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A couple observations: 1 – The SMTA engine can be prone to running on – especially if fitted with high performance pistons and/or an LPG head. The lower quality unleaded gasoline we get these days doesn’t help. Solutions include setting the slow idle RPM as low as possible and making sure the carb idle mixture is set properly. Sometimes switching to a lower heat range plug – or changing plug brands – can help. Fully close the throttle then wait until the RPM settles before hitting the kill switch. And if the engine has been working hard let it slow idle for 2 – 3 minutes before shutting down. Finally using premium gas can help. But fuel is too expensive as it is – I’d rather let it run on a few seconds or drag it to a stop with the clutch than pay extra for 93 octane! Also keep in mind that while running on may be annoying it really doesn’t hurt anything. (Incidentally ignition timing has no effect whatsoever on running on. Once the ignition is turned off, “ignition timing” is a meaningless concept….) 2 – Backfiring out the exhaust under sudden no-load throttle closure (which also happens while going downhill with the wheels driving the engine) is pretty much normal. However if backfiring is excessive look for timing out of spec (set too late) or an inoperative centrifugal spark advance inside the distributor. 3 – Finally a single, shotgun-like report up the exhaust after the engine finally stops rolling on shutdown is not unusual either – consider it part of the “charm” of an running old tractor! (Though I’ve had it split a muffler shell before…) It can be minimized/eliminated doing the things listed in #1 above.
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