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simonc

help needed with 300 2 stroke

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Can anyone give me some insight into a problem I had today.

2011 beta evo 300, been running great all morning, kicked it over after watching some guys doing a section and it just revved to full throttle, pulled kill switch, still revving, pulled plug lead still revving. pulled fuel pipe and by this time someone had come over to help, then it  finally  stopped.

Guy suggested I pull the plug out to check, after a long push to the van pulled the plug and it looked fine, so loaded up and went home with expensive  repairs in mind.

Got home, refitted everything, kicked over and started fine, did this multiple times and all seems fine.

So what do I need to be checking for/looking at as something was very wrong but I am new to two strokes so any advice greatly  appreciated.

Thanks in advance

 

 

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The only thing that will make an engine run away like that is something that holds the throttle slide in the carb open. A couple of common scenarios occur, 

Throttle tube sticking due to a fall or grip rubbing on end of handle bar. You may have noticed a lot of riders use "bar ends". Those things that sit in the hole at the end of the bar and require you to cut open the ends of your grips. These are popular for two reasons, First is you tend not to rip up grips as quickly. Second, in a fall the bar end takes the hit and you are less likely to mash the throttle tube further into the bar or even damage it so it doesn't turn freely. I highly recommend you get a set of bar ends and open the end of your grips.

The other thing that is common with any bike is the cable getting pulled out of either the throttle housing at the top or, in the case of the Beta, the "noodle" that comes out of the top of the carb. Here's where it's important to get into the habit of  before you start the bike twist the throttle and let it go and listen for the "thwock" sound that comes from the carb as the slide hits the idle adjust bolt. If you don't hear it DON'T start it. Fix it by making sure the grip is loose and the cable hasn't pulled out. Make it a habit. I don't even think about it anymore I just do it by reflex and it's kept me from the full throttle start a few times. It also has the tendency to seat a cable that isn't fully seated if you twist to the full throttle position.

Now as for not being able to kill a runaway engine at full throttle that is a fairly common experience. At that speed there is more than enough heat and free radical chemical compounds to keep an engine going without any spark. As for pulling the plug wire, I did it once. Never again, I actually felt my heart stop until the engine wound down. Scary as S***. As the wise folks above said stuff something over the exhaust. A glove a boot whatever. It will usually kill or slow down the engine enough for the kill switch to work again.

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Water /moisture on the throttle slide can freeze, holding throttle on, plating on slide can flake off (ask anyone who ever owned Honda CR). Both these examples usualy result in you being spat off, then when you pick yourself and the pieces of bike back up and blip the throttle, the engine ticks over as if there's nothing wrong, with no evidence of a fault. Till next time....... 

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You should pull the slide apart and make sure the retaining clip/piece for the needle is correct. Also the needle could be damaged. Better find out now before another exciting ride.

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Hi,it sounds to me like the engine could have been flooded and was running on the fuel in the crankcase,had the bike been laid on its side or is the float valve sticking or float height incorrectly set? When it stopped it had probably used up all the fuel that was in there.

Edited by topline620

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Ok guys think this through. Keihins don't have plated slides, just anodized aluminum. He parked the bike to walk a section so the engine was warm so no ice. A bike can't run on fuel in the crankcase without air mixed in the proper ratio. The bike ran well before and after so the needle holder and needle jet were in correctly. It is possible to put the needle jet in upside down  in the Keihin but the bike won't run. That leaves dirt in the slide causing it to stick to which the Keihin is very resistant. Strong spring, small slide.

Pretty much narrows it down to throttle sticking or cable pulled out. Both fairly common. All you have to do is momentarily catch a piece of clothing on the throttle cable to yank it out of the twist grip or carb noodle not to mention the excitement of hooking a branch on the loop trail. On the Beta you also have the potential of pulling the cable out of place by moving the wire bundle that comes up through the top plastic cover.

I don't think this is anything unusual Simon. You just hooked a cable and now it's got you a bit freaked out. Happens to everybody at some point. 

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Hi guys, thanks for the response's, I have had the cable get stuck before but was able to shut off via the kill switch, what I cant work out is why it was still revving flat out even after I pulled the spark plug lead off!

would it be worth having a look inside the head for carbon build up ?

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8 hours ago, simonc said:

Hi guys, thanks for the response's, I have had the cable get stuck before but was able to shut off via the kill switch, what I cant work out is why it was still revving flat out even after I pulled the spark plug lead off!

would it be worth having a look inside the head for carbon build up ?

Dumb luck. Conditions were just right to keep it dieseling along. There could be a weakness in your kill switch or wiring. When an engine is running full blat the ignition coils are putting out significant current and any resistance in the wiring that shunts the current to ground will keep some current flowing into the CDI. This is why I like to run the ground wire of the kill switch back to a hard ground on the frame. Usually one of the screws that mount the ignition coil. Years ago most bikes had a one wire kill switch. Ground was achieved by shorting to the handlebar. Good in theory but bad in practice because a runaway engine would pump out enough current to overwhelm the contact through the steering head bearings and not only not shut the motor down but electrify the handlebars in the process. Was great fun in the rain since hundreds of volts occur in the primary ignition circuit.

Take a look at the wiring for your kill switch and if the ground doesn't come from a hard point on the frame you need to correct it by running another wire back down through the harness to a good frame connection.

Beta's not known for their wiring prowess.

Edited by dan williams
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