This pops up frequently on the forum, so here's a tech post on it.
What is it? You start your ski on the trailer and the engine just takes off, screams to the redline and stays there. You didn't touch the throttle and when you try to turn it off, it won't stop it.
So you panic and yank the lanyard. Still going.
Next, you probably pull off the plug wires. Still going!
How is this possible?
This happens for a few different reasons:
1. The motor has an air leak on the intake side or possibly the crank seals.
2. There's air in the fuel lines.
3. The carburetor throttle plate linkages are not adjusted right (for dual carbs)
Technically, what is happening?
Normally, the engine gets air through the carburetors. The flow of air is controlled through the throttle plates. The air going through pulls fuel from the carburetor fuel passages. An air leak after the throttle plates lets air into the motor uncontrollably and does not pull fuel from the carbs. This unaccounted air is not matched with fuel to provide the appropriated fuel to air ratio. Running this lean increases the the temps at the crown of the piston so high that it no longer needs the controlled spark from the spark plug. Once the fuel starts to compress it automaticlly ignites the lean mixture that follows each revolution. That's why the stop button, lanyard, and pulling of plug wires will not stop the motor.
So how do you stop it when it happens? Few different approaches, with the far best one listed first.
1. Push and hold the stop button, pull the throttle wide open. While it may sound counter-intuitive, it works great. The stop button prevents the plugs from sparking. Pulling the throttle wide open allows an inrush of cool fuel and air. This cools the combustion chamber and stops the self-combustion right away (in absence of spark)
2. Prime the
out of those carbs. Keep pumping and pumping, eventually all the excess fuel will cool & drown the motor out.
3. Put something on the carb throat(s). Achieves the same as 2.
4. Block the exhaust outlet. The excess exhaust will feed back into the motor and stop it.
To reiterate, 1. is the best and fastest option.
As mentioned previously, such a runaway can also be caused by air in the fuel lines - it hasn't been run in a while, it's just been put together, or maybe there's a split or crack in the fuel line. In this case, the engine usually slows down by itself after a few seconds.
With dual carbs, it's possible that the throttle plates are not synched right. One plate may be shut and the other one is open. This only leads to really high idle speeds and can be stopped the normal way.
So what do you do about it?
1. Check carb synch
2. Check fuel lines for air bubbles and splits
3. Inspect the motor for air leaks.
3a. The most typical spot is the carb gaskets or intake gaskets. Visually inspect those and make sure they are 100% intact.
3b. The base gasket may also let air in. You typically only find those with a leak down test.
3c. The crank seals may be damaged or even partially hanging out of the cases. You can easily do a visual check on the coupler end. The flywheel end is trickier, but also far less common.
If you can't find anything with these visual checks, perform a leak down test. Essentially you pressurize the lower end of the motor and watch for leaks.
What is it? You start your ski on the trailer and the engine just takes off, screams to the redline and stays there. You didn't touch the throttle and when you try to turn it off, it won't stop it.
So you panic and yank the lanyard. Still going.
Next, you probably pull off the plug wires. Still going!
How is this possible?
This happens for a few different reasons:
1. The motor has an air leak on the intake side or possibly the crank seals.
2. There's air in the fuel lines.
3. The carburetor throttle plate linkages are not adjusted right (for dual carbs)
Technically, what is happening?
Normally, the engine gets air through the carburetors. The flow of air is controlled through the throttle plates. The air going through pulls fuel from the carburetor fuel passages. An air leak after the throttle plates lets air into the motor uncontrollably and does not pull fuel from the carbs. This unaccounted air is not matched with fuel to provide the appropriated fuel to air ratio. Running this lean increases the the temps at the crown of the piston so high that it no longer needs the controlled spark from the spark plug. Once the fuel starts to compress it automaticlly ignites the lean mixture that follows each revolution. That's why the stop button, lanyard, and pulling of plug wires will not stop the motor.
So how do you stop it when it happens? Few different approaches, with the far best one listed first.
1. Push and hold the stop button, pull the throttle wide open. While it may sound counter-intuitive, it works great. The stop button prevents the plugs from sparking. Pulling the throttle wide open allows an inrush of cool fuel and air. This cools the combustion chamber and stops the self-combustion right away (in absence of spark)
2. Prime the
out of those carbs. Keep pumping and pumping, eventually all the excess fuel will cool & drown the motor out.3. Put something on the carb throat(s). Achieves the same as 2.
4. Block the exhaust outlet. The excess exhaust will feed back into the motor and stop it.
To reiterate, 1. is the best and fastest option.
As mentioned previously, such a runaway can also be caused by air in the fuel lines - it hasn't been run in a while, it's just been put together, or maybe there's a split or crack in the fuel line. In this case, the engine usually slows down by itself after a few seconds.
With dual carbs, it's possible that the throttle plates are not synched right. One plate may be shut and the other one is open. This only leads to really high idle speeds and can be stopped the normal way.
So what do you do about it?
1. Check carb synch
2. Check fuel lines for air bubbles and splits
3. Inspect the motor for air leaks.
3a. The most typical spot is the carb gaskets or intake gaskets. Visually inspect those and make sure they are 100% intact.
3b. The base gasket may also let air in. You typically only find those with a leak down test.
3c. The crank seals may be damaged or even partially hanging out of the cases. You can easily do a visual check on the coupler end. The flywheel end is trickier, but also far less common.
If you can't find anything with these visual checks, perform a leak down test. Essentially you pressurize the lower end of the motor and watch for leaks.
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