250r rpm runaway

[PAPA250r]

I have a ported and piped 250r . sometimes I have a strange problem. if I'm getting low on gas and forget to hit reserve. the engine rpms will run away in neutral after the ride.

I mean REALLY run away almost to the point I feel a pitson is going to go through my leg..lol

holding the kill button has no effect and choking it does nothing even makes it a little worse .

what gives ? when I put it on reserve it calms down once it gets gas again .

this only hapens on the rare occasion I run the gas down too low.

I'm runnin 32:1

thanks for any insight !

[RodKnockRacing]

its because its runs very lean when you run out of gas in a 2 stroke

[Vealmonkey]

The infamous leaned out death rev!

[Billy Golightly]

The above posters are correct, to break it down a little further:


A higher air mixture going into the fuel, is more volatile, causing the engine to rev higher. When the tank is running out of fuel, your getting air into the system, and not a continous stream of liquid fuel. This "leans" out the fuel mixture and makes the engine rev more. This is a very bad thing. After a certain point, the engine gets hot enough that it "pre-detonates" which means that a certain point in the motor (on the inside of the combustion chamber, probably on the underside of the head) is getting so hot, that, that point is igniting the fuel mixture before and WITHOUT the spark ignition of the spark plug. Technically its similar to a diesel motor, and it might be referred to as "dieseling" by some people. So then the engine is "firing" faster, and its getting a higher potency fuel that makes it continue to climb RPMS until it either flings itself apart (Remember, the spark ignition isn't running it, so turning it off does nothing, AND its not RPM limited by ignition timing advance/retard abilities anymore either!) or it just runs out of gas. The best thing to do is at the first hint of it running out of gas, turn the reserve on.

[PAPA250r]

I don't like the sounds of 'death rev' lol

I'm going to make an effert to keep it toped off at all times so this never happens again . .

could this happen while riding ? it would sure be a wiked ride more like a missle


thanks for the replys

[Vealmonkey]

Usually if it happens while riding, uncool things happen.

[Jason Hall]

This will sound Wrong, but If you hold the throttle wide open It will shut off almost Instantly (pull the clutch In of course If your riding It LOL). The sudden burst of air will kill the engine right Now, and will Quickly cool the cylinder.

[mrtwidster1]

Ah Yes!!
The old DEATH REV!!
Had it happen on my first 84 R. Scared me real bad!! Toasted my top end!!
My Son had it happen on his 84 also, Same result. Scared real bad and toasted top end!
Thanks for the explanation Billy! That was one of the best I have seen so far.
Mr Hall that is a great tip! Just hope I never have to use it.!

[Macdiesel]

or you could have an airleak mine did the same stuff when i had the base basket goin bad

[Jeepermc]

One of the reasons I never leave camp without a full tank.... Never.

[rdlsz24]

I tried to tell my brother not to run the gas out of the carb before putting away his CR125R because of this. He would turn off the gas from the tank and ride the bike around till it died. I thought he understood that on a 2-smoke it if isn't getting gas then it's not getting lubricated either but maybe he didn't. In the short time he had the bike he did get lucky and not have it happen though

Rob