20" Gas lawn mower runs for 20 min - stalls out

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Today I was cutting the lawn and 20 min into it, while stationary on level ground, it just stopped running, like it ran out of fuel. Tank was 2/3 full though! It did this 2 weeks ago but since it was close to being out of fuel, I figured that was the cause. It also fits with another symptom I've noticed over the past several mows, the mower is not quickly getting up to running rpm once it starts. It barely turns over, then over the next 3-5 seconds it sputters up to speed, then acts fully normal. For the past 2 years this Poulon Pro mower has run fine without any of these symptoms. It's 2 years old now and still on all original parts and filters.

B&S 126 cc engine. No choke. Primer button only. Fuel is about 6 weeks old. Considering I've run this mower fine on 6 month old fuel several times in the past, I don't think it's the fuel. This same fuel was fine 4 weeks ago. Another "issue" is that I'm pretty certain the "control run/start bar" has changed position a bit. It doesn't seem to get any resistance until it's almost fully depressed. It didn't feel like that a month ago. Still, it starts and stops via this lever w/o a problem. I don't think that's related to the stopping issue. Been running a little bit of Techron on every fill up this season to help keep the carb clean.

So what are the MOST often seen causes the would shut down the mower after running perfectly fine for 20-30 min, once it gets hot? It will restart, but only run 1 minute the next time, and 5-20 seconds after that. Here are some things I found on line to check - though I've never had any issue on any of these. Never changed them either, never needed to. Change oil...clean air filter...fresh gas...and go.

Spark plug? Clogged gas cap vent? Too much oil? (no, it's just right) Clogged air filter? (not likely, but I washed it very thoroughly anyways for the next attempt). Carb (air/fuel ratio)? Anything else?

This is a first for me. I've had these little inexpensive gas mowers for 20 yrs and the only failure mode in the past has been hard starting to no starting. Any help would be appreciated. Not a fan of pulling apart the carb as the last time I did that, the mower never even started again.
 
I would check the ignition. It may be on the fritz. Getting hot and cutting out on ope says ignition. Maybe change the plug to see if it helps lesson the load on the coil. A new plug gapped has the least resistance and should be easier to fire on a weaker coil. Just a guess on replacing the plug...start cheap
 
I'll check the spark tomorrow/just replace it. I probably have some new Champion car plugs out in the garage from previous cars (reasonably interchangeable?)

I've never needed to change a plug on one of these in 20 yrs. A new plug never made one run any better. Figured they were bullet proof for how little use they get. The electrodes always look good. Only 2 yrs old too. Considering I can instantly restart the mower after it dies....suggests there's ample spark. The sputtering and slow startup rpm might be indicative of a weak spark. Though wouldn't it sputter and hunt and full rpm if that was the case? I have no issue with running rpm being nice and smooth....until it just dies like it ran out of fuel?

Float bowl? Haven't resigned myself to looking at the carb yet. Does that bowl unscrew easily without ripping the carb out of the system? I've played around with the various internals on automobile carbs on 1968-1970 models (muscle car tuning), which were no problem. Not sure why lawn mower carbs didn't work for me.
 
Originally Posted By: Syntheticuser
I would check the ignition. It may be on the fritz. Getting hot and cutting out on ope says ignition. Maybe change the plug to see if it helps lesson the load on the coil. A new plug gapped has the least resistance and should be easier to fire on a weaker coil. Just a guess on replacing the plug...start cheap


I think you are on to something. I had a string trimmer that would do this and it ended up being the spark plug. The owners manual calls for a resistor type spark plug and some dope (me) had installed a non resistor plug that said it was interchangeable. If it you are using a Champion plug, make sure the part number starts with an R. If you are using an NGK plug, make sure it had an R below the NGK logo. Other brands I am not sure how they identify a resistor type spark plug.
 
Originally Posted By: WobblyElvis
Re torque the head bolts, might find the bolts a little lose.


Once I find them. I'll give that a shot too. Thanks.

Learned about spark plug "reach" tonight. Never knew that...or what any of these letters/#'s designated. Now I know. Neat.

RC12YC is what would work on my B&S.
 
Car plugs cannot be used in a mower engine. Can be used to test for spark though.

