Engine dies randomly at idle setting

My 912 ULS was new in 2001 and was never even out of its plastic sealed bag until a year ago when I first mounted it in the plane. All rubber and metal surfaces appeared like new condition with rubber components still pliable and no signs of age. During recent first starts at my home's shop it started quickly and idled great. At my shop I only ran it between idle and mid range rpm, both ran smooth. Oil pressure and temps have all been well within normal ranges. I very recently brought my new finished plane out to my hanger to do its first taxi runs prior to first flight. As before idle and mid range rpm operations were smooth and uneventful. During the initial taxi run while bringing the throttle down from midrange rpm down to idle (2000rpm) the engine died suddenly. The engine restarted fine with a little choke and I thought it was just a random event. However, a mag check at 3500 rpm showed a drop of about 500 rpm on one side. After taxiing uneventfully for another 15 minutes with random throttle settings at idle I thought everything was fine then it died again while bringing the throttle back to idle from mid range rpm. After that occurrence it turned over fine but didn't want to start right away, seemed like it was firing not on all cylinders. Rather than over stress the starter I pulled it back to the hanger to see if anything obvious presented itself. I pulled the carb bowls and they were both full of gas. Any thoughts on what I should look for? Any ideas are appreciated.

Larry, Long time for those carbs to be sitting unused. Not too worried about the engine but wondering about rubber parts inside the carbs. Also make sure your "chokes" are both returning to full off.
They may need a little lubrication also after all those years. Might end up rebuilding both carbs which is not a difficult job. Good luck. Can't think of anything else at this time to look into but maybe others will have some ideas for you to check into. Best of luck and be careful.

Larry - will it idle at 2100? If so, see if it will keep running on each (individually). If it won't run on one, does that match the side that had the 500 rpm drop?

I reran the engine today and discovered that I had set the idle at around 1500 rpm. At my shop it would run there but did run a little rough. On the grass taxiway at our airport coming down from mid-range rpm to that low idle probably had enough inertia in the decelerating prop to dip even lower than 1500 and cause the engine to die. I reset the idle today at 2000 rpm (as recommended by Rotax to minimize gearbox wear and tear) and every thing runs nice and smooth with no engine dying during speed transitions. I read a lot of discussion on this forum about idle speeds during landing approaches and the desire to get it below 2000 rpm to minimize float on landings. I also read about others that had their engines kill on final because of low idle settings. From what I can deduce from all that there doesn't seem to be any "perfect" idle setting that is preferred by most Kitfox pilots with 912 ULS engines. Or am I wrong on this point?

I also reran the mag checks and one side only drops 250 rpm and the other side drops 400 rpm (not 500 as previously stated). I don't remember what the acceptable range is right now, or what it means when its over limit, or what to check or adjust to get it back in limits. I suppose if there's even a small issue here it could impact how low I can push my idle setting without repeating my engine dying scenario. Please remind me what to look for to resolve this. Thanks

I believe the most rpm loss you should see is 300 rpm. Not sure if this has anything to do with it, but I'll throw this one out there. Yesterday I leaned out the needles one notch on the needle/circlip, (went to the top notch) cause my cruise EGTs were running 1275-1325 and I thought it could be a bit higher. The two cylinders I'm monitoring often read within a few degrees of each other, but lower then I thought they could be. Was going to go for a flight last evening, and had a mag drop of 450 to 500 on each side. Usually it drops 200-250 per side. Needless to say, I didn't go flying then and reset the needles to where they had been today. Didn't get a chance to make sure the mag drop is back to being acceptable again, but I'll bet it is. JImChuk

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Originally Posted by avidflyer

I believe the most rpm loss you should see is 300 rpm. Not sure if this has anything to do with it, but I'll throw this one out there. Yesterday I leaned out the needles one notch on the needle/circlip, (went to the top notch) cause my cruise EGTs were running 1275-1325 and I thought it could be a bit higher. The two cylinders I'm monitoring often read within a few degrees of each other, but lower then I thought they could be. Was going to go for a flight last evening, and had a mag drop of 450 to 500 on each side. Usually it drops 200-250 per side. Needless to say, I didn't go flying then and reset the needles to where they had been today. Didn't get a chance to make sure the mag drop is back to being acceptable again, but I'll bet it is. JImChuk

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Jim, My EGTs are running lower than yours (around 1100 F) but I'm still only on the ground taxiing so far. Are you thinking that leaning out the fuel/air mixture may change the mag drop, as the evidence on your engine seems to indicate? I don't have that needle/clip arrangement on my Bing 64 carb but I'll look in the manuals to see if there's a mixture control screw on the carb.

All Bing 64 carbs have the needle/circlip I mentioned in my post. There are three mixture circuits in a Bing carb. Idle circuit, which is controlled by idle jet, midrange mixture is controlled by the needle I mentioned, and main jet which controls full throttle mixture. (there is a bit more to it then that, but mainly that's how it works) At low RPMs, EGTs will be lower, I was seeing the 1275 to 1325 EGTs at 5200 rpm or so.

