H Backfire Problem

Lately, when I shut my 1951 H engine off with the kill switch I get a very strong backfire. What is the likely cause of this? This tractor has been running very well for a long time, so I don't understand what could have changed to suddenly cause this repetitive problem. Any tip you guys can offer that may help me solve this problem will be most appreciated.
 
Hot spots or carbon build up and cause that or you do not idle it down and let it cool for a bit before you shut it off. Best way to stop it and be sure it is not going to do it any more is shut the gas off and let it run till it dies and the back fire will not happen. One the ignition is turned off the engine still spins over for a few more turns which pumps raw gas into the cylinder and then out into the manifold where they hit hot carbon so you get a big bank
 
Old's advice is right on!

Set the idle as slow as it will go (450 - 500 RPM max).

Then when you are done with the tractor, let it slow idle a minute or two before hitting the kill switch. It will shut down quietly.

Or do as I do - shut off the fuel at the sediment bulb and let it run until it quits. Don't forget to turn off the ignition after it stops!
 
If you are running the tractor hard, let it cool down 5 FIVE minutes at 700-1000 rpm, then idle down to 450-500 rpm and push ignition button in.

You may have a leak in the exhaust manifold letting raw air loaded with oxygen into the manifold & muffler. Might have been caused by the pressure of the backfires.

Dad's Super M-TA used to backfire like that, also diesel backwards if shut off too hot. Let it idle 3-5 minutes and no problem.
 
If you are not running the tractor hard, then something else is causing it to heat up excessively. Running lean, timing being too advanced or retarded, can cause the engine to run much hotter than normal even under light loads.

Also tractors need regular maintenance. A tractor can run well for a long time, but if you do not perform all the regular maintenance you can only expect that suddenly without warning the tractor will start running differently than you expect.
 

If I've been working my 450 and forget to let her cool down before killing her for lunch she will diesel and pop some. Plowing with a Super M as a kid my boss told me to always let her cool down a few minutes before turning the key, but sometimes I'd forget and I can still see him shaking his head.

At the end of the day I'll turn off the gas and let her slowly stop on her own.
I hope this helps.
 
With a carburated engine, when you turn the engine off, you are only turning the spark off. As the engine slows down & stops, it's sucking in gas into the system. Hot carbon in the cylinder can trigger the explosion.

As others have mentioned, always let a hard working engine......of any kind (tractor, lawn mower, pressure washer, car)........cool off a bit at idle before shutting it off.

Your car does that pretty much automatically as you slow down from the road and slowly drive up the drive to the house.

Pete
 
the thing is ... you need the idle adjusted as slow as it will chug. less than 400 rpm. if that dont work then put throttle in idle position and backout idle screw on carb till engine stops on its own. this guarantee's that the throttle butterfly is completely closed,as thats what you want to prevent fuel being sucked into the intake. then when using tractor and idling it for a bit just close throttle and hit kill switch and it will stop. a very easy fix that only reqires a screwdriver!
 
The last military vehicle restoration I did had a 817 cu in Waukesha gasser with two big one bbl carbs. To shut it down you pull a knob that kills the ignition but it also opens a valve in the intake manifold that lets in air so it doesn’t have enough vacuum to draw through the carbs and can’t diesel. It won’t help your tractor but it’s an interesting concept.
 
I hope this comes out clear enough to read.
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Could also need the valves adjusted if they are not shutting completely can cause that problem. As well as all the other things listed.
 

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