Change the air filter anyway, and the gas filter if it has one.
Does filling up the gas tank fix problem?
 
Check for spark immediately when it dies. Maybe magneto is breaking down. Give it a shot of ether and attempt to restart. If all checks out, valve clearance could be out of specs, especially when everything is warm.
 
Had a Tecumseh carb recently that would run from 5 to 20 or so minutes, with all the bouncing around would stir up crud in the tank, eventually clogging the main jet passages.Finally cleaned the tiny carb orifices correctly and solved the problem.It is an Eager 1 6hp. Its a late 90's Sears mower that runs with great power.Brand new carburetors are being sold on E-bay for cheap these days, cheap enough to buy a spare.
 
Thanks for the tips. The answer must be in one of those. Let's hope for one of the easy ones. Today, clean air filter, new spark plug and check/clean/remove vent cap.

A failing magneto? Didn't even know it had one.
 
Originally Posted By: dwendt44
Car plugs cannot be used in a mower engine. Can be used to test for spark though.

Change the air filter anyway, and the gas filter if it has one.
Does filling up the gas tank fix problem?
My Honda mower takes the same BP6ES plug my SAAB 900 took.
 
Originally Posted By: wayne50
I second ammolab and that's the easiest to check first anyway.


When it stopped yesterday after running for 20 minutes, I checked the fuel level. It was fine. That should have relieved the built up pressure. If it is the vent, how did that pressure build up quickly (1-3 minutes) when it stalled out again on the 2nd try? Going out shortly to play with it again.
 
Does it have a small plastic diaphragm fuel pump.
If so... they are junk. My JD Briggs acted the same way.
I now keep an extra on hand.
I also find adding a bit od 2-stroke oil with each fill helps
prolong the life of the Chinese made pumps.
 
Originally Posted By: 69GTX
Originally Posted By: wayne50
I second ammolab and that's the easiest to check first anyway.


When it stopped yesterday after running for 20 minutes, I checked the fuel level. It was fine. That should have relieved the built up pressure. If it is the vent, how did that pressure build up quickly (1-3 minutes) when it stalled out again on the 2nd try? Going out shortly to play with it again.


Sounds to me like an ignition coil failing. Starts to fail once it gets too hot. If it was the fuel cap, opening the cap to check the fuel should have relieved pressure and allowed you to mow for 20 more minutes or so.
 
Replaced the spark plug with the identical brand and model. The old one looked fine except for a little oil blowby past the threads on to the ceramic portion of the plug, obscuring the part #. Gaps on both approx 0.022. While I had the lid off I cleaned up all debris around the High Voltage module, fuel inlet/pump/carb, exhaust manifold. It was all pretty grungy. That could have helped retain unwanted heat.

Regardless of all that, it's now hard DOWN. Started for 1-2 seconds with improved RPMs and died almost instantly. Tried again and no joy. No run. Just a brief "cough" without much amplitude. Pulled the spark plug out to see if I could see a spark while cranking (my wife watched for this...saw no spark). The obvious choice here is now failed ignition system. If it were the carb or fuel pump, how did it run for 20 minutes yesterday....only 2 seconds today?

I hope the OHV module doesn't cost more than $35 or so, as this 2 yr old mower is worth no more than $75 now ($168 with tax). At $85/yr, it's almost cheaper to just buy a new one. And I get a new carb, starter, engine, chassis, etc. The guy that sells these up at the local hardware store says he buys a new one every year. His wife does the mowing. And he doesn't want to hear a single complaint about hard starting or stalling out....lol. This Poulon Pro ain't no "pro." Then again, its the Briggs and Stratton 126cc engine that is the problem.

Any other thoughts before I research the price and availability of the OHV coil assembly? Look like it just unbolts pretty easily.
 
Engine ignition trouble shooting

I'll read up on this later. Could even be the rotating magnets on the fly wheel.

Dumb question, when I removed flywheel/engine cover, does that require a specific alignment to ensure proper timing? I just plopped it back on with appropriate pull cord tension and screwed it down.

A trip to a B&S/small engine dealer could easily cost $50-$100 in labor. A new mower would be preferable.
 
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