All gas A shutdown

I have a 1948 A that my father bought new. The OM states to stop the tractor simply pull the throttle all the way back. That is how it worked from new (per dad) until I rebuilt carb in 2005. Our all gas B did too (both magneto ignition). I used the specified drill bits during rebuild. Now it just drops to a perfect idle when I throttle back. With all passages open, I do not see how it ever stopped the tractor with the throttle closed. Although I do not like any of the other shut down methods for brief stops, I am mainly just curious what is missing from the new design intent.

That is an excellent question and I wish I had the answer. I have a 1952 A that has a distributor that of course is shut off with the key. It will idle slowly but will not shut off with the throttle back. A friend has a 51 A with a magneto that will shut off with the throttle. You could always hook a kill switch to the magneto to stop your tractor. I hope some of the carburetor experts will chime in and tell us what is going on.

Check for a worn throttle plate and bushings. If the plate is worn it will let air flow by and also pull fuel into the engine.

The gas only "A" will, and is designed to, shut down with throttle all the way back. The throttle plate, if all is adjusted correctly, will completely cover ALL the bore ports for fuel, shutting it down. If it did it before your re-build and not now , and , assuming you didn't change ANY linkage adjustments, let me ask you this. Did you replace the throttle shaft? There are some junk shafts out there (e-bay) without the correct throttle disc to lever orientation which could prevent the disc from covering the very last bore port to shut it down. In a good quality shaft, the ports will be all hidden by disc when top lever is against the stop. Make sure whatever you used for a keeper pin in the rod you put into top of throttle lever on carb is put in from clutch side to flywheel side so head of pin is towards clutch. Otherwise the head will wedge against the bushing casting boss and prevent correct and free shaft movement. I assume you are aware that a good re-build will also include a new throttle disc? They are always worn out on the bottom and lets it sit low in the bore and leaks air all around the top....acts same as throttle not shutting completely. Take it out and you will see the "step" worn on bottom. No other reason it should not shut down with lever all way back. Let us know what you find.

Essentially a worn throttle plate will make the engine run like the throttle is in the idle position when all the way back.

A lot of farmers got tired of their kids or themselves inadvertingly shutting down the tractor so a lot of them bent the stop shaft slightly to prevent this....was a bad ideal as this was the only safety shutdown feature...if tractor was idling and power called for the governor would open up....now the all fuel would not shut down as they had a bypass port to prevent shutting down and fouling the plugs...

50+ years of killing our Gasoline B's has not hurt them...

"Safe emergency shut down"?..just pull out the Choke..!

Thanks RB. The correction must be one or a combination of the items you listed. The tractor otherwise runs excellent. It will likely be winter before I check and correct as you described. I am also in MI.

It looks like a poor idea on a distributor tractor as one would forget to turn the key switch off and burn up the points.

Yep proper shutoff procedure depends on whether you have a mag or a distributer. I have a 1950 JD A and the factory oerators manual.

Mag:
a) JD operator manual specifies the proper procedure is to turn the gas off using the dash lever and let the tractor die out that way. (IMOP: This is most important on a dual fuel as it drains the carb bowl and it will be ready for an easy start on gas next cold start.) Regardless the manual specifies the same procedure for gas only as dual fuel. I do it this way when a rapid kill is not required.
B) When I need a fast kill then I simply pull the throttle all the way back.

Dist:
It is important to turn the switch off

I can assure you that if you are in trouble and panicing to find a quick shut down you're not looking to take your hands off steering wheel and bend over and turn a key off. You might grab for the throttle and this is why they had the throttle shut down. You could shut down with a key or magneto grounding key under normal circumstances. But like I say a lot of folks bent or cocked the stud over just a little to prevent a shut down. As they came from the factory the butterfly would completely close and cut off fuel. I'm not exactly sure what the choke would do if the butterfly was completely open. I think some of the tractors had a choke spring that would make the choke partially open on start up. But I can tell you from my experience underload, it doesn't matter where the throttle control lever is set while running as rpm lowers that butterfly will open. It will go full throttle from an idle position if it is underload. This is why folks go it trouble as they couldn't get away from the governor while they were in trouble. Those engines have so much torque that they will start climbing rpm when loaded up. They are not likely to stall out. From low idle to high idle is not very different in rpm range.