Mower quits after short time.

I will assume that you are hsc something at honda spree.

You ask why a mower engine would quit. Several. Spark plug or ignition coil acting up when warmed up. Valves lash gone and valves open when they expand.
I had one that had a clog at the carb. inlet. Was full of grass and seeds from previous owner
not cleaning the gas can or got a lot of grass in the can.
Fuel cap not letting air in to replace gas being used.  Carb. flooding.

What type engine? Flathead or overhead valve?   Does it starts easy when cold?

its a flathead, starts easy when cold. it has 80psi compression, tested, has new starter, and new battery. it stops after about 15 minutes just kind of dies. I figure, its the compression. im guessing it is but no scientific tests done by me. anyone with experience everyone around here, has experience untl yuou have a particular problem. i end up experimenting when i can figure it out and do it myself, most of the time.

If it is not smoking I would say valves. Or ignition coil acting up when warmed up. Compression may be better spinning backwards because of built in decompression on the cam lobe for Briggs and a counter weight if Tecumseh. Compression has to be done when it quits because the valves are expanded and compression is lowest. Valve clearance is .008 intake and .010. exhaust. The book says .005 for Briggs but then you have to do the valves sooner so I make it a little bigger because it is the intake that looses clearances fastest. I also grind the intake sealing surface till it is flat with no gulley. Then some valve grinding compound to get a grey area all the way around. Then grind a little off the stem to get your .008 just past TDC so you are past the decompression part on the cam. Then clean all the compound off valve and block.
Oil the valve stem and install spring and keeper.
Check the head for flatness. .003 max. in low spots 80 grit on sheet of glass on a sturdy flat surface then 120 when you get less the .002 feeler gauge under straight edge. A little water on the 120 for final pass. Brush off sand paper frequently. use the proper tighten sequence and torque value for your particular engine.

I am guessing you don't have a fuel problem if it stats easily each time and runs 15.

does all of that still hold for a flat top engine? its not an overhead, and i read that an overhead you have to set the valves.. somewhere before or after the compression stroke because of some cam deal. and you where saying cam.

about your scooter and it cutting out, the idea I heard, that negative pressure has something to do with it, sounds a bit like a conspiracy to me.

I've noticed something before with a less free flow air filter on my elite. But never nailing down the carb completely. But, people do take the stock elite carb airbox and open up air holes in it. So the big story you heard, about negative air pressure I believe is a made up story. Since an elite, runs faster with a bigger main jet and an airhole made by cutting a tab off of the airbox. the airflow through the elites air intake on the airbox, is about or pretty much the same, the same, .. airflow as the stock carrburetor can do. So the story about negative pressure, Im not falling for it. Ive not heard of it. Ive not seen it proven, wrote anywhere. Ive noticed my problem, but never nailed nothing down, which doenst tell me nothing. heres a video about tuning a carburetor with the air filter off then on, and no rpm drop. That would tell you that its not restricting the flow of air that that carb can pull in, no negative pressure, https://www.youtube.com/watch?v=XFYaC1vncQA

So, I would think but not tested it, the compression which is why i said on mine. Yours I would just totally guess, may be running too hot on your bike, without an upjet of the main jet. i would still filter the dust but, its everyone own choice what to take. theres some guy diospeeddemon who runs a horn on the back of his carb. he has shown videos of this bike for years, with this. You don't see him showing rebuilds a lot lately. Who really knows but. he does it.

Briggs. Tecumseh, Clinton etc. had decompression even on their 2 to 3hp mower engines
in the 50's. It made them easier to rope start for women, kids, old or weak people.
I would generally set the clearance .008 to.010. Instead of the .004 to .006 in the books.
This gives a low compression worn engine more compression to start easier and it also
lengthened the time till next valve jobs.

Overhead valve engines need decompression because they have a much higher ratio than
flat heads. Instead of using a bigger starter and battery, they all have decompression to
make them easier to start. This is a big problem now because it is a common fail point on
OHV engines. I prefer the older flat heads 50's to 90's that have the intake valve on the
top of the exhaust. New ones have the exhaust on top due to some EPA regulations  B.S.  

