WOT Upon Starting??

[fxnteeth]

Hey guys, my 86 250R goes crazy when started. I pulled the carb off because it was running a little rough. Cleaned it well. However, my intake boot is a little soft and has a few tears. Im guessing I could be sucking air from this. Needle definitely not sticking. It functions properly. As soon as I kick it revs like crazy. I have to hold the brake and let of the clutch. Please help thx

[rdlsz24]

I would imaging your suspicion is right. You are getting death rev from the leaking intake boot

Rob

[webbch]

If it's a death rev that didn't exist before he removed the carb, I'd want to make sure the carb slide isn't installed 180 deg out and sticking up too high in the bore.

[emmie357]

If the boot is soft and torn I would replace it, even if you find something else wrong in the carb/throttle cable.

[El Camexican]

my intake boot is a little soft and has a few tears. Either that or you torqued the idle screw to 8 ft. #'s when you put it back together.

[fxnteeth]

Ok guys, got my new intake on. Fired it up and it was perfect. Rode her for about 20 minutes. Shut the gas off and let her sit for a few days while I worked on other ones. Fired it up today and it death revved. What the hell? Sounds carburetor like. It's not going to be an air leak Throttle was fine the other day. Any suggestions? What in the carb would cause this?

[El Camexican]

Put me down for $10 bucks on a bad stator side crank seal.

[fxnteeth]

can a sticking float valve cause there to not be enough fuel for pick up, leaning it out and making it scream?

[El Camexican]

Can you hear a nice sharp metallic click every time you open and release the throttle?

Did you use new gaskets when you put the boot and reed cage on?

How did you shut down the "death rev" this time?

If you think it could be the float just remove the bowl drain before you try starting it and if gas comes gushing out and it still revs for the moon when you fire it then it is doubtful your floats are hanging up. However, if the fuel was trickling a drop at a time due to a plugged filter or petcock I guess it could still run and create a lean condition.

Pull the stator cover off and look for any signs of seepage, or oil stains around the crank seal. That would be a red flag for a leak.

To sort of answer your question: possible yes, probable no.

[nd4speed]

I have had the death rev a time or 2 and never changed any gaskets or seals, mostly it was a lean carb issue and the motor "dieseling" cause of a hot lean fire. Kill switch of no use- just drop it in gear with brake held to stall the motor.

Once it was throttle cable pulled on/ not returning raising he carb slide. Float height problems. Always seemed to be a carb issue.

When I had a crank seal go/ crank fry the thing ran funny and smoked and power was on and off, but it may have been clutch side seal, then crank locked when bearing came apart. I thought I fried the top end because I knew it was leaning/richening and just kept riding it WOT to its death, but piston was fine.

I had an air leak between the case halves direct under the crank in a 1984 KX250 and it ran great in the winter. The motor was competely loose and probably original bearings with a fresh piston kit. lol, everything was all loose and sloppy but 3rd gear on the trans got jammed up with metal flakes and wouldnt slide on the shaft.

My friend had a LT250R motor that had a high rev on cold start, that motor leaked and weeped from every seal, he never worked on it. It ran great really fast and made me mad when I would take good care of mine and his was still faster. My pipe and tire choice were really holding me back. And his was ported.

[Ryanq55]

Put me down for $10 bucks on a bad stator side crank seal.

Is this really a possibility? How come that would make it rev to the moon? Not being sarcastic, can you explain why it could possibly be the cause of his problem?
Thanks

[El Camexican]

Is this really a possibility? How come that would make it rev to the moon? Not being sarcastic, can you explain why it could possibly be the cause of his problem?
Thanks

Think of it this way. The engine is an air pump. On a reed valve 2 stroke fresh air is drawn through the carb because of the vacuum created by the rising piston. On the way through the carb it mixes with fuel. How much depends on a number of things, but in a perfect world the ratio would be about 15 parts of air to 1 part of fuel.

The vacuum that happens on a reed valve engine can only occur when the reeds are closed and the transfer ports are below the piston rings as it moves towards the top of its stroke. During this very short period negative pressure builds to the point that the reeds are literally sucked open allowing the air and fuel to rush into the crankcase.

