Revving Very High When low on gas, Couldn't Kill the Engine!!!!

[heavy_d]

I was out on my zilla tonight, and started it revving a little high, so i figured it was starting to run out of gas, so i switched it to reserve, then it started running full throttle,(throttle wasn't sticking) screaming, i pushed the kill switch, turned the key off and it was still revving like hell, so i turned the gas off, it didn't stop till it ran out of gas in the line, it was constantly revving for about a total 10-15 seconds straight, thank god it didn't seize, has anybody else experienced this problem when running out of fuel or anytime? It really makes you mad when you sitting there and you can't do a damn thing until it runs dry in the carb

[DantheEXman]

I would have pulled the cap off the sparkplug :)

I have no idea why this would happnen.

Check your carb.

[Sprks450r04]

air leak!!! it happened to my friend on his blaster and he had to pull the choke up for it to stop

[nicky slee]

I had the same thing happen last week when i pulled my carb apart to change my needle setting and i accidently left the throttle cable wrapped over the throttle reel that the throttle cable sets in. The thing that really suked about that was i just got it back from getting it built and the piston was no where near broke in

[heavy_d]

Originally posted by nicky slee
I had the same thing happen last week when i pulled my carb apart to change my needle setting and i accidently left the throttle cable wrapped over the throttle reel that the throttle cable sets in. The thing that really suked about that was i just got it back from getting it built and the piston was no where near broke in

that would make me sick:( this was rebuilt about a year ago, the spark plug cap is hard to get to and i would've burnt the crap out of my hand and i had one hand on the clutch and didn't want to guess if it was in neutral

[02Yellow400]

This happened to me on the last 2 stroke i had. Check the carb very good for any cracks or leaks. Over winter i managed to get a crack in the car somehow and it was drawing in tons of air to make it rev way up just like yours. Let me know how you make out

[86atc250r]

It's called lean runaway.

You won't do anything but shock the crap out of yourself by pulling the plug wire. Your best bet is to keep your cool, put it in a tall gear, let the clutch out as you hold the brakes.

Check your mixture and plug heat range (once you know you don't have any vacuum leaks).

Sounds like you're on the edge of being too lean and/or you have too hot a plug in the beast.

[heavy_d]

Here is what the spark plug looked like after the high revving, it looks okay to me, the factory manual says NGK B8EGV which is not available any more and i've been running a NGK BR8EG which is the same thing, i have ran a 9 before, but it was harder to start and i needed a hotter spark, I'm wondering why the kill switch or key didn't work though, is it because the current is jumping or something?

http://www.imagestation.com/picture/sraid114/pf333fc2af02f6bd94efaa915f81c16ba/f8d342bf.jpg

[amosclifford]

:confused: the reason it would keep runnig is it is so lean it cause enough heat to keep firing itself no spark needed, in a sense it is deiseling

[wilkin250r]

Yep. Lean runaway.

When you start to run out of gas, your fuel/air mixture can lean out. A lean mixture will increase combustion tempuratures. So basically, when you lean out, your bike will begin to produce more heat.

This extra heat creates a hot spot inside the combustion chamber, usually the tip of the spark plug. It gets RED hot, hot enough to ignite the fuel without a spark. Now it's running on it's own, which creates even more heat. More heat means the tip of the spark plug gets even HOTTER, which ignites the fuel even easier, which creates more heat, ect... This is why it is called a runaway, it feeds on itself. Hitting the kill switch or pulling the spark plug wire doesn't fix the problem, because it's not running off the spark anymore.

The most important thing to do when this happens is to realize WHY it's happening, and fix it. You don't need to panic, 5 seconds of revving to the moon won't kill your engine, but you DO need to act quickly, you don't want it to go on like this forever, because your piston is getting VERY hot in there.

There are basically two things you can do to fix the problem. Slow your motor down, or cut off the air. To slow your motor down, you just put it in a high gear, lock the rear brakes, and slowly let your clutch out. If the motor can't rev, the problem will die. The other solution is to cut off the air. This is why the choke solution worked. If your motor can't pull in enough air, the problem will disappear.