The "A" holes on hondaspree rather rudely and crudely informed me that I can't use regular
logic with a spree and only they know what is best. There is a guy on y-tube that is
running up and down the street without a filter at 33mph. This is a highly modified rebuild.
He, like me, never heard of "a spree won't run without a filter". Those bastardized designs
should never have been. Even if the "A"  holes quote H.D and Bombardier etc. use that
bastardized system. I use what experience has taught me for most things.

ah.  they never changed the oil still in the mower, since the carb was leaking into the crankcase.  the compression is low, but yea ill get to that.  adjust the valves.  its a murray 11hp.  

Everything in that topic about your airfilter and scooter, had nothing to do with your scooter.  

They said EPA and filter at the same time to scare people into thinking, oh yea, well I'd probably get a filter, they might as well put BUY OUR FILTER because the EPA said so, and even if you got a filter, I bet the EPA knows its no good.  The EPA is not concerned one bit aobut the environment.  Non-trademark.  I have to say non-trademark, because they will next try to include global warming, blaming everyone for everything in their next arguement. yea. The epa has not one bit of concern about the environment, or how or IF your bike runs. That was a statment by that user to get credibility for all his other claims to give himself some thing to stand on.

I 100% believe that entire forum is ran, in a secret section that somoene linked to once, asking someone to explain something to them in there more.  They must of let people look at it but not join it.  I clicked the link and never seen this before or ever on that forum, it said, your not allowed to see this section or part? of the forum.  

I dont believe they let anyone say any information as things might be discussed and other methods to work or use these scooters will be brought out.  And its becuase they have their sellers and its so they can sell it with complete control of all the chat about the scooters.  I think other forums are ran in the same way.  They deleted my posts and locked that topic without explanation.  That guy who locked it too, is dying of cancer or something.  man must make you feel bad to help another person to sell things.  I also don't believe service is a viable living

i figured out what it was, its not my mower so. i dont know why i forgot about this. the blade safety switch is messed up and when its not pressed in, the engine shuts off.

i got a scooter problem. im trying to run a 17.5 mm carb on my stock elite with an aftermarket exhaust. i can make it idle nice. i have no idea whats going on. seems im thinking its got an air screw when its a fuel screw. it cuts out, unless i turn the screw to no more than like 1 turn out. if i go past 1.5 turns out then it quits. its hard to start.

my last 21mm carb i ran a 44 pilot jet, which is pretty big, on this stock motor. it started easy and revved back fine with no cutting out, wouldnt die. but i couldnt stop it from overheating. the needles wouldnt tune at all. i tried to richen and richen the 1/4 needle part, and it seemed to never stop jumping up and down at around 1/4 throttle. i think.. that i finally got it lined out like, with an entire set of needles, like at 1 notch from the richest ssetting they have on the entire needle series .including the needle notches.

like, they had 6 setttings of needles for 1/4 throttle. then 4 notches. so go from leanest, richen the clip 3 notches 3 notches more, then go up 4 of 5 series of needles. there it stops cutting out or jumping back and forth up and down at 1/4 throttle. but that leaves me with barely any 3/4 needle options. and at that point, it still overheated. i just gave up on that piece of crap. just forget it, too much stuff, i didnt know where to go.

i put a 17.5mm carb, pretty much sold for the elite on it. arreche. they fit the manifold. and airbox. i have the japanese version for the elite motor on mine. you8 can get a 16mm keihin pb carburetor they have adjustable pilot jets. have to have the manifold thou and airbox both to fit and look stock. they sell one manifold online but its reed cage i am sure is shitty. i think it can be replaced but u have to cut the holes to match better. they sell 19mm kehin pb too and they have the stock also 14mm ones, which i have that too. just wanted more carb since some bikes in this size have 16mm stock size carbs. its not too uncommon. so im using the manifold and airbox, and a 17.5mm carb and having problems with it.