Mystery mower fuel problem

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begreen

begreen

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Nov 18, 2005
104,666
South Puget Sound, WA
Our 7 yr old 18hp Kohler powered Craftsman yard mower started having fuel issues this year. It's a simple gravity feed fuel system. The mower runs out of gas when the tank gets down to about 1/3 full if I am mowing uphill (even as small amount of incline). If I back up the same hill it will run fine. If I wait a few minutes then restart it will run again fine for sometimes as much as 10 minutes more. If I fill the tank it will run fine. The first thing I checked was to be sure the vent hole in the tank cap was clear. It is. The problem will even happen with the cap off.

I have replaced everything from the tank to the engine block and the problem persists. When it stalls out of gas you can see that the fuel filter is almost empty. When the mower runs it runs great and cuts very well still. Power is good even uphill cutting heavy grass. Any thoughts on what's happening?
 
Strainer or outlet in fuel tank clogged?
I believe even gravity feed systems use a fuel pump on some larger motors. Maybe a bad diaphragm or clog there?
 
Most of those tanks will have a water screen at the bottom. Completely drain and dry the tank and see if that helps. Compressed air from the hose barb side will help dislodge anything on the screen.
 
No fuel pump. I don't see any screen in the tank, just a plastic nipple at the bottom. When I had it apart replacing the fuel line I did try using compressed air, no change. This is really baffling.
 
Hmmm...no fuel pump on an 18 HP seems odd to me. IF there is a screen it is a very fine mesh and sometimes very hard to see without a flashlight. Not all have that.

Any heat shielding missing or removed causing vapor lock?? The cooling down thing has me kinda of baffled. One of two things are happening there. Either the "time" of cooling down is allowing for the fuel to refill the hose/carb/line or the actual cooling effect is causing something to happen. Interdasting.
 
Gas goes from tank right behind the engine, to fuel filter, to carb. Same setup as a model T. There has been no heat shielding removed. I bought the mower new and this is the first year I have done any engine service outside of regular oil and air filter changes. You can see that the fuel filter is not filling when it stops running.

I will take the gas tank out again and will closely examine for a micro screen. Maybe I missed this. Tis indeed wierd because for the first 2/3ds of the tank or about an hour it runs great.
 
For what it is worth (not much) - I had a 12 hp that had a fuel pump go bad. It is also now a simple gravity feed and it will not empty the tank before the engine will cut out from lack of fuel. If you refuel right away, does it fire right back up and run for another hour? If so - this tells me that the extra weight of the fuel is pushing enough past whatever restriction you have (that is why I was thinking about the water catching screen.)

Another test...when it dies, shake the crap out of the machine and see if it will run. That is another way to get water to move.
 
begreen said:
Gas goes from tank right behind the engine, to fuel filter, to carb
Click to expand...
I've seen the fuel pumps installed on the carb itself. The intake manifold vacuum pulses at the carb move a diaphragm that pumps the fuel.
 
Yes, if I refill the tank it will go on for another hour. That is how I had been running it last month.

No fuel pump on the simple Walbro carb that I know of, just a simple float system. Changing the old to the new carb made no difference in behavior.
 
Another thought. You said it still happens even with the cap off? And that the filter will actually almost empty? Does not compute. That would insinuate a vacuum or a blockage. When the filter is almost empty if you pull the line from the carb, will flow begin again? You could use that method to isolate which side is the culprit. Pull line from carb and if it runs fuel - carb side is causing it. If it doesn't run fuel - line/tank side is causing it.
 
Long shot - is something shifting and pinching the fuel line when you're headed uphill?
 
No pinching or shifting, the fuel line is 1/4 heavy rubber hose. It runs through a chassis grommet to keep it neatly on it's path. The total run is only about 2ft.
Jags said:
Another thought. You said it still happens even with the cap off? And that the filter will actually almost empty? Does not compute. That would insinuate a vacuum or a blockage. When the filter is almost empty if you pull the line from the carb, will flow begin again? You could use that method to isolate which side is the culprit. Pull line from carb and if it runs fuel - carb side is causing it. If it doesn't run fuel - line/tank side is causing it.
Click to expand...
Can do. Though odd and frustrating if two carbs, old and new, have the same symptom.
 
fuel line should be replaced with the newer line designed for use with the ethanol fuel, not rubber line the fuel causes it to rot from the inside out. you have a fueline blockage, does the unit have a shut off valve at the tank? could be blocked with some crud. Some of the metal tanks were lined with something to prevent rust but the dang lining breaks down in the presence of ethanol. Makes a gummy mess that can block any type of screen quick. also I hope that 1/4" fuel line was the id size and not the od as that would be pretty small line for a 18hp motor. Most units that size have a fuel shut off sol. if it loses power no fuel flow.
 
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I too have a 19.5 HP engine in my craftsman mower. No fuel pump, no fuel shutoff. Fuel tank also under hood right near engine. This is just like a motorcycle setup. Gravity feed to the carb where the float lets fuel in to keep the bowl full. Your problem is upstream of the fuel filter towards the tank. Something is preventing fuel from leaving the tank and making it to the filter. The open float valve in the carb is allowing the fuel to drain from the line and air to refill the fuel filter from below. A screen, a pinch, debris, etc. Your clear fuel filter is helpful in this diagnosis.

Start by disconnecting the fuel line upstream of the filter and try to let the tank drain into a gas can. I suspect the flow stops (or slows way down) before the tank empties. At that point, I would blow backwards through the fuel line and see if fuel begins to flow again. If not, then I would remove the fuel line from the tank. Does fuel pour out of the nipple on the tank bottom? If not then pull the tank and investigate what's plugging it. Then a nice drill bit would be running up through the outlet nipple and clearing that obstruction.
 
I have drained the tank twice from the 1/3d full level and it drains fine, no stoppage. The old fuel line was fine, no blockage but just in case I replaced it all. The fuel line is all new. I'll try running a drill bit through the tank outlet when I do this riff all over again.
 
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So the fuel tank is free flowing to the fuel filter at all times but the fuel filter has no fuel in it. I know my craftsman OEM fuel filter was not clear. Could it be that your aftermarket fuel filter is designed for a system with a fuel pump? Regular automotive fuel pumps make 3-6 psi for the carburetor and could handle some restriction from a relatively tight filter. Your gravity system does not make 3 psi. It takes 83 inches of water column to make 3 psi and 12 inches only provides less than 0.5 psi.

Remove the fuel filter. Test.
 
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Good point, that thought crossed my mind too although I told the mower shop what the purpose of the filter was. The previous filter was the original and like yours opaque white and a bit larger cylinder. The mower had the same symptom with that filter. Also the gas seems to flow fine if I drain through the new filter. But I can order the Sears filter, it just wasn't in stock at the time.
 
begreen said:
Good point, that thought crossed my mind too although I told the mower shop what the purpose of the filter was. The previous filter was the original and like yours opaque white and a bit larger cylinder. The mower had the same symptom with that filter. Also the gas seems to flow fine if I drain through the new filter. But I can order the Sears filter, it just wasn't in stock at the time.
Click to expand...

The absence of fuel at the filter, yet full flow from the tank to the filter is pointing to the filter as the restriction. Is it possible to eliminate the filter with a short section of tube or a longer fuel line?

Carb float valves can get sticky too so when you have the fuel filter out I would recommend blasting down the fuel line to the carb with carb cleaner followed by a little air in case you have chunks of junk there. I know you swapped carbs but carb cleaner is like 96 cents per can.
 
I'd put a short piece of tubing in place of the filter too, with very little fuel in it to see if it eliminates the problem. When all else fails, process of elimination. Very frustrating too.

When it dies, if you blow into the tank to pressurize it, and it fills the filter, that is a sign you're overcoming the resistance of the filter the same as more weight of more fuel pushes it through. I'd bet on the original filter getting slightly clogged causing the problem and the new one having more resistance causing the same problem when fuel is low............... I put the old glass bowl style on when there's enough room.

Some Model T's do that too; Henry Ford designed my 1921 Center Door Model T with the fuel tank under driver seat. On long steep hills, the tank becomes lower than the carb intentionally to run it out of fuel. No oil pump with splash feed allows the front bearing to dry out first on steep uphill grades. To protect the front bearing, it was designed to run out of fuel before oil ! You must turn around and BACK up steep hills to keep the tank higher than carb and oil to the front bearing.
 
Um, and then what about the rear bearing?
 
The oil is very deep in the rear and the flywheel splash provides plenty of oil there.

p16s.jpg


What would now be called the transmission was called the hogs head that holds the same engine oil in the rear.
 
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LOL That thought actually crossed my mind.
 
ok begreen, are you sure its a fuel problem ?......i had a situation with my ford lgt14d a while ago where it would stall out on the hill but only when going in 1 direction. Happened a bunch of times before i figured that one out. Turned out to be a broken motor mount on the lh side and when traveling the hill with the rh side of the tractor on the lower portion of the hill the lh side of the motor would actually raise up enough to lose the ground connection and stall out. Replaced the motor mount and been fine since....
rn
 
Get any small engine tank. Garbage pick or whatever. I keep one around from a vertical shaft Briggs. Rig the tank and a fuel line right to the carb. Isolate the existing tank and plumbing and see if there's a difference.
 
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