Dies After 45 Minutes Running

[Back44L&G]

".........the fuel tank is clean and rust free (I only use real gasoline, not car gas)"

All metal fuel tanks can/will rust.
All fuel tanks "sweat" ie condensation develops . This is especially so in climates that have a wide daily temperature range, at certain times of the year. The condensat collects in the bottom of the tank. Plastic tanks will not rust of course however metal ones will - it may take along time but nature will take its course.
The only long term preventions are:

• Drain tank regularly (at least once per season)
• Keep tanks a minimum of 75% full

Water may be worse with ethanol blends due to the hygroscopic nature of the ethanol.

I have used non-ethanol mixed with Sta-BIL at the time of purchase for the past 30 years with no fuel issues. I mix it with an extra measure of the stabilizer for the stated two-year life which lasts about 18 months to turn over my supply. Non-ethanol without any stabilization will not last seven years. That’s wishful thinking as it’s only good for about sixty days. All of my equipment is stored in a utility barn and not left outside in the weather. Since using Sta-Bil, I have not experienced any carburetor problems.

 

[Goby1234]

Probably the fuel pump. They get weak with age and display those symptoms

 

[Catenary Kid]

Many years ago I had a Simplicity mower with the same problem. For years it would run great until it got good and hot (usually about 2 hours, depending on ambient temps) then just quit. Nothing I did would make it run until it cooled down, when it would start and run just fine for exponentially decreasing times until it quit again. I tried everything with no success until a mechanic acquaintance told me it was vapor locking, which I found was indeed the case. I finally solved the problem by trading it in on a new mower.

 

[Back44L&G]

Many years ago I had a Simplicity mower with the same problem. For years it would run great until it got good and hot (usually about 2 hours, depending on ambient temps) then just quit. Nothing I did would make it run until it cooled down, when it would start and run just fine for exponentially decreasing times until it quit again. I tried everything with no success until a mechanic acquaintance told me it was vapor locking, which I found was indeed the case. I finally solved the problem by trading it in on a new mower.

About six years ago, I had a vapor lock issue with a 2007 Troy-built pressure washer with a Briggs & Stratton 8.75 HP engine. It would run when it was cold but when it ran out of fuel and was refilled, it was always hard to start. I called the local dealer and was advised that B&S had redesigned the gas caps. I ordered new gas caps for all of my B&S engines and haven't had any more issues. Some years ago, Honda had a similar problem with their walk-behind mowers. There are several videos on YouTube on how to retrofit Honda gas caps so they vent better.

 

[SamB]

".........the fuel tank is clean and rust free (I only use real gasoline, not car gas)"

All metal fuel tanks can/will rust.
All fuel tanks "sweat" ie condensation develops . This is especially so in climates that have a wide daily temperature range, at certain times of the year. The condensat collects in the bottom of the tank. Plastic tanks will not rust of course however metal ones will - it may take along time but nature will take its course.
The only long term preventions are:

• Drain tank regularly (at least once per season)
• Keep tanks a minimum of 75% full

Water may be worse with ethanol blends due to the hygroscopic nature of the ethanol.

I am not 100% sure , but Snapper didn't have anything but polyethylene gas tanks, like well....forever.
If your Snapper is an RER, It is worth getting running again.

 

[TobyU]

Briggs and Stratton Engine 12H802-2677B1 Engine on a lawnmower starts and runs. However, after about 45 minutes of running full speed, depending upon the outside temperature, it will die and not start until it is full cooled off.

I have;
• de-carbon the head and top of the piston
• buffed the rust & corrosion off of the armature / magneto and the flywheel; gapped & reinstalled

But it ran the same.

I suspect a weak armature / magneto because the engine is 24 years old.

Am I heading in the right direction?

Okay, first of all stop taking heads off of engine to decarbonize anything because it's just a waste of time. Lol
Secondly, always diagnose a problem before you do anything to it or change any parts.

First thing you should determine when this happens is whether it doesn't have fuel or spark because there's a 99% chance one of those two things is the culprit.
It's actually more like 99.9999%, but occasionally you will find one where when it warms up or when the sunlight gets the handle hot, the kill switch cable moves just enough to tickle the kill switch but like I said it's rare.

So get it hot and wait till it dies and then give it a little bit of gas into the carburetor I pushing the primer bulb if it has one or a little bit of carb cleaner spray in there or dribble a teaspoon of gas into the carb and try it again.
If it runs at all you know it's fuel but my guess is it will not because it sounds like you have a coil on its way out but they typically will not run 45 minutes until they fail.
I run 12 to 18 and that's like clockwork.

Regardless, then pull the plug out and get another used plug or whatever you have or another known good plug and check for spark.
If you don't see a nice snappy spark when someone pulls the rope and you hold the plug and the wire next to the block then you can almost bet it's coil.

But then before you replace that, take the small kill switch wire off the coil and check for spark again because sometimes you have a bare spot on a wire and when things vibrate and get just warm enough it will pinch that wire just enough to kill itself out and then once it cools off it will move a little bit the other way and be okay.
Temperature and vibration do a
weird things.