Watch out, I think he's going to blow!

[TVB]

On the latest road trip, my Buddy 50 performed like a champ, ran for hours, got me to the Mackinac Island Ferry with 1 minute to spare, blah, blah, etc. But I did have one "incident" with it, that I'd like some feedback on.

I stopped the scooter to take a picture of the herd (flock? pride?) of wild turkeys in the road ahead of me. I was kinda in a hurry to get the camera out, didn't plan to stay still for long, and I wasn't thinking really clearly, so I didn't shut off the engine. A mistake, but let's let it go for now. Realizing that I'd need both hands for the camera, I again left the engine running (I know, I know) and kicked it up onto the center stand.

At that point, the engine started revving, then revving fast. I was not touching the throttle. I hit the kill switch. It kept going, getting faster. Thinking the kill switch had failed, I reached down and turned off the ignition. Still going. I seriously thought the thing was going to explode itself. The one idea I had left was to grab the brakes, and in doing so, I knocked it off the center stand. The rear wheel skidded briefly in the dirt, then stopped.

Somehow I maintained control of the bike with that last maneuver. After a minute of being glad I was too dehydrated to piss myself, I tried starting the engine again, and it ran normally. That was about 200 miles ago (yesterday), and there's been no change in the engine's behavior or performance since then.

So... WTF happened?

The weather was in the 80s, and I'd ridden the scooter about 160 miles in the previous 8 hours (speeds ranging from off to 40mph), so the engine was hot... but not horrifically so. The only other clue I have is that in the process of putting it on the center stand, I bumped the kickstart lever and heard something for an instant. Any ideas?

[charlie55]

Sometimes an engine will go wild when it's about to run out of gas and the fuel level in the carb's float bowl gets low enough to result in an extremely lean mix. Happened to me once with the CB, but hitting the kill switch did stop it.

Since your restart was normal, and you've been OK since the incident, perhaps (long shot) the float in your carb got stuck and the bowl started to run dry. The bump when it can off the center stand may have dislodged it thereafter.

As to the run-on after you hit the kill switch, was it an exceptionally hot day? If so, it could be that the combination of high engine/ambient temps and an extremely lean mix allowed your engine to "diesel" (i.e., achieve combustion without spark simply via heat and compression). It's a stretch, but I can't think of anything else.

You're right though, a runaway engine is damned scary.

[Stormswift]

Sometimes an engine will go wild when it's about to run out of gas and the fuel level in the carb's float bowl gets low enough to result in an extremely lean mix. Happened to me once with the CB, but hitting the kill switch did stop it.

Since your restart was normal, and you've been OK since the incident, perhaps (long shot) the float in your carb got stuck and the bowl started to run dry. The bump when it can off the center stand may have dislodged it thereafter.

As to the run-on after you hit the kill switch, was it an exceptionally hot day? If so, it could be that the combination of high engine/ambient temps and an extremely lean mix allowed your engine to "diesel" (i.e., achieve combustion without spark simply via heat and compression). It's a stretch, but I can't think of anything else.

You're right though, a runaway engine is damned scary.

Either that or you some poltergeist got a hold of your Buddy. I don't know much about combustion but I did have a brand new camera go psycho on me after I turned it off. I know how scarry that was so I cannot immagine what it was like to have a scooter go postal on you.

[jrsjr]

Sometimes an engine will go wild when it's about to run out of gas and the fuel level in the carb's float bowl gets low enough to result in an extremely lean mix. Happened to me once with the CB, but hitting the kill switch did stop it.

Since your restart was normal, and you've been OK since the incident, perhaps (long shot) the float in your carb got stuck and the bowl started to run dry. The bump when it can off the center stand may have dislodged it thereafter.

As to the run-on after you hit the kill switch, was it an exceptionally hot day? If so, it could be that the combination of high engine/ambient temps and an extremely lean mix allowed your engine to "diesel" (i.e., achieve combustion without spark simply via heat and compression). It's a stretch, but I can't think of anything else.

You're right though, a runaway engine is damned scary.

Yeah, the only time I ever saw that happen was on a 2-stroke Vespa whose carburetor had been leaned way out on a hot day. We thought it was going to seize before it stopped running but it eventually stopped and subsequently ran fine after it cooled down.

[TVB]

The fact that the engine was revving with the rear wheel up didn't surprise me too much. Even though I wasn't twisting the throttle, the brake was off and the wheel was free to spin, so there was nothing to hold it back from "idling" really fast.

I can't quantify it, but the conditions were definitely "hot". It was late in the day, sunny, and it was farmland without much shade. It was after a rather long day of riding, and I hadn't stopped very recently. So it's not much of a stretch to stipulate that it was "hot enough".

The log for my next fuel stop says I added just over a gallon, which was probably about 15 miles later, so I was "low" but still a ways from empty. Maybe putting it up on the center stand simulated an "almost empty" situation?

The fact that both kill-switch and key failed to stop it makes this "dieseling" hypothesis seem pretty likely. That's two places where the spark should've been stopped, and I can't buy the idea that both of them failed to open the circuit. So the gas must have been igniting without. The theoretical physicist in me always knew that was possible, but I'm not much of an engineer, so I didn't think it was actually... possible.

While the poltergeist theory is simpler... I am a skeptic at heart.

[Lokky]

Sometimes an engine will go wild when it's about to run out of gas and the fuel level in the carb's float bowl gets low enough to result in an extremely lean mix. Happened to me once with the CB, but hitting the kill switch did stop it.

Since your restart was normal, and you've been OK since the incident, perhaps (long shot) the float in your carb got stuck and the bowl started to run dry. The bump when it can off the center stand may have dislodged it thereafter.

