That is due to some unburned fuel going out of the engine after the ignition has been shut off. The innards of the muffler are still hot enough to ignite these gas fumes, but only after the engine comes to a stop.
It is possible that your engine has an anti-afterfire fuel solenoid valve that has stuck in the open position. If the carburetor has one or two wires leading to a small cylinder shaped thing on the float bowl, it has the anti-afterfire valve and it is likely not closing when you turn the switch off.
You can try letting the engine idle for a couple of minutes (time it to a true 2 or more minutes) which will cool off the muffler (as well as reduce the amount of fuel going into the engine) before you turn off the key or rise from the seat.

125 or L 125 or LA 125.
A 125 is listed as a skid loader with a Continental engine.

Assuming you have an X125 model JD.

As Mownie stated. the "after pop" is caused by unburnt fuel passing into the hot exhaust after the engine is shut off. It is nothing to worry about and is normal. I have three Cub Cadets from the 70 and 80's that do it.

May try shutting the engine off at 1/2 throttle. Sometimes it helps.

My JD tractor model is 125. You are right. If I do not immediately shut down the engine after mowing the lawn, there is no pop sound. But I do not understand why the new tractor does not have such a problem after it mows the lawn and shuts down the engine immediately. Why does the old tractor have such a problem, loud pop sound after shutting down the engine? WhatâÂÂs the difference in the problem occurrence between the new and old tractor? Do I have to change some parts to solve the problem? Can you explain it? Thank you.

READ mownies response!

Newer engines have a fuel shut off solenoid that shuts off the fuel flow when the engine is shut off.

Older engines were fed fuel by the vacuum formed inside of the combustion chamber by the piston. When the key is shut off the engine keeps spinning and cycling just as it did with the key on EXCEPT their is no spark at the plugs to ignite the fuel/air mixture. That mixture passes through the exhaust valve and collects in the exhaust pipe and muffler. Then ignites from the heat inside of the muffler.

Basically the same thing that Mownie wrote.

I had a Craftsman that did it. On mine just had to throttle it down to a idle for minute or so before shutting off.

Is it possible get ride of such a loud pop sound problem by change of some parts? Since the new purchased tractor does not have such a problem, I am think some parts may be need to be replaced so that the tractor can work like the new purchased tractor without a loud pop sound after shutting down the engine immediately.

Do you understand the suggestion about letting the engine idle real slow for a minute or two to see if that is all you need to do?
Of course you can put some parts on it that would upgrade it if it never had the parts to do this before.
But you may think that would be cheap and easy.
If it never had the parts on it ever, you would need the following parts.
If your present carburetor is made so that it can be fitted with the fuel solenoid, you would have to buy a fuel solenoid made for the specific carburetor.
If your carburetor cannot be fitted with a fuel solenoid, you will have to buy a carburetor that features an anti-afterfire fuel solenoid valve, and the carburetor must also be compatible with your engine.
And either option you can choose will require additional wiring from your key switch that will activate the fuel solenoid when you start the engine and also deactivate it when the key is turned off.
You will have to get the work done at a professional shop unless you are very mechanically inclined, and I'm not getting that vibe.

This post was edited by mownie on Sat, Apr 12, 14 at 0:43

I know letting the engine idle real slow. Then there is no pop sound when shutting down the engine. But, I am comparing with the new purchased tractor. Since the new purchased tractor does not have to let the engine idle real slow but no pop sound when shutting down the engine, there must have some parts no longer work effectively as the new tractor does. I think I may need to replace some parts. Thus, the tractor will work as the same as the new tractor does without the loud pop sound when shutting down the engine. If someone knows to replace the parts can resolve the loud pop sound when shutting down the JD 125 tractor engine, please share.

You can't compare with a new tractor.

Was there a time that it didn't "pop"?
IF So, something's different.
When's the last time the cooling fins have been cleaned? An overheated engine will have a hotter muffler.

I don't know if the core of what I have been trying to say is clear, or not. But Bill did hit on a different way of saying it.
Once a long time ago, there was no such thing as an anti-afterfire fuel solenoid on any brand of outdoor power.
Back in those days, everybody's outdoor power engines might pop if you shut them down without a short cool down idle period.
Some engines and applications did not pop as much as others, due to different configurations of the exhaust muffler or length of exhaust pipe.
Then suddenly, back in the late 1980s and early 1990s, the anti-afterfire solenoids began to appear.

So, the only way to solve your problem is to have a true professional, well experienced, small engine technician/mechanic evaluate your old tractor and tell you whether or not the tractor ever has an anti-afterfire solenoid on it.
If you can remember a time when it did not pop when you shut it down, there may be hope that there is something faulty with an existing solenoid.
But if the tractor never was equipped with the anti-afterfire solenoid valve system in the first place, then you are going to have to spend some money to have it retrofitted to behave like a modern tractor.

It's kind of like if you want a 1935 Chevrolet with air conditioning on it, you are going to have to pay someone to put it on there because they did not have air conditioned cars back then.

Thanks for the trip down memory lane mownie. I had forgotten about the retrofit air conditioners for the 1950's that had the evaporator attached under the dash. A real knee-knocker. Or the cooler devices that you filled with ice and ran a fan off of the cigarette lighter. Those were the days.
OP, I was once told that putting a new exhaust on might solve the problem. Cheaper to let the engine idle a few minutes before shut down. Plus isn't a cool down recommended for engine life anyway?

Yah the 11 hoss B+S I/C flat head does it . ,Has been doing it since I got it free back in '01. Thanks for the explanation. Idling it down doesnt really help. But now I'm gonna worry about it even less than I did before. :)