Jim,

Sorry for doubting your statement that the Bing 64 has a needle and spring clip. I'm used to the Bing 54 carb with a needle and spring clips on my previous Rotax 503 powered Challenger and I had assumed that the Bing 64 had done away with it. When I looked at the 912 illustrated parts list there it was just like you said. So much for my assumptions. My mag drop tests today with the rpm set at 4000 (per the Maintenance Manual) showed 200 and 300 rpm. Previous checks were done at 3500 rpm (based on my past 503 engine experience) when one side was high. I'll keep checking it to see if the reading are consistent over time and at various engine temps.

Forum experts,
This morning I was running the engine again and doing a carb sync with my CarbMate. While running the engine with the CarbMate hooked up I plug its tubes into the manifold cross over tube (option 2 in the Rotax maintenance manual). The engine was idling fine at 2000 rpm during the first few minutes of the sync check when all of a sudden the engine started to run rough and I noticed that fuel was spraying out of the vent tubes on both carbs. When I brought up the rpm to 3000 rpm it stopped spraying out of one side but continued out of the other so I shut it down. Later today I rewatched the Rotax videos on carb sync and they suggested I start the sync testing at 2500 rpm, which I'll try again at tomorrow. I've read that fuel overflow spraying out of these vent tubes can be due to too high fuel pump pressure. I currently don't have a port in the fuel pump output line to check fuel pressure but I could add one if this is the most likely cause. Is that the only cause for overflow and why would that suddenly happen when all previous idling tests didn't experience any overflow at any speed?

I am no expert, but my 912UL began running similarly to what your describing, hard starting, dieing and a strong gas smell at times. Checked carb floats and needles, they were fine. Eventually found a wire to one of my ignition modules broken inside the insulation, no sign of a break on the outside. Repaired that and took care of the problems. This was at about 25-30 hrs. since new. Something to check, may or may not be a problem.

Birdseye,

Might be a bit hard to diagnose without being on site and hands on; but, maybe try a couple ideas. Always try the cheap stuff first.

On the fuel system with a new engine and plumbing there is always the chance of crud making its way down the pipes to screw up the needle and seat assembly or plug up something that doesn't need to be plugged up - and it won't necessarily happen right away. If you haven't taken off the float bowl, that might be a thing to do in order to see if there is some dirt that has been washed down into the fuel bowl. At the same time, it is easy enough to pick out the needle, flush a little gas through by opening up the fuel shutoff valve and see if anything comes out the bottom - a coffee filter under the carb on each side will let a person see if any crud runs out. I don't recall if you mentioned about checking if the idle mixture is set to Rotax's initial setting but that may be something to check. It can happen that a small piece of dirt can wedge in the idle mixture adjusting needle as there isn't a lot of space there - backing off the idle mixture screw 2 or 3 turns and allowing some fuel to run through can occasionally flush out a small piece (or taking the mixture screw out completely). A decent diagnostic fuel pressure gauge is useful to determine if the fuel pressure is correct - most automotive diagnostic gauges come with an assortment of adapters so a person can tap right into the line anywhere they want to. Rotax has managed to manufacture some out of spec fuel pumps on occasion (as well as bad fuel hoses).

An additional thing a person can try on the ignition is to find a inductive automotive timing light which will allow a person to determine if consistent spark is going to each of the 8 plugs. DO NOT UNDER ANY CIRCUMSTANCES TRY TO CHECK SPARK USING THE "BRIGGS AND STRATTON "METHOD OF PULLING A PLUG WIRE OFF TO CHECK EOR SPARK - GUARANTEED TO KILL YOUR MODULE(S). I have seen on more than one occasion where a new spark plug was bad or damaged - if you have a resistance check multimeter - checking the resistance of the plug's core can uncover a problem - should be around 5,000 ohms plus or minus 1,000 or so -a bad plug will usually show up as infinite resistance. Secondary ignition wires (although not likely) can also be checked with the meter to see if they are all the same.

Don't know if any of this will help, but wishing you good luck.

According to my research on this forum's history fuel leaking out the 912 carb vent tubes around the idle rpm is quite common. Mine was doing this a lot and installing a new carb overhaul kit (level 3 service pack) helped but didn't eliminate it completely. The floats were visibly floating and not sinking and they were below the spec dry max weight of 7 grams a set. My engine was getting hard to start as well. Then someone on the Rotax forum suggested I lower the float levels by bending the tabs on the float valve armatures. The idea was to reset the float levels to get the fuel level in each bowl at around half full at idle. Each time I had previously pulled my float bowls off after the vents were seen leaking the bowls were always full to overflowing. So I bent the little tab on the float armature accordingly and lowered the fuel level in each bowl. After that, no more leaking out the vents and the engine starts right away and idles smoothly down to about 1800 rpm. Below that rpm the left carb still vents a little gas occasionally so I probably need to lower the float level in that carb just a little more. Bottom line for me seems to be that the fuel float level was originally adjusted too high and adjusting it down appears to have solved my hard starting, rough idling and fuel venting problems. I suppose there is a fuel float level setting that is too low and may cause other unforeseen problems, but for me the setting that equates to a fuel bowl level around half full appears to have solved all my rough idle operating and starting issues. The engine seems to run smoother overall and I haven't even done the pneumatic carb sync yet. At the moment I'm a happy camper.