As to the run-on after you hit the kill switch, was it an exceptionally hot day? If so, it could be that the combination of high engine/ambient temps and an extremely lean mix allowed your engine to "diesel" (i.e., achieve combustion without spark simply via heat and compression). It's a stretch, but I can't think of anything else.

You're right though, a runaway engine is damned scary.

Yeah, the only time I ever saw that happen was on a 2-stroke Vespa whose carburetor had been leaned way out on a hot day. We thought it was going to seize before it stopped running but it eventually stopped and subsequently ran fine after it cooled down.

hindsight is 20/20 but wouldn't you just turn the fuel tap off if that happened on an old vespa?

[JohnKiniston]

Sounds like it could be an air leak. That would explain the kill switch and the ignition switch not doing anything.

[agrogod]

Sometimes an engine will go wild when it's about to run out of gas and the fuel level in the carb's float bowl gets low enough to result in an extremely lean mix. Happened to me once with the CB, but hitting the kill switch did stop it.

Since your restart was normal, and you've been OK since the incident, perhaps (long shot) the float in your carb got stuck and the bowl started to run dry. The bump when it can off the center stand may have dislodged it thereafter.

As to the run-on after you hit the kill switch, was it an exceptionally hot day? If so, it could be that the combination of high engine/ambient temps and an extremely lean mix allowed your engine to "diesel" (i.e., achieve combustion without spark simply via heat and compression). It's a stretch, but I can't think of anything else.

You're right though, a runaway engine is damned scary.

Yeah, the only time I ever saw that happen was on a 2-stroke Vespa whose carburetor had been leaned way out on a hot day. We thought it was going to seize before it stopped running but it eventually stopped and subsequently ran fine after it cooled down.

hindsight is 20/20 but wouldn't you just turn the fuel tap off if that happened on an old vespa?

It would still continue to "diesel" until the carb ran out of fuel.

[BuddyRaton]

Yeah, the only time I ever saw that happen was on a 2-stroke Vespa whose carburetor had been leaned way out on a hot day. We thought it was going to seize before it stopped running but it eventually stopped and subsequently ran fine after it cooled down.

hindsight is 20/20 but wouldn't you just turn the fuel tap off if that happened on an old vespa?

It would still continue to "diesel" until the carb ran out of fuel.

Pull the clutch, stomp on the brake, shift into first and stall it out...don't forget to hang on while you do it! I had a vintage with a bad kill switch for a while and this was the routine way of shutting it down.

[TVB]

Sounds like it could be an air leak. That would explain the kill switch and the ignition switch not doing anything.

Could you explain this a little more?

[jrsjr]

Sometimes an engine will go wild when it's about to run out of gas and the fuel level in the carb's float bowl gets low enough to result in an extremely lean mix. Happened to me once with the CB, but hitting the kill switch did stop it.

Since your restart was normal, and you've been OK since the incident, perhaps (long shot) the float in your carb got stuck and the bowl started to run dry. The bump when it can off the center stand may have dislodged it thereafter.

As to the run-on after you hit the kill switch, was it an exceptionally hot day? If so, it could be that the combination of high engine/ambient temps and an extremely lean mix allowed your engine to "diesel" (i.e., achieve combustion without spark simply via heat and compression). It's a stretch, but I can't think of anything else.

You're right though, a runaway engine is damned scary.

Yeah, the only time I ever saw that happen was on a 2-stroke Vespa whose carburetor had been leaned way out on a hot day. We thought it was going to seize before it stopped running but it eventually stopped and subsequently ran fine after it cooled down.

hindsight is 20/20 but wouldn't you just turn the fuel tap off if that happened on an old vespa?

He did turn the fuel tap off. It was running off the fuel in the carb and, I guess, what was in fuel line leading up to the tap. Also it may have had something to do with the fact that he was running a prototype of one of Hot Rod Al's carbs and we were at pretty high altitude at the time, I don't know. Scoot Richmond Chelsea would probably know more about Robert's setup than me because this was on the 2004 Cannonball that staged at her shop. I do know it was really pretty scary listening to the motor run away at extremely high RPMs.

[BuddyRaton]

Sounds like it could be an air leak. That would explain the kill switch and the ignition switch not doing anything.

Could you explain this a little more?

I was thinking air leak too. An air leak is just like running with an uber undersized jet.

Here we go....the piston goes up...increasing the volume and decreasing the pressure inside the crank case. The crank comes around and allows air, fuel and oil from the carb to get sucked in...if you are jetted correctly...you are getting the proper mixture. The mix is transfered to the cylinder on the down stroke of the piston and it starts all over again




Now...if you have an air leak say at an oil seal or the carb base, when the pressure in the crank case drops to suck in your mix from the carb it will also pull in additional air from where ever the leak is. The result is you are running way too lean and you will be running way too hot!

If all that happened was dieseling you got away lucky! Usually what happens (and has happened to me more than once) is that the top end gets way too hot. The circumference of the piston starts to expand from the heat until it doesn't fit in the cylnder any more. That's when the motor siezes..the piston just gets too big for the bore and gets stuck.

Several things can happen when the motor siezes...none of them good!

[charlie55]

While it could theoretically be an air leak, they don't usually tend to be transient. But then again, everything's speculation until you actually get in there and start tearing things apart. Perhaps we'll never really know for sure.

If I had to pick a diagnostic starting point, I'd probably pull the carb and give it a once-over, paying special attention to the float, the fuel cutoff valve that it operates, the valve's seat, and the bowl in general.