Runaway RPMs in Driveway..What gives???

[12Footer]

89 Force50HP . I had just returned from saltwater, so had the muffs on, started the engine to flush, and went about stowing the gear...When all of a sudden, the engine started racing..I mean, like someone had put the binickle wide open,or at least around 4000RPM. I had to kill the ignition! I started it back up, and it was fine,then it sneezed ,and ramped-up again!!<br />I've never had an outboard do this before, and I have no clue why it's doing this.<br />Anyone know WTF is happening here?

 

[Dhadley]

Re: Runaway RPMs in Driveway..What gives???

I had a V4 cross flow do that once. Took me about a week to find the cause. You got 6 days to go. LOL! Sorry - I couldn't help it. I'm sure you have verified all linkages are in tact, and to the idle position with the throttle plates closed. No sheared flywheel key and so on. <br /><br />Here's what I found - The small plastic yellow oil line plug that's supposed to be clipped on a hose came off and got sucked into the air box. As the owner was running it got pulled thru the carb throat and into the reed box. It wedged one of the leafs open. It was "idling" at about 2500. Even with all the linkages off. I finally pulled the carbs to switch them with the other engine and just happened to look in the intake and there it was in the corner of one reed box. Pulled it out and all was right with the world again.<br /><br />There might not be anything wrong with your reeds but it's a thought.<br /><br />Good luck! Really!

 

[jegervais]

Re: Runaway RPMs in Driveway..What gives???

Yep, seen it once and heard plenty of horror stories about it. The 40/50 JohnnyRudes are good for it too. There's no backpressure on the exhaust, it allows it to run too lean - just barely touch the throttle and she's off! The only way to stop the JohnnyRudes is to flood it out with the primer or put your hands over both carb throats - pulling the kill switch, turning the key off, even pulling plugs wires sometimes has no effect, its pure heat and crankshaft rotation.<br /><br />The Laws of Physics - Go Figure...<br /><br />-John

 

[DJ]

Re: Runaway RPMs in Driveway..What gives???

John is correct. In effect, a two stroke engine can run without spark induced at the spark plug. As long as there is a sufficient amount of heat (glowing carbon) and compression. Just like a two stroke Detroit Diesel. In some cases they'll run backwards as long as there is a source of fuel-excess fuel in the exhaust housing.<br /><br />Carbon build up adds to this phenom.

 

[petryshyn]

Re: Runaway RPMs in Driveway..What gives???

Interesting topic! Hey 12'er, just for the heck of it, next time its doing it, stretch a plastic bag over the prop exhaust. See if it stops ramping up with some back pressure.<br /><br />

 

[chumbucket]

Re: Runaway RPMs in Driveway..What gives???

Footer sounds like one of those steam driven boiler feed pumps in fireroom that lost suction or leslie reg. stuck <grin> head for the hills !!!!!!!!!!

 

[ODDD1]

Re: Runaway RPMs in Driveway..What gives???

12 footer... common on the 2 cyl force/chrysler motors....if you get the motor warmed up an try leaning out the idle mix, you can even get them to do it in the water, just not as bad.<br /> you need all the conditions you had, plus a lean mix in the cylinders to cause this...fatten up the mix and it will go away...just hitting the choke will usually settle em right down....have wondered about the mechanics causing this, the best I have found is an abnormal combustion process dealing with 'free radical' hydrocarbons...I'm still lookin for a good explanation........

 

[DJ]

Re: Runaway RPMs in Driveway..What gives???

They taught me, way back when, that when a Detroit "takes off" "best run parallel to the crankshaft".

 

[DJ]

Re: Runaway RPMs in Driveway..What gives???

ODDD1,<br /><br />When a two stroke diesel "takes off", one of two things happens. 1. The "rack" is so far out of adjustment that the engine decides how much fuel it needs and will run at that fuel setting until someone either: a; kills it, or b; it destroys itself.<br /><br />The worst scenario is when the engine starts running backwards using its own crankcase oil as fuel-no control, no way to shut it down. Just run parallel to the crank and look for cover. Engine parts to follow! Outboards can (theoritically) do this for brief periods using unburned fuel in the exhaust housing.<br /><br />Two stoke OB's are a little bit different. They will run without spark. They still continue to get a mixture of gas and air as long as the pistons are moving in the bore. Since there are no valves, just ports, this mixture is going to get into the combustion chamber anyway. As long as there is something hot enough to set it off, the whole cycle starts again. <br /><br />Granted, there is no POWER in this phenom., just rpm. Maybe enough to use the old adage. What do you do with an OB? Can't go up, dig a hole-quick!

 

[12Footer]

Re: Runaway RPMs in Driveway..What gives???

Reading the replies,and putting 2+2 together here...<br />I did do an inordinate amount of idling (dern sea cows!!!). Mostly,the river I was in is one big manatee speed zone this time of year. And I was just chuggin-around, lookin for holes to record for future fishing trips. These Force engines carbon-up faster than a lit cigarette on a Harley as it is. I usually decarb it on the way to the dock,but was out of juice on this trip...AHA!!<br />That is the one thing that sets this last trip appart from the last 3 trips out. Carbon built-up, and it ran away on it's own "glow plugs". No wonder the ignition had no effect without hitting the choke too!<br />I'll keep a spray bottle of water nearby from now on,so I can at least steam clean the jugs when (and if) it ever decides to whack-out on me again.<br />Thanks, y'all. <br />It's a frightening thing to see your engine do this, and be powerless to stop it.<br /><br />

 

[DJ]

Re: Runaway RPMs in Driveway..What gives???

12Footer,<br /><br />You got it-carbon=glow plugs!

 

[tteal]

Re: Runaway RPMs in Driveway..What gives???

12Footer,<br /> I had the same symptoms as you described just recently with my Force in the driveway with the muffs. Did you ever figure out what was causing it and also how did you correct it? Did you just adjust the air/fuel mixture screw or ????<br /><br />Tks,<br /> Tim

 

[catfish1]

Re: Runaway RPMs in Driveway..What gives???

my 85 force 50hp did it whenever i started it on muffs, i took it to a buddys house(he was a marine mech for 25yrs) he said that it was because of the single carb feeding both cyl, he says any single carb dual cyl will do it. but he says the force motors were the worst for it and they are very sensitive to backpressure on the exhaust, it wont do that if its in the water! i think he's right because my dual carb twin cyl evinrude wont do it.

 

[tteal]

Re: Runaway RPMs in Driveway..What gives???

Yeppers... took the boat out today and it did not do the runaway idle in the water and only with the muffs. Yaaaaay!!