run away xp 1996

[jasonham41]

Hi all, had a problem today that i thought was technically impossible on a 2stroke.
My ski ran fine all day i then came back to the beach and trailered it, i then fired it up and it was running like a dog fauled plugs type of missfire, i blipped it a few times and it picked up and then it took off and revved way past the rev limiter. i whipped the key off.... nothing still going like hell,
so i yank the seat off and rip the plug leads off.... nothing still going
so i switch the fuel off.. nothing still going
last resort i then pulled the choke and tryed to cover the air intake and this eventually killed it.
i have now pulled the carbs and rotary cover, as i expected maybe the rotary oil seal had gone but it seems normal in there although if you rotate the crank you can see the high spots are saturated with oil.
your thoughts and advice much appreciated,
Jason.

 

[Dr Honda]

Sounds normal.

They can run away, and you use the coke to kill it.

Clean the carbs, and open the low mix screws about an 1/8th of a turn.

 

[mattydipps]

yupp what doc said^^

first time my ski ran away on my scared the heck out me i thought i was going to loose it... until someone ran over and pulled my choke...

 

[jasonham41]

wow, really that simple. so it is possible for it to "diesel" on a lean mix of petrol??
many thanks doc, i really though i was going to have the motor pulled out by the end of the weekend.

 

[Dr Honda]

wow, really that simple. so it is possible for it to "diesel" on a lean mix of petrol??
many thanks doc, i really though i was going to have the motor pulled out by the end of the weekend.

Absolutely. The higher the RPM's... the harder it is to control. The only way to kill it is to have a slightly rich mix to keep it cool. One thing to remember... we tune these engines for a heavy load, and without it... they are already idling high. (around 3000 out of the water) So... if you aren't tuned properly... it will take off. It's very common for someone to have the idle extra high, and than have a lean mix. That condition in the water will idle properly... but out of the water, it will want to rev.


FYI... that will never happen in the water. (whit a load)

 

[SabrToothSqrl]

Kill it with coke! sounds expensive

My last run-away, I didn't know the choke method and burned the top end, which I replaced, only to have the crank go ONE tank later! :'(

Now I don't run these out of water unless I have no choice...

I did same as you, stop button, removed lanyard, pulled plug wires, fuel off... it didn't even die then and there, it ran 2-3 more tanks, but water was getting in to the combustion chamber and apparently killing my HP, then eventually killing the engine...

 

[jasonham41]

Kill it with coke! sounds expensive

My last run-away, I didn't know the choke method and burned the top end, which I replaced, only to have the crank go ONE tank later! :'(

Now I don't run these out of water unless I have no choice...

I did same as you, stop button, removed lanyard, pulled plug wires, fuel off... it didn't even die then and there, it ran 2-3 more tanks, but water was getting in to the combustion chamber and apparently killing my HP, then eventually killing the engine...

eek dont say things like that!!! it seemed like an eternity at the time but it was probably only 20seconds or so. i hope it is ok it was running around 150psi on both cylinders.

 

[Dr Honda]

One thing to remember... when they run-away... they can go WAY past max RPM's since they are no longer relying on the ignition. SO... in a worst case... they can scatter a crank or rods.

 

[jasonham41]

it's all still intact at the moment (rods ect), i am rebuilding our boat at the moment but as soon as i get time i will get the filters out and cleaned. where is the low mix screw on these carbs, is it the small screw on top of each carb below the fuel lines?

 

[vaser21]

how was that thing running even after he pulled the spark plug boots off?

 

[Chester]

It does a thing called glow plugging. If it's running lean or the throttle is open more than it should be at idle, it'll run hot and a piece of carbon or the tip of the spark plug will get so hot it acts like a glow plug. The engine will then run away uncontrollably.

Chester

 

[Dr Honda]

it's all still intact at the moment (rods ect), i am rebuilding our boat at the moment but as soon as i get time i will get the filters out and cleaned. where is the low mix screw on these carbs, is it the small screw on top of each carb below the fuel lines?

Nice thing with a 2-stroke is that the mass is low, and runaway damage normally doesn't happen as long as the engine was healthy to begin with.

how was that thing running even after he pulled the spark plug boots off?

As Chester was saying... the fuel is igniting from heat.

If you remember back to high School Chemistry or Physics... there is something called the natural gas law. (PV=NRT) When you compress a gas (V=volume) it is directly inverse proportional to the Temp (T). So... if you already have a hot engine, and you compress the air 10:1... the temp will also go up by a factor of 10. This rise in temp... coupled to a hot spot in the head, will cause the fuel to auto-ignite.


The way to combat it is to cool the combustion gasses. We do that 2 ways.

1) A slightly rich mixture. Basically, as the fuel goes from a liquid to a gas... it absorbs heat

2) A properly set squash band. This allows a quenching of the piston edges, and the high velocity air, helps cool the piston crown.


If you are not aware... Diesel engines are compression ignition engines. They don't have spark plugs. They just inject fuel at the proper time. And some of them... like my old Cummins 2500 didn't even have glow plugs because it was a very efficient engine.

 

[griz400]

actually the cummings diesel didnt use glow plugs, but, they use a heater grid, to pre heat the air going into the cylinders --

 

[vaser21]

wow thats crazy

 

[vaser21]

thank you for the chemistry lesson hahaa

 

[Spimothy Leary]

one thing I haven't seen in this thread, is for those of us with a primer kit
i'm pretty sure that you pull the tether
then
hit the throttle/primer.

i remember hearing that holding the stop start switch while wot works too.

 

[Dr Honda]

actually the cummings diesel didnt use glow plugs, but, they use a heater grid, to pre heat the air going into the cylinders --

Yes... but ask me the last time it was hooked up?? Also, the stock computer didn't even turn it on unless it was less than 40 degrees.

I removed my heater grid, and it would start with PA winters without an issue. The only time I had a problem was one morning it was -10 degrees, and it grumbled... but it started. BUT... I only made it about 300 ydrs from my house, and it shut off because my fuel gell'ed. :blush:

Basically, if it was warm enough for diesel #1 to flow... it wasn't needed.

For they guys who aren't diesel heads... Diesel fuel has paraffin in it... and it will go solid when it started to get cold. Normally we use Kerosene, or Diesel #2 to keep it flowing when it gets around freezing.

 

[griz400]

lol, i know how it works

 

[Dr Honda]

one thing I haven't seen in this thread, is for those of us with a primer kit
i'm pretty sure that you pull the tether
then
hit the throttle/primer.

i remember hearing that holding the stop start switch while wot works too.

Pumping the primer will normally get it to stop. You basically douse the heat with fuel.

Holding the switch won't work since you are no longer dealing with the electrical system.

lol, i know how it works

I figured.... but it was for the info for the other guys. :thumbsup: