What *REALLY* happens when you run out of fuel?

[Andy in Ottawa]

For many years, I had trouble with my B&S mower, having to start on low throttle and increase gently as it warmed up because if I put it full on, it would sound hollow, lose power and die. Then, three years ago, a friend took the carb apart and cleaned it. It was like magic! For the last three wonderful years, it has started first time (even in spring) and worked perfectly. Until TODAY, that is. For the first time since my friend fixed it, it ran out of gas. Now, the problem is back, and it's worse than before. It will usually start if I prime it, but it only runs for a few seconds, and I can't get it to full throttle to actually mow without it dying again. So, I want to know, "what happened"?

I have heard a story that if you let an engine run out of fuel, then it sucks in the debris from the bottom of the tank, and this gums up the carb. It fits the evidence, but I have a VERY HARD time trying to imagine how the fuel intake would work, that it would carefully ignore the gunk for three years, and then suck it all in when the fuel runs out. It doesn't "ring true" as it were. Either it sucks fuel from the bottom, or it doesn't. It's not going to take fuel from the middle normally, but bottom-feed if you dare to let it run low on gas.

It seems more likely to me that there's an air bubble in the line, or something, that's stopping the intake from getting a full flow of fuel. If so, how can I avoid it or fix it?

Are either of the above true, or could it be something else? Can I do anything other than take apart and clean the carb like my friend did?

Do the rest of you always avoid letting your mowers run out of fuel to get around this problem? It doesn't say that in the instructions, but it seems to turn a fully functional lawn mower into a brick. (I am careful to check the content of my can of fuel, by the way, and don't pour the last bit into the mower's tank, so the fuel should be pure, and I always use premium fuel.)

I appreciate any comments that experts can offer.

 

[Boobala]

In the past 3 "wonderful" years have you changed your air filter, fuel filter, fuel lines, shut-off valve (ALL should be changed ANNUALLY ) for peak performance, while I'm listing things, have you at least checked your blades & belts, cutting deck spindles greased, steering greased ?? I assume all that was done to have a mower perform so well for 3 years, I think another thorough carb cleaning a replacement of filters and lines is in order, remove the fuel tank and clean it thoroughly as well, look forward to 3 more happy years .. :thumbsup:

 

[bertsmobile1]

To answer your original question
Running out of fuel causes the engine to run lean for a few seconds and that is good for a lot of engines, particularly 2 stroke ones because it burns off the crud that builds up on the plug.
As for why your mower is not running properly see the previous post

 

[Andy in Ottawa]

Thank you for the answers so far; I have not done much preventative maintenance because the mower gets little use. It's a push mower, so no belts that I know of, nor steering. I'm pretty sure it's not greasing the axle because it spins freely when I pull the starting cord. I also don't think it's the air filter because it wouldn't have changed suddenly and not much air has passed through it and it's clean air here anyway.

It has to be something to do with the fuel, because that's what changed. It could be the fuel filter, I'll have to find out where it is.

I figure that taking the carb apart and cleaning it will fix it (again) but I'd like to know why. Whether it's air locks or dirt, doing what my friend did will probably cure it, I'd just like to understand, and avoid this in future.

P.S. I am rigorous in checking the oil level prior to use, and putting fuel stabiliser in every fill. The premium fuel is shared with the snowblower.

 

[bertsmobile1]

Nothing sinister about a mower running out of fuel.
FWIW I fit taps to every mower that comes into the workshop and advise people to turn the tap off and starve the engine off rather than turning it off from the throttle/ignition.

And if you do not tell us, we do not know.
You have the mower sitting right in front of you, we just have words on a screen.

 

[Boobala]

In the past 3 "wonderful" years have you changed your air filter, fuel filter, fuel lines, shut-off valve (ALL should be changed ANNUALLY ) for peak performance, while I'm listing things, have you at least checked your blades & belts, cutting deck spindles greased, steering greased ?? I assume all that was done to have a mower perform so well for 3 years, I think another thorough carb cleaning a replacement of filters and lines is in order, remove the fuel tank and clean it thoroughly as well, look forward to 3 more happy years .. :thumbsup:

My apologies, I did'nt notice it was a push mower, as stated in your Avatar area, but the advice of cleaning the fuel system still applies, and the air filter too.. your mower probably does NOT have a fuel filter or shut-off valve for that matter, I believe you may have a fuel pickup with a fine screen on it and that may be the problem, if you post your engine info. we might be able to better help you, .. see EXAMPLE, .......

 

[itsgalf]

Your lawnmower probably doesn't have an inline fuel filter. In the fuel tank itself there should be a screen that covers the outlet where fuel line connects - may want to check that or just dump the fuel into a container and try cleaning the tank. I wouldn't think there would be any air bubble problems since usually the fuel line run is pretty straight from tank to the carb inlet without any loops in the fuel line and the tank is the highest point in the system, and your fuel tank is vented through the cap so all air should work its way out.

Only thing I could guess about why running out of fuel started your problems may be something like your friend suggested. I'm no fluid dynamics guru (just straight up guessing), but could be when the jets are sucking out the very last parts of fuel from the carb bowl the flow is more turbulent than usual and if you haven't been cleaning your fuel system of gunk for years, some of that gunk works it's way into the small jet clearances and clogs them. Something like when you're drinking something out of a straw and you get to the very bottom and suck hard the fluid is kind of turbulent coming up

Heh, probably just talking out of my rear. Clean the carb

 

[Andy in Ottawa]

Nothing sinister about a mower running out of fuel.
FWIW I fit taps to every mower that comes into the workshop and advise people to turn the tap off and starve the engine off rather than turning it off from the throttle/ignition.

And if you do not tell us, we do not know.
You have the mower sitting right in front of you, we just have words on a screen.

I can't find anything useful on the mower to describe the engine, other than Briggs & Stratton 5.5HP. The mower is a "Murray 20456x31A - Walk-Behind (1998)". It looks almost identical to this one:

https://offerup.com/item/detail/303184985/

except that it's not self-propelled. (Sorry, with only two posts, the forum won't let me make it a link.)

You know, at risk of confusing my story, it occurs to me that I had an almost identical problem with my snowblower. Due to an old leaf blower having a tube rot inside, I got in the habit of turning the fuel tap on the snowblower off before I walked it to the storage place, so that any fuel in the carburettor was used up and not sitting there {allegedly} rotting things. But it was always hell to start, requiring 20 or 30 goes on the electric starter before it burst into life. However, last year on a whim, I decided NOT to turn the fuel off in advance, and BINGO it was easy to start on the first turn. What was different about the snowblower however, was that once it started, it was working normally.

Incidentally, a commentary on the lawnmower again: my friend said "Oh, it's so old, it has the proper Briggs & Stratton engine in it, you should hang on to that one because they put crappy engines in the newer ones."

 

[Boudreaux In Eunice La.]

Hello Andy .......... I read or scanned your posts I should say and noticed you said older engine....... About how old and please describe your engine to us so I can tell you where your Model number is located....

I will help you along here....... Where is your carb at ???? On top of the gas tank with 6 screws holding it on or 2 bolts holding it on with a 5 inch by 5 inch air filter box with a Primer Bulb ????

Let us know Mon Ami ~!~!

 

[Boudreaux In Eunice La.]

Andy........... I made a short video on you tube... I didn't make one like this before so I thought I would make it to see if your carb looks like this one......

If it is, then you are in great shape.... I show the kit numbers for it... You probably won't have the welch plug on the outside like this one does but if you do the kit comes with one.....

Sorry for the bad video I was in a rush to make it. ......Plus Tard Mon Ami Let us know something ...