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Lean run away

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I am trying to decide if I agree with the "lean runaway" diagnosis. They claim that the bike running out of fuel will lean out and cause a lean run away. I don't believe this to be possible. Before I ever heard of such, I had determined that it was running off of residual oil in the crank case, that is expected to be there, as a diesel would. Here is my post from another forum before I ever heard of "lean runaway". I referred to it is WOT sticking. 

"My slide spring is much stiffer than stock, so what ever it is that triggers the wot should be the subject of investigation. But for sure, without air, this high rpm is not possible, so we know that the slide is up and being lifted. But what triggers it???? I only have my riding of the time before it happened. If I knew others it would help so much. It is a mere guess at this point. Anyone who would give description of their riding 5 minutes before the WOT stick would be appreciated. But I will guess. I was riding ultra slow, like a trials rider for 30 minutes before my WOT stick. Mostly trying to stall the bike on extremely low rpm's on a hill. Working on my clutch skills, etc. Rpm's never met 1/2 throttle or close. 1st gear 80% of the time. I think I was getting a build up of unburned oil in my case. The fuel burns leaving the oil suspended. What I would like to do is start studying how much oil puddle is remaining in my crank case after different throttle ranges, maintained for 20 minutes. up to 1/4 throttle. 1/4 to 1/2 and 1/2 to wot. At 80:1 at mid main, I don't expect much puddle. It burns it almost instant. 1/4 to 1/2 likely perfect for 80;1. But under 1/4 throttle, I am expecting that oil is building up, in need of being "cleaned out". As oil remains, it begins to dam up/restrict fuel sent from the carb, through the crank case, working expondential to block clean fuel. Almost like flooding the case. During this time, your engine should have a degree of engine braking. At this time is when you run the potential of the surprise  reverse, running backwards,  because resistance is on the crank to reach TDC. Now, just as I did, After awhile, I wanted to rip off into the sunset. 200 yards into the wot run, I have now heated up the piston enough to turn my 2t into a diesel engine. Or it could simply be the trigger to start the lift of the slide that is fired by the plug? This is why the kill button will not kill it when it sticks. As soon as it burns out the excess from the flooded crank, it should go back to normal. I have heard trials riders use the term, "clean it out". Now I know what they mean. So, this could be way off, could be close, but should be a good starting place for discussion. It's worded like it is fact, but it is actually me speculating, thinking out loud, trying to see if it adds up or not. It may be enough for someone else to figure it out. But we have to figure it out"

As I later studied the topic I see it is called "lean runaway". I can see how the bike can increase idle for a moment when the fuel starts running out, creating a leaner burn, most 2t being tuned rich. But the WOT stick last for much more than a moment. It is clearly diseling. I can see how a vacuum leak can do the same but many of the post on the net found nothing wrong after the incident.  My bike after the incident was ran another 20 minutes to make sure all was OK.  I don't understand how a cold motor does it since it does not have continuity with my theory. Several post on the net is regarding a cold motor on start up. What I suspect now after much study is that a "trigger" happens to start the diseling. The fuel  either the crank residual pool oil, or fuel being vacuumed out of the carb due to a lifted slide from vacuum, it has to have air, lots of it for near WOT with either crank oil or carb fuel. But the "trigger" that starts it is what I wonder most. We know that the kill will not kill it so the combustion chamber, either plug, piston or other is acting as a glow plug . Again, running out of fuel would be a split second of lean, not 45 seconds or more of shear terror thinking it is going to blow. If it does happen to you the best reaction is to gear up, brake hard and dump the clutch to stall. If yours is a case leak,  you can simple hold the kill switch and throttle open to pull in more fuel to change the lean condition because your not likely full blown WOT due to highly restricted air. Again, I am talking as if this is fact but it is all speculation on my part, thinking out loud, looking for contradictions, prodding others to thought, hoping for continuity. So, several questions, is it fuel from the carb or  residual oil from the crank that is burning? This could be answered if we knew how long the lean runaway would last? Many said they killed it by pulling the plug wire. Not so, they only thought this fixed it. I think it ran out of useable oil???? We know when it happens that hitting the kill button will not stop it and this is the same as pulling the plug wire. Fuel from the carb would go on and on. Unless you were one of the crowd that says theirs triggered from running out of fuel. Yet I have and many others run out of 2t fuel many times without this triggering and when mine happened I was low on fuel but still have plenty after another 20 minutes of riding???? Some say trash in the carb can do the same. I have a hard time with this because a partial blockage will lean up the mix as if it were a small jet, but then it should stay this way until the carb was taken down and cleaned. Not it did it, now I finish my ride. Next specific question.... what then is the trigger. Something is the trigger to start the diesel situation. This assumes a hot something acting as a glow plug... or does it? If you search the net, all sorts of answers exist as to why it has the so called lean runaway. But nothing so far sounds conclusive. Although the running out of gas does give a "trigger" for the theory where no other theory offers a trigger. So, I am rambling, thinking out loud. Someone more experienced than me with real experience, rather than me speculating, can take this over from here. I will  link  an interesting chart about oil migration dwell times that might apply

