Random Question - Revving on the muffs

[hoeser]

I know personally that it is a bad idea to let an engine run beyond even 1500rpm on muffs due to lack of load and exhaust backpressure, but I was cruising through youtube a little while ago killing time at work and I just happen to stumble across a bunch of videos with people just hammering down the throttles on their engines on muffs, some even for ebay sales videos!! We're talking spinning them at least 4000rpm...

I'm just wondering, does anyone have any personal experience with failures related to running at high revs on muffs?

 

[tashasdaddy]

Re: Random Question - Revving on the muffs

i have not had the unlucky experience, but there have been many rookies, here on the forum who have. it causes runaway engines. you can shut them down, only way is to remove the fuel line and let it run out of gas. it's a good way to get a face full of connecting rod.

 

[jtexas]

Re: Random Question - Revving on the muffs

There was a thread here a few months ago from a guy whose motor ran away - turned off the key, pulled the emergency lanyard, yanked the spark plug wires, even tried slamming it into gear...finally disconnected the fuel line and ran the carbs dry. His engine didn't break apart - but I believe that it could have.

Not a personal experience, but I'm convinced it actually happened.

 

[tashasdaddy]

Re: Random Question - Revving on the muffs

that reminded me i did the follow up post on that. there is a link in this link, that describes the event. http://forums.iboats.com/showthread.php?t=195016&highlight=classic+runaway

 

[hoeser]

Re: Random Question - Revving on the muffs

That seems to me like a pretty bad scenario... I was thinking along a different line.. such a premature wear due to running without a load or something. The whole runaway business is just frightening.

 

[offshore100]

Re: Random Question - Revving on the muffs

Yep, I've seen it happen twice, very scary, rpm's so high you're sure she's gonna bust. One was a 150 or thereabouts Merc, the other was a 150 Evin rude, 60 degree.
Luckily, in neither case, did they explode, though the screaming noise is appalling. Both times they were on the ears, and revving too high with no load, then they took off.
Here's a way to stop them that hasn't been mentioned (it worked for us anyway): rags in the carb throats. Cuts off the air, voila, no combustion.
(Another reason I dislike breather covers.)

 

[hoeser]

Re: Random Question - Revving on the muffs

I think if that happened to me I wouldn't be fumbling around getting the engine cover off to air-starve the carbs for fear of catching a rod in the face or something. I'd probably just run back and yank the fuel line and hope for the best, while getting the hell away from it.

 

[offshore100]

Re: Random Question - Revving on the muffs

In both the cases I mentioned, covers were already off but, yeah, cutting fuel supply'd probably be the safer way.
So, then, when this runaway event occurs, spark has nothing to do with it?

 

[iwombat]

Re: Random Question - Revving on the muffs

Well, see there's a couple of issues at play here.

1. The engine is hot enough that the fuel can flash off from compression alone (dieseling).

2. Magnetos produce enough juice to not ground out very well at high RPMs.


I have a nice motorcycle story including the old stuck throttle, broken clutch cable, can't ground the magneto trifecta to back that up.

 

[tschamp20]

Re: Random Question - Revving on the muffs

even pulling the fuel line might take a minute to run out of fuel.mine ran for darn near 45 seconds before starving.not a runaway motor but still.