Primer Kit vs Runaway

[TEXAN]

So I'm minutes away from ordering a primer kit when I came across a post about runaway and primers. My goal is to completely do away with my choke cable and use the primer kit instead. Is there another way to prevent runaway without the use of a choke cable? I'd love to do a kill switch somewhere close to wheel for easy access. Thanks!

-Tex

 

[suke]

I have a primer kit on both my skis and never had a run away issue. Also never heard of this "issue". Not saying it doesn't exist, just saying I never heard of it. I'd venture to guess if installed right there shouldn't be any issue. I premix and use the oil injection ports for the primer injection. Works fantastic. Couple of pumps and she fires right up, first time......every time.

 

[purple_people_eater]

its more prevalent on reed induction engines. and can really only happen when the pump is unloaded. basically what happens is something throws the delicate balance our skis run on out of wack, usually an air leak some where or wrong carb tunings. the engine runs lean and produces excess heat. a hot spot forms on the domes of the head or tip of the spark plug becomes overheated and the compressed fuel air mixture is ignited by the hot spot or plug tip. so your ignition system is no longer controlling the detention of fuel and air. this is also know as "dieseling."

now why this usually doesn't happen on the rotary value motors is because of the rotary value itself. since its controlling when fuel is delivered and how much, then closing the port, the reaction is not allowed to get to badly out of balance. unless the air leak is in the cases or head, still however, its some what controlled, where as with reeds there is nothing stopping new fuel from continuously being drawn into the combustion chamber.

now to stop the "runaway" you have to upset the reaction that's taking place. and there is more than one way to do this. as you mentioned this is normally done by pulling the choke because it does 2 things toward ending the reaction, and does it quickly. first it restricts incoming fresh air and floods the chamber with fuel cooling down the hot spot and flooding the engine and restricts incoming fresh air.

now if you don't have choke plates, you still can disrupt the reaction that is taking place, its just not as simple. the easiest and quickest method is to just pull the lanyard and open the throttle wide open. the sudden influx of cool air and fuel will flood the engine and re stabilize it and so long as its not getting ignition spark, will stop the runaway.

 

[Luv2Camp]

If you have the primer system and it starts to run away, just give it a few pulls of prime and the extra fuel will bring the rpm's down and also may stall it out

 

[Sportster-2001-951C-Stock]

Yep, I think if your engine does run away you can probably upset that "rare" circumstance by pumping the primer. If that doesn't work though, opening the throttle completely to wide open is another approach that might work.

 

[TEXAN]

Thanks for the info guys. I encountered a stuck throttle blade after purchasing the boat so this stuff is always in the back mind.

 

[Sportster-2001-951C-Stock]

In that case, it might help to know a stuck throttle blade is not the same phenomenon as an autoignition runaway.

In the case of an autoignition runaway the engine is igniting the fuel despite there is no ignition spark. the theory goes, the fuel is being ignited by a hot element inside the combustion chamber, this is difficult to prove of course, many have tried, another theory is the fuel mixture is ignited by compression ignition.

Anyway, shutting off the MPEM during one of these events makes no difference, the engine keeps racing at high RPM and can blow up due to overspeeding.

So if this happens to you, the first course of action is to shut off the MPEM using the stop button, or removing the lanyard key. This action stops the plugs from firing. If the engine continues running then it is indeed autoignitiing and this type of runaway can be stopped by chilling the hot element inside the combustion chamber responsible for causing the fuel mixture to ignite.

There are two possibilities for chilling the hot element, one involves adding a ton of fuel to cool it by using the choke (or in your case, the primer pump should work). The other unproven method involves adding a ton of air by opening the throttle fully.

It's not something that should happen often but has been known to occur under the right conditions in various high performance 2-stroke engines. In fact, there are laboratory research papers written on the subject, attempting to explain the mechanisms at work.

Suffice it to say, most of the time when this occurs it can be traced to a lean condition due to air leaks in the intake tract, this includes leaky worn crankshaft seals and carburetor/intake manifold gaskets. There's also at least one instance I'm aware of the owner reported runaway occurring from attempting to run with an empty fuel tank.

This can't happen if the boat is in the water, b/c the jet pump is submerged and provides enough load to avoid the possibility.

 

[TEXAN]

its more prevalent on reed induction engines. and can really only happen when the pump is unloaded. basically what happens is something throws the delicate balance our skis run on out of wack, usually an air leak some where or wrong carb tunings. the engine runs lean and produces excess heat. a hot spot forms on the domes of the head or tip of the spark plug becomes overheated and the compressed fuel air mixture is ignited by the hot spot or plug tip. so your ignition system is no longer controlling the detention of fuel and air. this is also know as "dieseling."

now why this usually doesn't happen on the rotary value motors is because of the rotary value itself. since its controlling when fuel is delivered and how much, then closing the port, the reaction is not allowed to get to badly out of balance. unless the air leak is in the cases or head, still however, its some what controlled, where as with reeds there is nothing stopping new fuel from continuously being drawn into the combustion chamber.

now to stop the "runaway" you have to upset the reaction that's taking place. and there is more than one way to do this. as you mentioned this is normally done by pulling the choke because it does 2 things toward ending the reaction, and does it quickly. first it restricts incoming fresh air and floods the chamber with fuel cooling down the hot spot and flooding the engine and restricts incoming fresh air.

now if you don't have choke plates, you still can disrupt the reaction that is taking place, its just not as simple. the easiest and quickest method is to just pull the lanyard and open the throttle wide open. the sudden influx of cool air and fuel will flood the engine and re stabilize it and so long as its not getting ignition spark, will stop the runaway.

Sounds very similar to cook off when dealing with firearms or weapon systems. As a Phalanx CIWS tech we always have to be aware of this when we do multiple gun shoots in a short time frame.