[Atreyu]

This happened to me when I had my blaster. It screamed like this for about 30 seconds before I tapped the throttle and it bogged out and shut off. This was after I burned the crap out of myself pullling the spark boot. I heard it was because I had run too high of an octane in it causing it to "predetonate" or "detonation." Thus causing the gasoline to "explode" forcing my piston up and down. Thats why my killswitch and spark plug boot didnt kill it. Dont know whether that's true or not though.

[86atc250r]

Actually the opposite is true - a higher octane will make the problem less likely to occur.

When lean runaway is happening you can try hitting the throttle to lean the mixture to the point that the engine just dies --- sounds like what you did.

Another thing to check for is carbon deposits on the piston - carbon deposits can act as glow plugs when the lean mixture brings combustion chamber temps too high, much like the tip of the spark plug.

[big-guy]

And heat rating on the plugs has to do with the insulation on the electrode and how much heat it dissipates to the head, not how hot the spark is.

[sparks400]

something like this happened to my blaster, u prolly hve an air leak somewhere, it coul be where the reed cage is, or the spark or even a head gasket, that if it is a air leak

[plkmonster2]

My uncle had a rm 125 that and the air screw on it was so far out from vibration that it started to act up. It started to rev a bit, so he hit the switch. No luck, he hit the choke. Since it is a plunger, which gives more gas, the thing went like it was full throttle. He gave it gas, and it bogged and died. The air screw was so far out that it started to lean, and get real hot. Pulling the choke worsened things more. Check the air screw also...;)

[Chanman420q]

this happend to both my friends with LT250Rs, 1 of them ran away because he thought it was gunna blow up:p and the other one got so frustrated he started to kick the cylender:eek2:

another thing ive heard was in a honda 250R that happenning and it went on to wear the piston cracked in 5 peices

[LT250Racer609]

yup...happened to me...i was riding my LT during the winter and it was about 38 outside...and i had my bike jetted perfect for the weather anyways...i was riding it and all of a sudden i was in neutral and it started screaming...i tried the shut off button...it didnt work...i pulled the cap off my spark plug and it still didnt shut off! next thing you know anti freeze is everywhere cause it blew my head gasket and it will stil screaming! finally i didnt kno what else to do besides kick the motor and try to free the piston so it didnt seize...i did that and finally it shut off.....because of this is whorped the head and scared the $h!t outa me!

[redrider69]

happened to me the other day at the sand pits it was because i got something wet and i let it sit for about a week and started up fine and has ran fine since i think it was my starter that jus kep turnin over and revin real high

[mc1984]

The same thing happened to me on my friends Blaster. I was the one unfortunate to be on it. When it happened I hit the kill switch but it kept running, at WOT, I yelled for him to pull the plug wire and he did. To my suprise it kept running wide open!!! I was shocked and trying to make a split second decision, luckily I got the engine to kill by letting the clutch out. I talked to a 2-stroke engineer at work and he said another name for it is Automatic Radical Combustion. It is possible with the right amount of air and fule delivery. It does not scavange enough exhaust gasses out of the cylinder and the exhaust heat reignites it. Wierd, I wouldnt have believed it if I hadnt seen it first hand.

[wilkin250r]

For those that don't know, this is exactly how a diesel engine works. Diesel engines don't have spark plugs, they have glow plugs. They create a hot spot inside the combustion chamber.

When you compress a gas, it heats up. Boyle's Law. When your piston is traveling upwards during the compression stroke, it is compressing (and therefore heating) the fuel/air mixture. If it compresses TOO much, it can ignite the fuel. The way diesel engine works, when the fuel/air compresses, that little glow plug adds that little extra heat needed to ignite the fuel. In reality, it is detonation, which would damage an ordinary 4-stroke motor, but diesel engines are built very heavy-duty to withstand the additional stresses.

