This morning I was using a borrowed lawnmower (my own had refused to start) at a customer’s home. The mower, which has a throttle bar you have to hold open while mowing, stalled twice while I was using it. (It’s a “Murray” rotary mower, with a Briggs & Stratton engine.) I had filled the tank with gas and when it first stalled I checked; still plenty of fuel. A few minutes later I started it again and continued mowing. I made two passes over the lawn, one at a higher level, one at a lower level. It stalled again on the second pass; I started it up again almost immediately. And it stalled yet a third time just as I was finished, so I didn’t bother.
I returned it to the person who owned it (another customer whom I have knowen for about 35 years) and told her it was stalling.
Assuming it’s getting the gas properly, what would make it do this? 
Lawnmowers are one of those things that are considered an appliance in the way that a toaster is, except you fill it with gas. This means that most people don’t maintain them. Since they usually operate in dusty conditions (grass clipings flying about, dirt from mowing over a clump) they can be very susceptable to carb problems… including things as simple as a dirty/plugged air filter.
Could also be ignition problems, but they are usually B&S are usually very reliable in this area. Any idea how old the mower is?
Ten years old at the max…and hey, you reminded me of something; Briggs & Stratton mowers have an air filter with a lid held on with a long bolt in the top; perhaps the spongy filter elment inside is dirty…I’ll be over to this customer’s place on Sunday and check it, unless the customer’s son has done so.
Thanks. 
Was there suddenly a lot of fur under the blade?
I have a number of older neighbors and I maintain their lawn mowers and those are often getting clogged air filters. It could well be the air filter. But the oil in lawn mowers work very hard also. When was the last time the oil was changed?
I think that a dirty air filter would slow down the rpms but not cause a stall. The mower did start up again. A test would be to remove the air filter and then start the motor and see if the problem is reproducible. My hunch is a stuck choke or possibly the linkage/springs located in the front right of the mower may have had somehow been modified by hedge trauma. Good luck!
It could be that the spark plug isn’t sealing correctly and you’re losing compression. I had this happen with a mower once. It would start and run fine while cold, but as soon as the motor heated up a bit it would start sputtering and eventually kill altogether.
was the engine running at the proper RPM when you were using it? if so, then that is very very odd, especially for a Briggs. if not, then it was probably running at a lower RPM because of carb problems, possibly running rich.
could have been bad gas.
i don’t think it was the air filter.
was there a lot of smoke coming out the exhuast?
oil might have been dirty. i need more info in order to tell you for sure.
also, could have had grass stuck in between the blade and the deck. was it a thick lawn you were cutting?
what do you mean “rotary” engine? surely it wasn’t a rotary engine, like in the RX7???
Rotary mower, Fuel.
I saw no smoke. I guess I’ll want to check the oil intake as well.
The grass was relatively short; in fact this customer has had problems with bald spots on the lawn (not caused by the mower scalping it). I had moved the adjustable throttle lever (not the one controlled by the spring-loaded crossbar) to nearly closed, the second time; then when the mower started I opened it, for full power. Part of the lawn is that thick St. Augustine grass, but that wasn’t very high. This mower is used much as my own; only when my customer’s son comes over, which is rare, or I myself use it at the customer’s home, which is rarer still.
I should probably also mention that I had put the mower into the trunk of my car, an '84 T-Bird, yesterday about 12:30 in the afternnon, and was not able to fit it in level. It was tipped to the left-hand side, opposite where I added gas and opposite where the air filter is. I got it to the customer’s home about 9 this morning, unloaded it, and struggled to start it; I sprayed starting fluid around the central air intake (where the starting rope is coiled), and finally got it to start.
My yard is also St. Augustine and I’ve been using the same mower to cut it every week for the past eight years. Very thick lawn cut 52 times a year for eight years. Every six months or so I replace the filter, plug, and change the oil whether it needs it or not. I am very happy with my 5.5 HP B&S engine. The only problem I’ve ever had with it is the same one dougie_monty describes. I was lucky to discover on my own that I had bent one of the linkage bars on the automatic choke by hitting the front of the mower on a hedge branch. She’s purred like a kitten ever since. Banging into things like hedges and car trunks may be the problem. Check the choke & again good luck.
[NITPICK]The bail across the handle that you have to hold in order to run the mower is NOT a “throttle bar.” Newer mowers are equipped (probably by law) with a spring-loaded crankshaft brake designed to stop the crankshaft – and the blade – quickly when the bail is released. Releasing it may also interrupt the ignition circuit, I’m not sure, but it is definitely NOT connected to the throttle.[/NITPICK]
I’d check the ignition system if it were mine. Specifically, I’d look for cuts or nicks in the spark plug wire that would allow current to arc to ground. (You did say that we’re to “assume” that it’s getting fuel properly, right?)
Good luck!
I usually put in a new spark plug, they are only a few bucks each & easy to do.
It sounds like there’s some water in the gas tank. If there’s any water in the tank a little bit will eventually get sucked into the carburetor passages and block the flow of gas.
Has the lawnmower been left out in the weather or in a damp shed? If so, drain the tank and refill. Alternately, you might want to add a little line dryer (or whatever the alcohol you get from auto stores is called) to the tank. The alcohol could possibly damage the rubber and plastic parts of the fuel system, so be sure to run the tank dry afterwards and refill with gas.
think this is the way to go, fuel or air starvation.
fuel and air, and a blocked needle jet to trunk (to boot in uk)
think motorbike and it becomes clear
i had the same thing on my scooter…it was a blocked air intake…
the only other thing may be a dud capacitor on the points where it is saturating and not taking off the unused voltage in time for the next spark so there is not enough differential to jump the plug gap…
really as someone said it is time for a top to toe check up.
i have had experience with tipping mowers over and it screwing them up.
mowers are very sensitive to that kind of thing. the carb got oil dripped into it probably. i had an overhead valve mower that really got messed up from tipping it over. we thought we had to buy new carb, but instead we just ran it with smoke pouring out and it running at half rev and it finally cleaned itself out.
a carb on a mower only has to work with a narrow range of RPM’s (as opposed to a weedwhacker) and a mower is usually sitting upright. so you don’t usually have carb problems on mower, unless it is tipped. and remember, it doesn’t matter which way you tip it… certain directions are worse, but any way can do harm.
my guess is there is no permanent damage, just run it.
When I finished with the mower I was able to get it back into the trunk a little more level, so I could close the trunk lid without using a bungee cord. Then I drove back to the home of the customer I borrowed it from.
At that place the mower has been kept inside a locked garage for quite a long time. I respect my customers’ property and I assume she wants me to return it intact to the place where I got it. (Several months ago she was the intended victim of a handyman who capitalized on her poor memory (she’s 83) to overcharge her, and left his tools in the yard and a horrendous mess in the garage. I took pictures–I used to work with a carpenter who cleaned up his detritus daily before leaving a customer’s home–and she showed the pictures to her daughter. who got quite angry and confronted this man; I’m surprised the daughter didn’t sue him or find a criminal charge to press against him! Such a thing would be unthinkable to me.)
run it through with a smidgit of carb cleaner in the gas.