You may not believe me, but can you explain why this happened

[Jogin]

So today a couple of my classmates my instructor and I are working on a 2000 Johnson 50 horse, we ended up pulling a plug wire off for a cylinder drop test, nothing changed. We tried the same on the other cylinder after plugging the other wire back in, still nothing changed. Well we proceeded to pull both plug wires off. I understand if this is hard for you to believe but the engine kept running for about 5-10 seconds after both plug wires were removed. My instructor who has been in the field for over 35 years has never seen this and is shocked of it, as are all of us who were standing around the boat. He is going to be calling BRP to see if they know what could cause such a thing. Well I decided to post on the forums here to see if anybody has heard or seen such a thing happen before, and if so what the hell caused it?

 

[Will Bark]

Re: You may not believe me, but can you explain why this happened

Possibly carbon build-up in the cylinders?

 

[viciousfishes]

Re: You may not believe me, but can you explain why this happened

maybe if the cylinders were hot enough the compression was igniting the fuel/air. Kinda like a diesel engine

 

[Jogin]

Re: You may not believe me, but can you explain why this happened

we had thought of the cylinder being far too hot and causing it, however we put a temp gun on each spark plug and they were not over 140 degrees Farenheit and the block itself was about 135, so I don't think it would be that cause of those temps. How could carbon build up cause this just out of curiosity? I guess if that is the case we will toss a bore scope in there and have a look see. Any other thoughts are very appreciated.

 

[Sixmark]

Re: You may not believe me, but can you explain why this happened

It's not that uncommon with the right combination of air/fuel and pressure, a lean condition.....

 

[boobie]

Re: You may not believe me, but can you explain why this happened

Is this mtr just used in school and run on muffs or has it had the hell run out of it at times??

 

[F_R]

Re: You may not believe me, but can you explain why this happened

High rpms under no load condition. Called run-away. Actually is is the same thing that causes model airplane engines to run with no ignition. They start 'em with a glow plug, and let 'er rev, disconnecting the glow plug and away it flies. Don't be doing that, someday you may not be able to stop it as it just keeps on revving.

 

[Sixmark]

Re: You may not believe me, but can you explain why this happened

I guess what really scares me is that an instructor of 35 years has never seen or heard of this ever happening.

The principle of the internal combustion engine has never changed, the only things that change is the way we introduce air/fuel/ignition.

 

[Jogin]

Re: You may not believe me, but can you explain why this happened

it is a customers motor, He is a very good teacher and really knows his stuff, it may just be that he has never seen it before. So what I am understanding is with an improper air/fuel mixture it will function much like a diesel, by igniting purely off compression? in having that it would have to be an extreme amount of fuel in the combustion chamber in order to operate like that right? We also know it wasn't the proper way to adjust the carbs by not putting it in the test tank so it was under a load, ultimately what ended up happening is it was just going to be an idle stop adjustment since it was far too high of rpms? I am loving all the input, thanks again guys and or gals

 

[Bayliner1952]

Re: You may not believe me, but can you explain why this happened

I could see carbon heating up and holding it for combustion. Still sounds wierd though.

 

[F_R]

Re: You may not believe me, but can you explain why this happened

Your thread title..."may not believe it"....well I do believe it. In fact it is so apt to happen on certain motors thet the mfr equipped them with vacuum cut-out switches to prevent getting it into that run-away mode.

The fuel/air mixture does not have to be wrong. In fact a perfectly normally adjusted motor can do it. It is a combination of flywheel momentum, high vacuum, compression, etc.

 

[Sixmark]

Re: You may not believe me, but can you explain why this happened

The point is that under the ideal conditions it can happen and does.

 

[scooper77515]

Re: You may not believe me, but can you explain why this happened

Common in 2-stroke high compression engines (Seadoos). Just a small air leak somewhere and you get run-on.

My Volvo 4.3 does it as well, if I don't run supreme, or if i use regular and don't let it idle for a few secs before cutting the engine after a hard run.

The seadoos are scary, since they don't just run on, but actually rev up to 7200 rpms after you pull the key and plugs. Only way to stop it is to actually GIVE it gas, or choke it. Giving it gas cools the cylinders down enough to stifle the dieseling.

 

[dew2]

Re: You may not believe me, but can you explain why this happened

Carbon when hot enough will act like coal, charcoal. ignited it will smolder till its robbed of oxygen,Actually charcoal is spent fuel leaving only the carbon

 

[Silvertip]

Re: You may not believe me, but can you explain why this happened

Common in 2-stroke high compression engines (Seadoos). Just a small air leak somewhere and you get run-on.

My Volvo 4.3 does it as well, if I don't run supreme, or if i use regular and don't let it idle for a few secs before cutting the engine after a hard run.

The seadoos are scary, since they don't just run on, but actually rev up to 7200 rpms after you pull the key and plugs. Only way to stop it is to actually GIVE it gas, or choke it. Giving it gas cools the cylinders down enough to stifle the dieseling.

That engine should not run-on either and you shouldn't have to run premium fuel. You have an issue -- fix it. Generally the problem is timing and/or idle speed set wrong.

 

[JB]

Re: You may not believe me, but can you explain why this happened

I agree with the other JohnnyRude old timers. You got the engine into a mild version of thermal runaway and were very fortunate that it dropped out quickly.

Don't fault your Instructor. He is probably used to treating outboards safely. If you do that you may never see a thermal runaway.

 

[scooper77515]

Re: You may not believe me, but can you explain why this happened

My 4.3 volvo is all stock. I haven't changed anything (other than new dizzy cap, rotor, plugs, and wires). And if you do a search, it is a common issue with this engine. The suggested fix is "after a long hard run, let it idle for 20-30 seconds before you kill it", but my fix is super unleaded.

I never did figure out why the seadoos did it. I had 3 of them, all 787s, and they all did it at one time or another. So I assumed it was "normal". Only did it if started out of the water, as the pressure of the water against the impeller and engine kept it at bay. Only did it on the hose in the driveway.

 

[viciousfishes]

Re: You may not believe me, but can you explain why this happened

You couldnt have said it any better

 

[Sixmark]

Re: You may not believe me, but can you explain why this happened

My 4.3 volvo is all stock. I haven't changed anything (other than new dizzy cap, rotor, plugs, and wires). And if you do a search, it is a common issue with this engine. The suggested fix is "after a long hard run, let it idle for 20-30 seconds before you kill it", but my fix is super unleaded.

I never did figure out why the seadoos did it. I had 3 of them, all 787s, and they all did it at one time or another. So I assumed it was "normal". Only did it if started out of the water, as the pressure of the water against the impeller and engine kept it at bay. Only did it on the hose in the driveway.

That is not a common fix, it's a procedure to help avoid the problem. Find the problem then fix it and you don't need to buy premium or let it idle for extended periods of time before shutting it down.

 

[jeffersondodson561]

Re: You may not believe me, but can you explain why this happened

I never did figure out why the seadoos did it. I had 3 of them, all 787s, and they all did it at one time or another.

Did these have the "Rotax" induction?