Melt Down

[Horicon Joe]

I am looking for a technical explanation on how this can happen. I did a lot of riding this weekend with the 80 Liq. Yesterday I was going wide open down a long straightaway and blew a belt. The engine over revved and I did not have a rev limiter on it. I tried the kill switch right away and it would not shut the high revving engine off.
I flipped the hood up and pulled the plug wires and no luck ..... It just kept racing for about 1 1/2 minutes until it seized up. I saw this type of thing happen a long time ago when a fiends dirt bike did the same thing but we were able to kill it by popping the
clutch with 3 guys holding the front of the motorcycle to kill it.

My guess is the plugs get so hot they enable the ignition part of it that cant be shut off. The next thing is where does it get the fuel to run at that kind of speed? Perhaps without a belt the idle speed fuel from the carbs can sustain a engine to run freely like that? Last of all ..... it needs lots of air to do this as well does it not? I'm thinking the PTO side seal got destroyed somehow during this that allowed the air supply? Would the Rev limiter have prevented this?? If so .... MY BAD !!!! Is there a technical term for this phenomenon?

Joe

PS .... yes this is the same sled I crashed a few weeks back ...

[ChickenRat]

This happened to me in the past with an older motocross bike three times but the motor didn't melt down. Brought it home, took off the head and there was a piece of carbon buildup on the top of the head. I was told that this can retain heat (glowing red) and continue to ignite fuel & air to keep it running with no functioning ignition. I scraped this off and it never happened again. I think it didn't sieze because this bike was premix so the gas/oil was mixed for the richest the motor would ever need and with your oil injection and finger off the throttle, it was only delivering enough oil for low speed RPM's and was way too lean and burned down. The only way that bike shut down was to shut off the fuel supply or air. I pinched off the fuel line and it shut down in a few seconds.
It is quite scary the first time this happens to you as you've never heard your engine scream like that.....

Robb

[Horicon Joe]

Would the rev limiter have prevented this? The book says yes but I am doubtful.

[JDYODA]

Joe - sorry to hear about the melt down -- Still had a good time on Sat. and Sun riding. Let me know when you tear it down I would like to see it JDYODA

[Danzig]

I would have to believe if the limiter was hooked up, this may not have happened. However, the burn down as you explained it sounds as if you already had it shut down, which leads me to believe, you then had no spark, thus your motor was burning itself down on its internals, that being carbon, un-burnt fuel/oil, then finally getting hot enough to start liquifying the aluminum and burning it. My experience with "rev limiters" as to govenors is kinda the same. However looking at the design of the rev limiters on these machines, the only thing that is stopped when the rev limiter is engaged is spark. Maybe I am full of BS too.

My opinion.....put a limiter on it, this burn down may have been prevented.


I think in the NAVY we called this a Class Delta fire.

[Matt]

I don't think the rev limiter would have helped. Whatever was happening to cause the pre-ignition was probably happening before the belt came apart.

I would also like to know how it continues running with only a little air. I don't understand that.

[ChickenRat]

Rev limiter would have made no difference. The carbon was the igniter and thus the limiter would have made no electrical difference. That motor was nothing more than a big suction pump sucking in all the air & fuel it could to continue to run. Probably shutting off the air source would be safer for the motor as it would still be getting fuel/oil and being lubricated. Shut off the fuel/oil and when it burns out the motor is still spinning at an incredibly high RPM and has a higher potential for damage as it winds down.

[Danzig]

My opinion, if the rev limiter would have been working to begin with, the motor may not have gotten to the point of igniting the carbon at all, possibly. It's history now.

[jdkevlar]

That was real interesting Joe, maybe next time I will have to take it easy on you at the intersections. JDKURTZ

[Horicon Joe]

This is the second time you had a hand in wrecking this sled...........
I need to learn to stay away from you.

As far as what could have or not saved this thing ...... maybe I will
learn more when I tear it down.
I wish I would have thought of removing the air supply when it happened.
I wonder it my hand over the air box hose would have worked? <_<

Joe


PS ...... Anybody feel sorry enough for me to sell me some pistons? :rolleyes:

[JRC ETD]

Fate...choking the air supply would have been to late, engine was injured already, don't blame yourself, not alot you could have done, they were made to do what you were doing.....just bad luck!

[Mr. Deere]

Wait what would have happened if he had kept the throttle pinned when it was off wouldnt that have kept the oil moving to the cylinders like at WOT. Would it have helped. BOB

[WatertownBob]

Whats the best action? Putting your hand over the intake hose to the airbox. Or would opening the choke work?

Bob

[427cobra]

I was thinking about this post.

I think the rev limiter would have prevented the engine from ever over rev-ing and getting to the point where you were experiencing the pre-ignition.

So what I believe would have happend if the rev limiter was installed is:

Your belt would have broke...rev limiter would have done its job....sled would not have over revved.....no melt down

So I would have to agree with Paul

just my .02

[Horicon Joe]

I would agree ...... The limiter would probably have saved saved it.
Another lesson learned.

Joe