long... 1 month or 2 years ????

Even though you did empty the tank in a jar without the cap on or off doesn't mean anything.......... The fuel system is still under pressure with the carb doing it's normal work..... Even without the pump like I said earlier....

You got me curious though on not a see thru shut off valve ..... I have some in stock 698183 and bust a pack open and check that out and reply in a lil while ...........

Let us know Mon Ami ~!~!

 

[jp1961]

Hmmm,,,the "significant slope" statement would lead me to suspect debris in the tank sloshing around in the tank and occasionally blocking the fuel supply.

Regards

Jeff

 

[Boobala]

I have the same problem with my mower at times, only when mowing the slopes in my yard, I'm sure it's the carburetor float so I just get on the other side of the slope for a few seconds, now I mow up & down the slopes sideways up one and like a "snake" to the other side ( like mowing an "S" pattern ) as far as the filter goes it's normal for the amount of fuel in it to change (unless it's blocked of course) it seems your issue is on the slope, as you stated doing front yard, it was functioning OK I have the same engine 31c707 series but a 17.5 HP.

I completed the front yard with no problems and went straight on to the back without stopping.
About half way through, the same fuel starvation problem began. The engine sputtered every time the slope of the terrain tilted the fuel away from the engine.