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I have a rider mower I purchased new in 1998. It has the Briggs and Stratton 14.5 HP I/C Quiet engine. This mower will run smoothly and mow my yard fine for about 40 minutes, then it starts to sputter and try to die out. I can play with the choke and get it running smoothly for another few minutes, then it sputters again. Play with the choke again and it runs fine for a few more minutes. Finally, it dies out and will not start again until it cools down for a couple hours. I have put every new part into this I can think of: starter, solenoid, magneto coil (properly gapped too), spark plug, gas cap, fuel lines, fuel filter, carburetor, air filter, you name it. The gas tank is semi transparent and I see no dirt or soot in the bottom. I have made sure there are no grass clippings stuck in the head cooling fins. What is causing this?

The local spares man must think you are a god.
Start with an inline spark tester.
install it and run it till the engine starts to play up.
If the misses coincide with no flashes then you have an electrical problem
If the tester flashes the same while the engine is dieing then you have a fuel problem.
The instant it stops spray some starter fluid down the carb and try to start it.
Starts & runs for a few seconds , confirmed fuel problem.

Do this and get back to us with your results.

If you don't turn up anything above, consider sticking valve. Try some top cylinder lube such as Marvel Mystery Oil per instructions on the container.

Walt Conner

Thanks for the replies.

I have already tried the starter fluid trick with no success. Next step will be the inline spark tester. If it were electrical, why would it purr like a kitten until about 40 minutes in every time like clockwork though?

atonys said:

Thanks for the replies.

I have already tried the starter fluid trick with no success. Next step will be the inline spark tester. If it were electrical, why would it purr like a kitten until about 40 minutes in every time like clockwork though?

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When it gets hot enough it stops working.

Here is the result of the test. I mowed tonight a little longer than normal, almost all my yard, by adjusting the choke multiple times to keep it running. Once it stalled out for good, I installed the inline spark tester. Turned the key and the light pulsed in time with the flywheel. Next, I pulled the breather and cover, sprayed starter fluid into the carburetor, and it started and ran great. In fact, I put the breather and cover back on and finished mowing the lawn. What do we make from that?

atonys said:

Here is the result of the test. I mowed tonight a little longer than normal, almost all my yard, by adjusting the choke multiple times to keep it running. Once it stalled out for good, I installed the inline spark tester. Turned the key and the light pulsed in time with the flywheel. Next, I pulled the breather and cover, sprayed starter fluid into the carburetor, and it started and ran great. In fact, I put the breather and cover back on and finished mowing the lawn. What do we make from that?

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Random fuel blockage.
Most likely debris forming a loose plug in the outlet of the fuel tank.
Remove the fuel cap and blow backwards up the fuel line.
I remove the fuel tanks sit them upside down and blow then out with compressed air.
Can also be failing fuel lines.
Modern fuel softens the butyl rubber linning making it go soft & sticky.
A mower of that age could do with the entire fuel line replaced.

Have we rulled out a blocked air vent in the fuel cap ?

Brand new fuel lines, gas cap, and fuel filter were installed this year. Could this be valve related?

If you can keep it running by manupliating the choke then that points to fuel or more to lack of fuel.
SO we have
1) not enough fuel getting to the carb
2) float level too low
3) air leak between carby emulsion tube and engine

I completely removed the fuel tank and fuel line (new this year) yesterday. I blew everything out with compressed air, but there was no internal dirt present to begin with. As an extra measure, I took the fuel tank and cap into the kitchen and gave them a good scrubbing in a sink full of soapy water. After everything air dried for an hour, it is all back together. Next test will come in a week or so when time to mow again. I suspect it will act the same. If so, steaming the carbon off the valves with water dribbled into the intake will be my next step.

atonys said:

I completely removed the fuel tank and fuel line (new this year) yesterday. I blew everything out with compressed air, but there was no internal dirt present to begin with. As an extra measure, I took the fuel tank and cap into the kitchen and gave them a good scrubbing in a sink full of soapy water. After everything air dried for an hour, it is all back together. Next test will come in a week or so when time to mow again. I suspect it will act the same. If so, steaming the carbon off the valves with water dribbled into the intake will be my next step.

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Way back in the olden days this was common thing to do the two strokes before we got special 2 stroke oils that do not build up heavy carbon deposits.
Use a trigger spray bottle set to the finest spray you can get.
Start the engine and run it for a good 10 to 20 minutes to get it fully hot.
Remove the muffler
Spray slowly with the engine at full speed and when you have finished run the engine for another 10 minutes for the engine to develope a replacement oil film on everything.

I give you a 50:50 chance of pulling this off without blowing the head gasket , cracking a ring of IF there is a carbon build up ( i seriously doubt that ) getting a blob of of carbon stuck under the exhaust valve.

Modern engines run really lean now days so finding a carbon build up usually means a blown gasket , leaking inlet valve seal, bad rings or blocked breather.