On the old Guinness canal boats, used to transport the famous beverage bearing the company name, the skippers could reverse the Bollinder single cylinder diesels by closing the throttle(though throttle isn't really the right term), until the engine was almost stopped, then "blipping" the throttle at the right moment. These engines only had a forward gear, and would run their bearings dry if left running backwards for long.
Many Years ago (more than I care to think about) A friend of mine was tweeking the ignition timing on his suzuki 250. Advanced it a little bit, started it, cracked the throttle and dropped the clutch. Did this a number of time - happy with the perceived increase in acceleration until he just went a little too far.
He started the bike (kick start of course) and received a nasty kick back to boot. The bike was running through, so he goosed the throttle and dropped the clutch. The next moment he was lying on the ground,hands cupped around a fragile and now very painful part of his anatomy.
The engine had started in reverse - need I explain further....
It is entirely possible you are putting us on. However...
Reed valve 2-cycle gas engines can run backwards and forwards, depending on the spark timing. Very small Cox Model airplane engines, for instance, run equally well in either direction and it's not un-common for them to start backwards as they come up on compression, bounce back without going over TDC (top dead center), and then fire and run in reverse.
I guess that you almost stalled the engine starting out, the piston didn't quite make it past TDC, and the plug fired just about the time the piston stopped. The piston was then pushed down and the engine began running backwards, with a delayed spark being the only symptom. You were probably too busy to notice the lack of power.
I suspect your little 50 cc engine was a 2-cycle engine. If for some reason the cranckshaft began rotating in the reverse direction - perhaps because of a near-stall on acceleration or a kickback from early ignition - you would have everything reversed. Bingo - your one-speed forward would become a one-speed reverse.
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