Engine runaway.

thatone123

Banned
Joined
Mar 7, 2009
Messages
707
Got one more question here, then I'll let it go. How common are runaway outboards where carbon in the engine gets so hot that it causes the engine to super accelerate until it runs out of gas in the carb if you are smart enough to unhook the fuel hose FAST and it won't stop otherwise. I understand this happens the most when the outboard is on muffs?
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Runaway is very common in motors running on muffs or in neutral/no load condition. But I don't think it is a glowing carbon problem. I asked an OMC engineer to explain it years ago. He said something about flywheel momentum and abnormally high intake manifold vacuum. Can't say that I can explain it 100% even yet, but have experienced it many times. It happens when for some reason the motor is revved up in neutral or out of the water. Usually because some unkowing or careless person did it. In fact, some motors are so prone to runaway that they even have a cut-out switch to prevent it.

Show me a guy high-revving an outboard in the driveway and I'll show you a guy that doesn't know what he's doing.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
The OMC Big Twins were prone to it if running Wide Open, had the Throttle suddenly closed and shifted to Neutral before the engine was at idle speed.
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
As FR stated its not carbon...if it was you would be seeing pre-ignition issues.
 

thatone123

Banned
Joined
Mar 7, 2009
Messages
707
FR, I have revved quite a number up in the driveway, but only maybe to 1500/2000 rpm and have never had it happen (yet) So that I won't become that "guy in the driveway" should I not even rev that high up? or whar?
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
As jimbo said, the 1950's-1960's era 35-40hp Big Twins were particulary prone to runaway when revving in the driveway. But they also were fitted with a vacuum cut-out switch which would slow them down (if it was working) Anything over 2000-ish was likely to initiate a runaway on those motors. I've initiated the runaway many times on those when working on the DC generator systems. You had to get them up to around 2000 before the generator cutout relay would close. So, you get the picture. I once had a 40hp Scott-Atwater go berserk and wouldn't slow down. No vacuum switch on those. I wound up grabbing a tool and busting a spark plug (in terror). So, bottom line, is it depends on the particular model. Some are much worse than others.

As for the comment about the guy not knowing what he is doing, if anybody reads these forums very long, he is going to see a topic where somebody says "It will run full wide open on the hose, but has no power in the water". Wide open throttle on the hose?? Gimme a break.
 

thatone123

Banned
Joined
Mar 7, 2009
Messages
707
Thanks FR, I will keep that stuff in mind. Wow, full on the hose...funny...
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
The bit i dont get is that it must be getting ignition from somewhere as the key is turned off, and if its hot enough to fire the mixture then it would then id of thought be hot enough for pre ignition, perhaps its the temperature of the head
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Actually, there are two types of runaway. There is the kind where ignition come from a glowing hot spot or high compression. Fortunately, that kind is pretty rare in an outboard and usually takes a very high RPM to set it off. Far more common is the kind I've been mentioning where the ignition is functional. That is what happens in the OMC Big Twins at 2000 RPM or above, and the reason the vacuum cut-out switch can slow it down by killing the spark on one cylinder. Many rope start motors do not have a key switch or stop button, and if the vacuum cut-out is missing or non-functional, things can get exciting.
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
Ah ok, ive only had the high rpm type- when i turned the key off and it kept on getting faster and faster and screaming louder and more shriller til i bungled around in fear taking the fuel hose off and took off around the house, was probably how those Stukka dive bombers use to affect
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,040
??----Using the kill switch and turning the key OFF is the same thing.-----Folks are clearly saying that they turn the key OFF but motor keeps running at very high revs !!
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Many rope start motors do not have a key switch or stop button, and if the vacuum cut-out is missing or non-functional, things can get exciting.

Consider it, Boredom Relief
 
Top