https://www.maximausa.com/pdf/Oil Migration Sheet.pdf

Edited by 1gr8bldr

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My 3 wheeler does this when it runs completely out of gas, I can't figure it out.  Also I blew up my jet ski because it ran away.  My jet ski had been sitting for about 10 years so I was trying to get it going again but it ran wot out of nowhere and I couldn't get it to stop.  I shoved rags down the intake and exhaust, kinked the fuel hose and pulled the plug wires and it was still running wot with no plug wires.  What happens is the spark plug gets red hot and starts acting like a glow plug and the motor will just run on whatever fumes it can scavenge up.  On a motorcycle you can just drop the clutch in gear and it will stop.  On my jet ski you can hold the throttle wide open along with the kill switch and it will stop.

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28 minutes ago, 96motorhead said:

My 3 wheeler does this when it runs completely out of gas, I can't figure it out.  Also I blew up my jet ski because it ran away.  My jet ski had been sitting for about 10 years so I was trying to get it going again but it ran wot out of nowhere and I couldn't get it to stop.  I shoved rags down the intake and exhaust , kinked the fuel hose and pulled the plug wires and it was still running wot with no plug wires.  What happens is the spark plug gets red hot and starts acting like a glow plug and the motor will just run on whatever fumes it can scavenge up.  On a motorcycle you can just drop the clutch in gear and it will stop.  On my jet ski you can hold the throttle wide open along with the kill switch and it will stop.

If your bike is running a bit rich and  you have the idle set accordingly, you can get a bit of rev out as it goes lean when it runs out of gas. You can get 'auto' ignition with anything two stroke or 4,  but it will quit running when you kill the fuel supply.  on a motorcycle you can put it in gear to kill it as stated.   Where two strokes are concerned, it can wind to the rafters but only when it doesn't have a load as there isnt' much power being made.   with a little bit of an air leak and a fuel supply,  it can go on pretty long.  choking or hitting the enrichenor can bring one back to normal a lot of the time.  once you get into the auto ignition mode (comes with heat usually,  but in the case of an air leak, not always) the kill button is useless since the rig is lighting without spark.  if you do kill the spark it some times brings the rpm down a tad since you lose some heat from the plug helping the auto ignition process.  I used to blame those all on the throttle sticking,  and a badly grounded kill switch.   I have seen plug wires pulled off and the engine was still singing away.

Some pwc mfrs' warn their owners about reving one out of the water for that very reason.   Bombardier powered ones warn people as to the rpm to subject them to (one man on a post said his came with a warning of 3000 rpm) without a load.  This particular man is a civil engineer and he posted that on a engineers forum where you must be a member to post on said forum. (anyone can go to the site and read,  you see they don't want to completely hide their knowledge from the rest of us),  but you have to be a member to post or answer.  I was suprised to see that this man got more than a dozen responses before someone was able to explain how a tuned exhaust system works and why it "runs away".  The OP was going on about how bombardier must have known about their design flaw to print the manual like that,  and how he couldn't believe they could sell them to the public knowing about the "obvious design flaw" that they were apprently admitting to.   The other engineers answers were likewise scary.  Finally a mechanical engineer explained how a two stroke exhaust system worked.  It was funny.   

 

2 minutes ago, Wild Alaskan said:

had an outboard jet do it once when the boat went dry unexpecteldly. was revved wayu up being out of the water with the ignition off, luckily the push to choke tamed it back down.

yep,  since it is a lean condition helping it along, the choke is the key.   

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Run away? No A 'Run Away' is when an engine either has a stuck throttle at wide open (gas) or alternative fuel supply (diesel).

A gas engine can be carboned up and glowing carbon can ignite the fuel as to render the spark (and therefore the kill switch) moot, foring you to still the engine from alternative means.

Running the carb out of fuel can cause it to rev 'to the moon' for a little while? Yes. Not good for the engine? Yes

You never want to rev an engine with no load for any extended period of time, it permits an excessively lean condition that if taken to an extreme, can damage an engine.

If the plan is to 'empty the carb', remove the float bowl and wipe it dry.

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