evinrude ghost

[turbosts]

After changing my powerhead and getting it adjusted, my 1989 evinrude powerhead ran flawless, the next day i started it for a test and it went crazy, it tached at about 5000 rpms and i couldn't shut it down. I removed the key, battery cables and even the spark plug wires and it continued to race out of control. I finally hit the manual primer to choke it out and it stalled. I reset the idle cam on the lower carb and now it idles fine, anyone have any input how this could happen???

 

[Gary H NC]

Re: evinrude ghost

Its called a runaway engine when it does that.It usually happens when the engine is revved up without a load on it.I happens when you rev it too high on the muffs.

 

[turbosts]

Re: evinrude ghost

Thank you for your input, I just started it and it took off, but now seems fine after I get this shift rod problem issue fixed, thanks again.

 

[reload]

Re: evinrude ghost

What do the muffs have to do with it? How could it run with the plug wires disconnected unless there is something glowing hot in the cylinder? I'm a little confused.

 

[turbosts]

Re: evinrude ghost

I used to be an engineer on power systems for general motors, and thats exactly what I thought, but the powerhead was just installed with no carbon deposits at all and it just kept running without spark or anything. I did go to a higher heat range plug after converting from my vro unit, but didnt think I would create a glowplug out of my spark plugs.

 

[JB]

Re: evinrude ghost

It's called thermal runaway and is in effect a diesel-like cycle with a severe ignition advance. It can stop two ways:
1. Remove fuel supply or choke it to death. This is a good way.
2. If it revs beyond tolerance of the parts it will explode.

 

[gss036]

Re: evinrude ghost

The only time I ever had that happen was on a 125cc 2 stroke Yamahs motor cycle. It was sucking air through an "O" ring on the lower end. My wife was riding it, she was scared (almost) to death. Thank goodness I was running synthetic oil. It peaked out over 10K rpm.
Just a thought on the cause.

 

[reload]

Re: evinrude ghost

Dieseling or engine run-on is a condition which can occur in spark plug gasoline powered internal combustion engines whereby the engine keeps running for a short period after being turned off, due to fuel igniting without a spark.

Dieseling is so-named because it is similar in appearance to how diesel engines operate, by firing without a spark. However the ignition source in a diesel is the compression of the gas in the cylinder, while in the dieseling phenomenon the compression ratio is not sufficient to ignite the fuel and it is a hot spot inside the cylinder which starts combustion.

This condition can occur for a multitude of reasons:

Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for sparking unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, non-smooth metal regions within the piston chamber can cause this same problem, since they can glow red. It has also been suggested that an improperly rated spark plug can retain heat and cause the same problem.
A carburetor that does not close entirely can contribute to running once the engine is off, since the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase can provide ample fuel for dieseling.
Incorrect timing.
An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to combust.

 

[F_R]

Re: evinrude ghost

They can also runaway even if not even warmed up (apperantly no hot spots). It's always been a mystery to me as to a logical explanation as to how it can happen. A two-stroke does not run with a completely closed throttle butterfly. It needs some air flow to make the system work as the charge has to be compressed in the crankcase to force it into the cylinder. That's why many butterflies have holes in them. During runaway I suspect that the incoming fuel charge with the abnormal RPM and mixture manages to be ignited by the outgoing exhaust gases. Just an unproven theory. I do know that it sure will get your attention.