912ul runs on

[egp8111]

My 912 wants to keep running for a time after I've shut off the ignition. I've checked the grounding of the ignition circuits and both work properly. The manual says this might happen due to the engine not being properly cooled prior to shut down but mine is doing it when it is relativley cool (CHT 180 and oil temp 150) any ideas ?

thanks,

EG
KF III/912ul

[catz631]

EG,
I had that problem with my 912UL. It would stop then run backward for a few revolutions. After doing the "valve clearance" check for possible air ingestion in the oil system so many times I got tired of it !(thought about replacing cowl fasteners with Velcro !)
After many fruitless checks including gearbox rebuild (after only 250 hrs)I replaced my prop with a much lighter smoother Kiev prop and that totally solved the problem !! Prior to the Kiev, I was using a Warp taper tip with nickle leading edges. Apparently the Warp was too heavy and causing the kickback/run on.
The "new" Kiev has been a wonderful prop ! Smooth as a turbine,good looking and the engine stops dead cold every time.
Dick Maddux
Fox 4
912UL
Milton,Fl

[Dave S]

Roberto & Dick,

I would keep some thoughts open that some additional constellation of factors, not only the assembly of a 912 and warp nickel edge taper, may be responsible for run-un. Our series 7 has this exact combination and there has never been any hint of a problem. Pretty sure others use this combo also.

Since I have never observed this on our plane, I obviously don't have direct experience solving this particular problem first hand.

With other engines, particularly high compression engines, a number of factors can cause run on, or "dieseling". here are a few I can think of right now.

1) Overheating can certainly cause it
2) Glowing deposits (lead, ash, carbon) on the plugs or in the combustion chamber
3) Incorrect timing (meaning too far advanced) which contributes to items 1 and 2 above
4) Having the idle speed set too high, which contributes to items 1 and 2 above
5) Kind and type of fuel used
6) loose p-leads or broken p-lead wires

I'd check the cheap stuff first.

Sincerely,

Dave S
KF7 912USL Warp

[jtpitkin06]

I remember in the early 80s when it was common for auto engines to run-on after turning the ignition off. It was always a combination of high compression and the new lower octane fuels. Nothing has changed in 30 years.

You didn’t say what kind of fuel you are using, mogas or avgas. If using mogas you might try switching to 100LL. Look to the fuel for the problem. Your supplier may have changed formulation without your knowledge. That dang “corn likker” is sneaking into fuels like soybean in burgers.
If using avgas, look at the plugs for heat indications… consider running a cooler plug. You might also look at idle mixture being a tad bit rich. I you don't have heat and you don't have fuel, the engine will stop.
As for putting a new prop on the airplane… the problem will not go away with a lighter prop, it is only masked. The lightweight prop doesn’t have enough flywheel effect to swing the crank over center at low RPM so the engine stops. That’s like saying your wheels vibrate after take-off so you now apply the brakes and there is no more shake. The underlying problem is still there.
JP

[Av8r3400]

John that is false assumption. The 912 flywheel is on the crank and the motor will run fine without a prop installed. Too heavy of a prop, like a nickel warp, will cause all kinds of problems with the motor. This may be one of them. If this motor was dieseling, like your 80s car, the motor would have distroyed itself.

[catz631]

John,
I also have to disagree with you. I have a mid 90's 912 UL 80 hp engine.(low compression) I don't have the slipper clutch. I use no ethanol premium auto fuel (not required) I have conferred with Ronnie Smith at SLLA who is a guru on Rotax engines in this area, for almost a year about the occasional run on problem. He suggested that I could have a gearbox problem as the prop could be stopping at the top of the pawls and swinging backwards at shutdown so we replaced the Bellview springs and shims all to no avail.
I have checked all the wiring,timing,fuel octane etc also with no good results. HOWEVER I did read an article about the Warp (somewhere) with nickle leading edges and how it was at the edge of the weight limits for my engine.(Roger in Tucson has also mentioned this)
I then opted to go for the lighter Kiev prop and walla,no more problem. The prop stops cold EVERY time and has for almost a year now since I installed the Kiev so I would concider the problem solved.
As for engine run on ,I own two late 50's Triumph TR3's and a 73 Stag so I am familiar with timing ,fuel etc in those old engines also.
I cannot say why the engine run on has also happened to me, but it did but then I also had a properly tighten oil filter come loose (fortunately on the ground) This ,according to Lockwood was not suppose to happen,but it did. I now safety wire the filter! Then too, another guy had this happen too and destroyed his engine. ( I believe reported here or on the Matronics forum)
Dick
Fox 4
912UL

Dave,
In my case ,I checked the idle speed,idle mixture,etc and I even bore scoped the engine for deposits. It was clean. I even used some octane booster. NOTHING worked untill I replaced the prop with the Kiev.
By the way I do have my excellent condition Warp,taper tip nickle edge for sale if anyone wants it. It is for a mid 90's hub which is different than the new hub. The prop is mounted on the wall and I don't think I will use it again. Makes a nice decoration though !
Dick

[jtpitkin06]

OK… I’m always willing to learn something. Maybe I don’t understand how an engine runs.

I have the impression it takes three items to get an internal combustion engine to run: spark (heat), compression and fuel. So please explain to me how changing a prop to a lighter version is anything but a change in the flywheel weight. It has no effect on compression ratio, valve timing, piston bore or stroke, ignition or fuel.

John Pitkin
Greenville, TX

[Dorsal]

Seems to me a heavier "fly wheel" could contribute to run on as it may drive a compression stroke that the lighter prop could not. This assumes there is some form of compression related ignition. I have certainly run across engines that would not run without a flywheel. Just an opinion, not my field of expertise.

[Monocock]

I too had terrible run-on probs with a heavier (6 kg/13 lb) prop. I switched to a Kiev (for lots of reasons, not just hard shut-downs) and after circa 200 shut downs I have never had a run-on problem since. When I was investigating the original problem my engineer swore that the prop weight had a direct effect on the shut down. I now have 2.5 kgs (5.5 lbs) less weight spinning on the hub.

Out of interest egp8111, what is your idle set at?

[catz631]

I have my idle set at around 1500-1600. This lower rpm will give me a lower rpm on approach(around 1900-2000 or so with the airload) On the ground I adjust it to 1800 for taxi and ground ops.
Dick

John,
I too was totally buffaloed by the prop/gearbox effect on run on but in my case it solved the problem and as mentioned I only stumbled on a possible solution by reading an article on the effects of prop weight on the Rotax engine. I wish I could remember where. I am always "in the books" over this engine as it was a completely different thing compared to the Lyc/Cont engines I was used to.
It is only in the last year or so that I have fairly well trusted this engine. I had many problems in the beginning and without you guys and other sources,I was on my own. I have attended 4 courses on the 912/2 stroke so far and have another renewal due this year. I still have much to learn !
I highly recommend that any and all of you guys attend an engine course if you can.Believe me they are very good and a wealth of information. As the man says," it's like drinking from a fire hose !" Take lots of notes !