[XANDADA]

very interesting, I never heard of this lean runaway. I wonder if that was what happened to me a while back? Right after I rebuilt the engine and installed a new carb, I thought my throttle was sticking open. Maybe it was this lean condition? It has since completely dissappeared. All I did was lube the slide and the throttle cable. It only ever stuck when it was wfo and I let off the gas. I think I used the kill switch and it killed the engine though.

[wilkin250r]

If cutting the spark killed the engine, then it wasn't lean runaway.

Lean runaway is very common when people try to "run the gas out of the fuel lines" They shut off their fuel, but keep the engine running to run out the fuel in the lines and float bowl. (I'm not sure why. Is the fuel going to hurt anything sitting in there?)

Think of how a carburetor works. The float bowl is at a higher pressure than the intake path, so the fuel is drawn into the intake through the main jet and needle. If the float level drops significantly, so does the pressure. Less pressure in float bowl results in less fuel drawn into the intake, leaning out your mixture.

When your mixture is lean, it creates more heat in your combustion chamber, hot spot, deiseling, runaway condition, blah blah blah.

So, the bottom line is, DON'T run the gas out of your bowl. If you are storing your quad for a long period of time and you are worried about the fuel going bad or gumming up thw works, then drain your tank and carb properly. Otherwise, don't worry about that little bit of gas sitting in your float bowl for a few days, it won't hurt anything.

[heavy_d]

great info, useful and interesting, i won't ever let the thing run low on gas again

[MichaelS693]

i thought this was common on all 2 strokes when the gas is low or almost out:confused:

[Ryan]

I've ran on gas really low but that never did it.


However, I have had the exact same problem as you did. Turns out the head gasket was put in backwords (from the factory). And the reason the key and killswitch wouldn't turn the engine off is because it was so lean, and hot, it was actually making its own spark. I got it to turn off by turning my idle screw all the way down.


Scared the crap outta me when it first happened. :o

[Mattman88]

hey everyone I am having the same problem as everyone else here is. By the way, sorry to bring back an old thread. But I am wondering could jetting be the problem. My blaster has always had a slight over rev problem but once I put stock exhaust back on without jetting, the motor goes insane and revs to the moon. Could jetting it correctly solve my problem??

[Mattman88]

bump?? I need help guys

[bradrenea]

If it was jetted for an aftermarket pipe and you put the stocker back on, it should theoretically be rich, not lean. Maybe you just need to turn the idle down. The best way to check if its too lean is to look at the spark plug. If its running lean the plug will be white. If its rich, it will be black. Try to get a tan color.

[markk]

THat happend to me about 5 years ago when i had an old old 1973 DT250 I found out that when i was riding that my bike was going wide open, I got her stoped, and did exaclty like u, i shut off the gas, and hit the kill switch and NOTHING, I ended up pulling it up into 4th gear and i dumped the clutch while holding the brake.

I found out that i had a BAD air leak, I had cracked my old brittle intake boot! Check that out first!

[Mattman88]

ahhh this sucks. I have no idea what could be wrong then. It always revved high but after I put stock exhaust back on it went crazy.

[racer94]

this is typical of a 2 stroke,they all will do it when too lean,or when the fuel gets low enough that is starts to starve and lean itself out...getting the right mixture in a 2 stroke is very critical and one of the key factors of the machine performing to its full potential..go to a colder plug or bigger jet,or raise the needle...good luck!

[bradrenea]

If its just doing it at idle, try adjusting the air screw and see if that helps. Might also check to make sure the throttle cable free play is correct. I noticed in your sig that your frame has been powdercoated, if the throttle cable was not routed back through the frame the right way when it was put back together it could be causing the throttle to stick. I would try the air screw first.

[mc1984]

Make sure the cap on top of the carb is tight, it is common that it gets a bad air leak there and the engine can run away.

[Mattman88]

we already checked, there is no air leak

[Mattman88]

just to get back to the person who was talking about my frame being pced, we already thought me throttle cable was sticking so we checked it to see if it was in correctly and routed correctly. We also bought a new one because the other cable melted to my pipe and we put that one in the right way to.