really, how bad is revving your OB in neutral?

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Home Cookin'

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I understand that you should not rev your outboard up in neutral or on muffs. I'm going to wrench on a 20+ year old Johnson 70 this weekend in the driveway, looking for a short that occurs at higher rpms (3-4K) so I would want to give it some short bursts while watching the circuit tester meter, etc.

Am I really asking for big trouble or can the motor handle it under these circumstances? Is the issue that it doesn't pull enough water and will overheat, or that without a load it can run up to extremely high RPMs? If so I can watch out for either circumstance. I'll add, too, that I don't know how accurate the tach is, but I have a pretty good ear for a motor that is over revved.

I had an old 70 OMC that was very cranky to start and often got that screaming start. Nothing bad ever happened that I could tell; got 20 years out of it.
 
JB

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Re: really, how bad is revving your OB in neutral?

Outboards revved with no load beyond some rpm around 2K may go into what is called thermal runaway.

In thermal runaway it is in a Diesel cycle and cannot be shut down by turning off the ignition and/or closing the throttle. Rpm can easily reach dangerous levels and can explode the engine. It is not a pretty sight and could harm bystanders.

The only ways to stop a thermal runaway are to interrupt the flow of fuel or air or both.
 
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Barnacle_Bill

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Re: really, how bad is revving your OB in neutral?

You may get away with it many times but it only takes once to ruin your day. And all the rest that you would have had.
 
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Home Cookin'

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Re: really, how bad is revving your OB in neutral?

Yikes. This is the info I am looking for.

There are lots of "rules" about boating, often pronounced around here as if they are all absolute. But in reality, they range from "probably not a good idea" to "fatal consequences" or from "not good for your equipment" to "certain to destroy it."

I also know that just because someone always did something with no ill effect, doesn't mean he wasn't just lucky, and that includes me. "The Lord looks out for fools and drunkards."
 
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tx1961whaler

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Re: really, how bad is revving your OB in neutral?

JB said:
Outboards revved with no load beyond some rpm around 2K may go into what is called thermal runaway.

In thermal runaway it is in a Diesel cycle and cannot be shut down by turning off the ignition and/or closing the throttle. Rpm can easily reach dangerous levels and can explode the engine. It is not a pretty sight and could harm bystanders.

The only ways to stop a thermal runaway are to interrupt the flow of fuel or air or both.
Click to expand...

It's scary to have to stuff a rag in the carb when the motor is screaming at 5K....
 
Silvertip

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Re: really, how bad is revving your OB in neutral?

A two stroke will stay together at 5K -- but getting above that is very easy and it makes an awful mess when it comes apart. And if you think that's bad, I once saw a picture of a huge Catapillar diesel generator that was being installed. Brand new rig. Fired it up and an oil leak in the turbocharger housing caused the engine to run-away. The pieces of the crankshaft eventually found their way out of the engine and rolled out an open overhead door into the driveway. The engine itself was reduced to scrap. Nobody was hurt but the installers knew immediately what was happening and cleared the area.
 
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Re: really, how bad is revving your OB in neutral?

scary. as many times as I hear people revving OB's I wonder how often it happens with them.
 
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tx1961whaler

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Re: really, how bad is revving your OB in neutral?

Home Cookin' said:
scary. as many times as I hear people revving OB's I wonder how often it happens with them.
Click to expand...
Usually nothing happens. It's that time when everything is "just right" when it takes off. The old OMC big twins had a vacuum cut out switch on the side of them to cut off one cylinder when it started to run away. Not foolproof, because once it goes into compression ignition mode you have to get rid of the air-fuel source to stop it.
 
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Home Cookin'

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Re: really, how bad is revving your OB in neutral?

why couldn't you just pull the kill switch lanyard and stop the spark to the plugs?
Or could you yank off the plug wires?
 
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Fl_Richard

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Re: really, how bad is revving your OB in neutral?

The issue is the carbon in the heads ignites and spark is not required. Hense the run away part.
 
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F_R

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Re: really, how bad is revving your OB in neutral?

The issue with the old OMC Big Twins is not really thermal runaway. It is high vacuum and flywheel action and can occur at the drop of a hat, even on a cold motor. Killing the ignition will stop it, that is why the vacuum cut-out switch is effective.

But that is not to say they cannot also experience thermal runaway.
 
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rodbolt

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Re: really, how bad is revving your OB in neutral?

F_R
we used to have 7-10 of the big twin 40s for pulling nets in beach launched dorys. they could eat the sand and puke it out and keep on rolling.
that vacum switch is there for high intake vacum no load conditions and will cut 1 cyl.
usually that condition arises from a sand bottom sheared prop pin.
atched a 40 bigtwin grenade one day due to that switch had failed,pin sheared and I dunno how tight it was turnig but we all jumped to the back of the dory.
found out a few things about runaway that day,one is its impossible to get the hood off,two is its amazing how quick it happens.
three is the spring loaded throttle plates can actually be pulled open by engine vacum and it doesnt take much ign timing to run that RPM with no load.
second runaway was on an 83 35 johnson,overheated ran away and we could not stop it until the headgasket blew and it finnaly lost compression and quit.
yes virginia,there can be a runaway.
 
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michaeldodd

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This is a very old thread, but I wanted to report that while I don't know the odd of it happening, what I come to find out is called "run away" is a real and potentially serious thing. It happened to me this evening with my 1983 Evinrude 2-stroke 25HP. The motor exploded throwing the cowling (on fire) over a pier landing 50 feet away. A fire engulf the motor and after throw the minnow bucket's water at it (to zero effect) we thought better and jumped over the side. We were fortunate, some one 3 docks down came running with two extinguisher and then someone got a hose on it. Very real. Very scary.
 
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909

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What caused your motor to explode.

Was it revving in neutral
 
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