What happens when a crankcase seal is damaged is that raw air (void of fuel) is sucked in during the moment that negative crankcase pressure occurs. Depending on how bad the leak is the ratio of air to fuel can rise greatly. This cases a lean condition which in turn causes the revs to come up, heats the engine up and causes all hell to break loose. Keep in mind that this air doesn't have any oil in it either.

If the leak was on the "wet" side of the engine where the gear that drives the clutch basket lives there would be oil getting drawn into the engine rather than just fresh air. It would run like crap and smoke a lot, but a lot less raw air would get in because of all the oil. A leak on that side is also less likely to occur because of the added lubrication in its environment. The stator side seal only gets is lube from the 2 stroke oil that enters the engine.

I read the other day that it is likely that more two strokes than not are operating with some degree of leakage and may never be damaged because of it, but once you start racing them and running them with near perfect jetting any leak will lead to a rapid failure. The onlt way to be sure if you have a well sealed engine is to seal and pressurize it. Lots of good youtube videos on this.

If you want to see this all first hand start your warm engine and shut the fuel petcock off. After a couple minutes you'll hear the RPM's start to come up as it starts to starve for fuel. How high it will get before it stops is hard to say. I usually hit the kill button before it gets too high. This is not good for an engine!

Something to put in your mental tool box is that you can shut down a runaway 2 stroke by opening the throttle all the way. I've never had to do it, but I have no reason to doubt it works.

[Ryanq55]

Think of it this way. The engine is an air pump. On a reed valve 2 stroke fresh air is drawn through the carb because of the vacuum created by the rising piston. On the way through the carb it mixes with fuel. How much depends on a number of things, but in a perfect world the ratio would be about 15 parts of air to 1 part of fuel.

The vacuum that happens on a reed valve engine can only occur when the reeds are closed and the transfer ports are below the piston rings as it moves towards the top of its stroke. During this very short period negative pressure builds to the point that the reeds are literally sucked open allowing the air and fuel to rush into the crankcase.

What happens when a crankcase seal is damaged is that raw air (void of fuel) is sucked in during the moment that negative crankcase pressure occurs. Depending on how bad the leak is the ratio of air to fuel can rise greatly. This cases a lean condition which in turn causes the revs to come up, heats the engine up and causes all hell to break loose. Keep in mind that this air doesn't have any oil in it either.

If the leak was on the "wet" side of the engine where the gear that drives the clutch basket lives there would be oil getting drawn into the engine rather than just fresh air. It would run like crap and smoke a lot, but a lot less raw air would get in because of all the oil. A leak on that side is also less likely to occur because of the added lubrication in its environment. The stator side seal only gets is lube from the 2 stroke oil that enters the engine.

I read the other day that it is likely that more two strokes than not are operating with some degree of leakage and may never be damaged because of it, but once you start racing them and running them with near perfect jetting any leak will lead to a rapid failure. The onlt way to be sure if you have a well sealed engine is to seal and pressurize it. Lots of good youtube videos on this.

If you want to see this all first hand start your warm engine and shut the fuel petcock off. After a couple minutes you'll hear the RPM's start to come up as it starts to starve for fuel. How high it will get before it stops is hard to say. I usually hit the kill button before it gets too high. This is not good for an engine!

Something to put in your mental tool box is that you can shut down a runaway 2 stroke by opening the throttle all the way. I've never had to do it, but I have no reason to doubt it works.


Hey thanks ElCamexican, I never would have thought about a bad seal would draw air in and cause that kind of problem. Thanks for the info!

[nd4speed]

A lot of the time holding WOT will kill a "runaway" two stroke death rev, but not always...trust me ..I know.

I run all my 2 stroke equipment dry of fuel before storage by letting them starve(empty) the carb and rev and die out. Have do it for years, well decades, and they all start and run fine the next season. I once let gas sit in my carb and it hardened an oil rock inside of one of my jet passages, the carb ran funny at high speeds and colder weather, then even after I figured out to get it unblocked it never ran the same again. I sold it for parts and started running my carbs dry if I remember.