2 stroke question

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Bill
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2 stroke question

Post by Bill » Sun Feb 12, 2006 2:37 pm

Just wondering if someone can help me resolve a question about 2 strokes engines.

Has anyone experienced the strange effect when either by crashing and having the bike still running on it’s side, or when they sometimes begin to run out of petrol the engine goes into a strange runaway over rev which you can’t stop by the kill switch or even taking off the plug lead. The throttle is closed no petrol in the carb but the engine suddenly revs it’s self to death sometimes.

I gather that a hot spot in the engine keeps it going without a spark but where does the fuel come from to keep it revving it’s nuts off for a couple of minutes.

Over the years I’ve read about this and seen it myself on a number of bikes including a bsa bantam, puch mopeds and some trials bikes, but try to tell someone this and you get that “aye right” response.

Does anyone know for sure the process involved so that I can resolve an argument elsewhere?

I'm hoping that maybe Mark, Stan, Ed or someone else could explain it fully.




Bill.

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kommando
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Post by kommando » Sun Feb 12, 2006 3:17 pm

Never had a 2 stroke but when I worked at Rover the MG metro Turbo had to have a fuel cutoff valve added to the fuel feed to the carb to prevent running on, the fuel was being ignited by a hot spot which took the place of the sparkplug so carried on ticking over with the ignition off.

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Post by Bill » Sun Feb 12, 2006 8:01 pm

Yeah Kommando this is what I want to get at, not all bikes will do this as it seems to require a hot spot from a few exposed spark plug threads or something similar. But once started what keeps it going, after all it’s running out of fuel or down on its side so fuel can’t get in but it revs like crazy.

Some have suggested vapour from the tank but nothing conclusive. Maybe I need to pick up an old moped and do some experiments.

Bill

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Post by DudeFromPhilly » Tue Feb 14, 2006 1:08 am

Couple of things. First, this revving is done with no load right? So it actually uses alot less fuel to wind out than under load. Second, The crank case is a chamber full of unburnt air fuel mixture. After you close the throttle, you still have that area to use up. Granted they try to make it as small as possible and fill it with the flywheels so there's little extra space to act as a reservoir but there's the whole under side of the piston along with the space that needs to be open for the connecting rod. Next, once you close the throttle, the engine can only get a percent of that charge each stroke, so there's always some left for the next stroke. Finally, It's not just a hot spot. An established technique for designing a deisel is to shoot a charge of the exhaust in to help ignite the fresh charge. When you close the throttle, you get more kick back from the exhaust. Any of this help? :grin:

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Post by Bill » Tue Feb 14, 2006 5:26 am

Ok I follow that dude, so if you were to open the throttle when it starts the fresh intake of air should flush it out and stop the revving.

Ok now to find a victim and put this to the test.

Cheers

bill.

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