Rotax 912 runs rough

[avidflyer]

I've been fighting with my 80 HP rotax 912. Here is the problem, when I do a mag check at 2500 or 3000, I seem to get pretty much the same rpm drop on both sides, about 150 rpm. If I do the mag check at 3500, one side drops about 150, and the other side drops about 500 rpm. I tied it off, and ran it to full throttle, and maybe 100 rpm drop at most on either side. Any ideas? Does it sound like ignition when it runs so smooth at full throttle compared to 3500 where it's very noticeable that it's not running right. It's not at all smooth at that rpm with both mags on. JImChuk

[desertdave]

Check your spark plug wires condition and if they are on the correct cylinder. Don't ask my how I know.........

[avidflyer]

I did put all new spark plug wires in, and today I checked for resistence from the wires that come from the same coil. All of them read good. That in a way would indicate that they were going to the right spot as well. I unplugged from spark plugs that were supposed to be driven by the same coil and showed the same amount of resistence on all 4 sets of wires from the four coils. I'll double check though just to be sure. Thanks for the suggestion. JImChuk

[Delta Whisky]

Maybe carb balance? It might be that they are slightly out of balance at the troublesome rpm and the mixture or power difference is affecting the rpm drop.

[avidflyer]

Thanks for that carb balance thought. I had balanced the carbs, but not at that rpm as I remember it. That is also pretty easy to do to see if it will make a difference. I'll try it today. JImChuk

[Dave S]

Jim,

With smooth operation high & lo RPM and good mag checks at that range, I believe that indicates the ignition is probably OK.

One suggestion based on an old timer's lo tech, cost free troubleshooting toolbox may help to determine if the engine is running rich or lean enough to cause the problem at that RPM. Then it is a matter of trying to figure out why that is the case.

If you have the standard choke ("starting carb" or whatever rotax likes to call it) try this - make sure the engine is reasonably warmed up first. Set the RPM at the troublesome range, then slowly pull the choke control and see how the engine responds - if the engine smooth out and mag check becomes better or OK with some degree of choke that indicates the mixture is lean at that RPM range (you may need to grow another hand or two to do that!).

If the engine slows down and becomes rougher with some degree of choke that indicates the engine is running too rich at that RPM.

Can't say I fully understand the carbs on this engine; however, most carbureted engines have overlapping systems for idle, full throttle and partial throttle. Normally the leanest range (For multiple reasons) is partial throttle but don't hold me to that on the Bing!

[avidflyer]

Thanks Dave, the choke idea is very easy to check as well, and I've used it often with two strokes. I'll have a look at that as well. I keep going back to ignition, but with it running so good at full throttle, I was thinking carbs as well. I'll maybe have a better idea by this evening. JImChuk

[jrevens]

Jim,

Do you have the springs on your throttle arms?

[avidflyer]

I do have springs on the throttle arms. They pull about 1/3 as much as the stock ones do though. I ran it up today, and tried pulling the choke. Engine was well warmed, and pulling the choke caused it to rev up about 300-400 rpms from idle up to 3500. Pulled the tops off the carbs and raised the mid range needles one notch, so the clip is on the third groove down from the top. They had been in that position before, and I had moved them to the factory setting of 2nd groove from the top. When I first started flying the plane, the EGTs were barely 1200 in cruise, so that's why I lowered the needles. At the factory setting, they were running at bout 1425. Raising the needles today did make it run better, although it still has the dead spot on the one mag at the 3500 rpm area. Reason I didn't go after the idle circuits in the carbs was I know they have the # 35 jets, and they have been out and cleaned recently. Also the air metering screws have been adjusted not long ago. I had also put new carb boots on, so didn't think that was the problem. I'll revisit all of those items in the next few days though. I didn't try to balance the carbs, it is 23 above for temps and about 25mph wind and didn't feel like standing in the prop blast to balance the carbs. I got plenty cold enough just changing the jets out. My thinking now is this. Raising the needles just helped some, but it's not the solution. I would guess an air leak somewhere, but the 1425 EGTs at the factory setting in cruise argue against that. I think I'll reinvestigate the idle circuits in the carbs and carb boots and intake manifold o rings where they attach to the heads. I do appreciate the suggestions, so keep them coming. JImChuk

[3 tracks]

I had this happen a couple of times with the 912 uls 100 hp. Turned out to be trigger coil gap both times, When you get the 500 rpm drop look at the egts. If there not all close to the same temp, i would check the gaps on the trigger coils. There is a reason Rotax want you to check Mags at 4,000 rpms i guess.

[Av8r3400]

Jim, What are you running for spark plug gap?

[avidflyer]

Jim, What are you running for spark plug gap?

Tried for .022, that is what the minimum in the manual is I believe. Just to double check myself, I tried .025 on each and it wouldn't fit. My feeler gauge has a .010, then .012, then .015 in a row, so that's why I did it the way I did. Hope that made sense. JImChuk

[Av8r3400]

I start with .020-.024" (.5-.6mm). I believe that Rotax is more than a little ambitious with their gap recommendation (.6-.7mm). My engine runs and starts much better with a tighter gap.

The other thing is to check the CDI harness grounds.

[Dave S]

I believe Rotax has also recommended tighter plug gaps where cold weather starting may be an issue.

Roger that on the CDI grounds for sure. Internal breaks on a wire or near a wire crimp can cause intermittent continuity and may be affected by vibration at one time/rpm or another. Don't know that in this case but not beyond possibilities. The earlier kitfoxes with reversed manifolds also creates a situation of modified CDI ground wiring that may be subject to vibrations and stress in a way the original rotax ground connections don't.

[avidflyer]

Not sure if I found the problem or not, but decided to see if I could check the trigger coil gaps. I was just barely able to remove the oil tank from behind the engine and that gives me some access to the trigger coils. But the expression "10 lbs of crap in a 5 lb bag sure applies". I sure didn't want to have to pull the engine, especially if it wasn't the problem. Anyway, here is what I found. The little round pickups on the the trigger coils were all rusty. I'm guessing that would have some effect. I don't know if they rusted since I installed the engine, I kind of don't think that is the case. Engine had sat for quite a while. But at any rate, I'll clean them up with some 400 grit sand paper, and try to check the gap. Hope this was the problem. Getting tired of wrenching instead of flying. JImChuk

[avidflyer]

Bit of an update, although the jury is not quite in yet. I did try to check the trigger coil gaps, pretty near impossible in that small space and my big hands. I did find out that .015" feeler gauge wouldn't go through on the ones I was able to check, so they don't seem to be real wide. I did sand the small round pickup points, and that was quite a chore as well, but they are cleaner then they were. Got it back together today, and fired it up. I didn't have the cowl on, so it didn't get as hot as it probably will, but at 3850 rpm, (which my manual says it the rpm for a mag check) I saw about 150 rpm drop on the side that was dropping close to 500 before. The other side seemed to be about 160-170 drop., It was the better side before. Tomorrow when I get a chance, I'll reinstall the cowl, and get it warmed up more, it seemed like it ran rougher once it was fully warm. Another change was when I put the choke on at fast idle, it didn't speed up like it did last time, and after a bit it started to stumble like it was choking out. Sure hope I got it, I'm kind of feeling like I found the problem (with a bunch of help from the guys on this forum) So thanks again for the comments and advice. Stay tuned.... :-) JImChuk

[Dave S]

Jim,

Thank you so much for sharing what you have found. Information like this is likely to be important to others down the road.

Keep us informed on what the final analysis shows.