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Panic Rev No Kill

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Just bought a very clean, low use '06 Honda CR250R.  This is my first 2T so still learning.  Was out on the track today poking around in 2nd gear, making my last lap of the day, when the bike started revving higher and higher.  It took about 10 seconds to reach max rev and was just screaming while I frantically hit the kill switch over and over and put it into neutral.  After I realized the motor was having no response whatsoever to the kill switch I jumped off the bike, shut off the petcock, and yanked the fuel line from the carb.  It took about 2 more seconds to start winding down and finally died.

I'm going to start looking for any air leaks and will report back on here so that hopefully this information helps someone else.  If I can't figure it out I'll report that and hopefully we can figure this out.

The real concern I have right now is will the motor be ok to run again?  It panic revved for probably a solid 20 seconds after already probably being hot from poking around the track in 100 deg, 80% humidity weather.  You think it's safe to just Fix the problem and move on?  Or do I need to tear the motor down and start inspecting?

 

 

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Ok so I'm not seeing anything obvious.  Spark plug is tight, carb boot looks sealed, air screw still in place, not seeing any cracks on the outside, throttle cable feels smooth.  Suggestions?

 

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Ok, I had the same issue with my '78 yz400. You want to do a leak down test. Basically you pressurize your top end with about 5 psi, and it is supposed to hold for over 30 minutes. If the pressure drops, you've got a leak somewhere. The way I did it was I made a leakdown tester with stuff from Home Depot and made it to where it slips in the carb boot. I made a plate to plug the exhaust where the manifold bolts on. It worked pretty well. I'll post a picture of it.

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It worked for my 36mm carb, I glued all the pipe fittings together to make sure it didn't leak. Also, you want to make sure you don't fill past 5 psi or you'll blow your crank seals. BTW when you ran WOT without control, was your gas tank empty?

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Interesting engineering there, I will look into this.  Thanks for the tip.

As far as the gas tank level, it was almost completely empty.  I can see just a tiny bit of fuel at the very back when I shake the bike forward.  Why do you ask about that?

 

 

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If you run out of fuel, you will run extremely lean and have a two stroke runaway. That's probably what caused it. When two strokes run lean they'll do what you described. Tell you what, put some fuel in the tank, don't forget oil lol, and see what happens. I bet that will solve your issue. I bet the bike got to a point here the amount of fuel in the tank wasn't enough to keep the carb filled due to it sloshing around, causing the float bowl on the carb to run out of fuel and lean conditions to occur.

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And your motor should be fine, it didn't run for too long. On my old 400 I had multiple episodes of lean runaway back to back while I was trying to get it to run right. In the end the top end was still good after about 5-6 episodes of runaway, actually nearly perfect. It's pretty scary I'll admit when it happens, especially on a old 400 smoker with no brakes and no killswitch lol. Your bike will be fine, just remember to check how much fuel is in your tank periodically.

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Wow, I would have never guessed running out of gas could cause that ... crazy.  Well, this is the learning process.  Would be a huge relief to discover that was all that was wrong so I will try this tomorrow and report back.

It's also a relief to hear you say my motor should be fine, thank you very much for your advice.

Also, I looked some more for any leaks by spraying the motor down with soapy water while I worked the kickstarter, I know this is not the leak down test that you suggested but it was a quick way to maybe see something major.  I didn't see anything doing that either.  Hopefully that's a good sign.

 

 

 

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Yeah you should be fine, it's pretty normal for two stokes to do that when they are out of gas. Multi cylinder two strokes are a lot better on avoiding that because they usually have multiple carbs and when fuel gets low one will run out before the other and the dead weight of one non functioning cylinder will prevent run away, my 350 twin does that.

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Update:  I tried draining the remaining fuel from the petcock but only got a few drops.  So I put some more fuel in the tank and filled up the carb, then shut off the petcock. Then I started it back up, and WAH LAH, runs perfectly normal again.

So I left the petcock closed and let it run for a min just trying to burn down some of the fuel in the carb (a practice I'm used to doing sometimes with 4 strokes to drain the carb if I didn't lay it over to drain it) when all the sudden I recalled doing this exact same thing the other day where towards the end the motor started idling faster. So I just shut it off and thought "WTH ... That was weird ... oh well."  And NOW it all makes sense!!!

I had no idea this happens, thanks ASP1227 for your help.

 

 

 

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Good to hear! Glad I could help. Just remember to drain your carb after you turn off the engine. It can be a chore but it's totally worth it. ;)

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If it runs away again and the kill switch doesn't shut it off, hold the brakes, drop it in gear and choke it down.  They say you can open the throttle wide open and it will shut it off but I have never tried it.

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That's not a bad idea.  At the time I thought about stalling it but was afraid it would loop out instead and go flying into the grand stands and still be screaming.  But I like the brake idea while hitting the kill switch.  Actually I like the don't run out of gas idea even better.

 

 

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