912UL Issues

[68niou1]

Hey guys- looking for some opinions. I just bought my 94 model IV, when I got it seemed to run rough, I chalked most of that up to sitting( I think it has been sitting most of its life imo) fixed a few squawks, changed plugs, rebuilt carbs with new gaskets and needles, checked float level, synced. The problem is this- at idle to 2500 the engine shakes violently and the gearbox is making lots of noise. It shakes so much the right carb pukes a little fuel from the vent line. From 2500 up it smoothes out, pulls cleanly all the way to 5250. I have triple checked the jets, sync is right on from 1400 and up. I think there is something wrong with the gearbox- seems weird it would be bad at 145 hours:confused: Any Rotax techs close to Spokane/CDA? Any input would be appreciated.

Thanks,

Scott

[Slyfox]

when I get this I know it's the middle range jet circuit. If you have seen the circuit you would understand. it goes through the jet and comes out in front of the filter or opening of the carb. the vent hole is about .030 real small. My advice is to remove the carb bowl, take this jet out and take some carb cleaner in a can or brake cleaner and squirt up into the opening for the jet and when you see a small stream come out the carb at the filter area you know you got it cleaned out, remove the filter of course. mine generally has a bunch of brown junk come out at first. than take some shop air and do a round about on the circuit, put it back together and should be much better. the jet is between main jet and the front of the carb twards the front of the engine.

[Av8r3400]

I've been noticing the same thing lately, only it seems to be intermittent. Runs smooth sometimes and other times it runs real rough at low rpm...

Been very frustrating. I'll try this technique. Thanks!

[kmach]

Hi, To me it sounds like your sync might be out. some Rotax people sync at 2500, some recommend and swear by doing it at 3000-3500 rpm. I have tried both and around 3000 works best for my setup .

I recall my first syncs didn't work out and I have had the fuel exiting the carb overflow. retried the sync , with some trial and error and experience syncing is easy and definitely smooths idle and lower rpms out.

[jamesmil]

hi guys, saw this a while back on a friends 912. syncsed the carbs, no help. rebuilt carbs, no help. installed new carbs, no help. after all that time and money it still ran so bad at low speed that we took the prop off so we could pull plug wires off while it was running to try and find the problem and lo and behold it ran great.
called lockwood and told them what we were seeing and they said to send them the gear box to be checked out. turns out the dog gear was frozen to the shaft and was not cushioning the low rpm impulses from the motor as it was supposed to. much money later and a rebuilt drive it runs great. hope this helps

[68niou1]

Jim Scott (Aircore Aviation) when I called him said the exact same thing. :D So I am pulling the gearbox and shipping it off. I will keep everyone updated.

Thanks for all the input-

Scott

[SkyPirate]

this is also a common low rpm thing with the B box's on a 582 but its the spacers and springs on output shaft that is the culprit,..when you start getting allot of low rpm chatter out of the gear box,..time for rebuild

[68niou1]

I thought I would post the end result on this so maybe it might help someone else along the line later on- :D It turned out to be the gearbox, sent it out to Jim Scott at Aircore Aviation in Arlington, (great guy and highly recommended) got it back in record time(would have been faster but parts were back ordered) bolted it back up, retorqued everything, safety wired, fired up and she runs smooooth. no puking carb, no weird noises. And my plane only has 145 hours on it- so go figure.:cool:

[Av8r3400]

I have the parts in hand to do mine now too. I hope it works as well.

As a side note, what prop do you have? I have a 3-blade 68" warp with tapered nickel leading edges. 450 hours tt on the motor.

[Slyfox]

fantastic. when i left the airport last night I saw you going out the gate with your airplane in tow. I take it you went up to deer park with it. did you get a chance to fly it yet. I know a few years ago they complained of a rough engine and it kept the airplane from flying. glad you found the problem. what parts did they change out, very curious.

[Av8r3400]

After diagnosing with techs at LEAF they thought (for me) it is a problem with the Bellevue washers. These are cupped shaped washers that make up the spring that retains the driven gear in the gearbox, similar to the two stroke boxes.

I was sent these washers, the retainer half rings and all the lock washers and gaskets to do the install.

I'll take some photos of the job when I get to it, and post them here.

[Slyfox]

back when I had the early 912ul, I replaced those bevel washers because they wore so bad. Not sure if it ran smoother after ward, don't remember. It was a few years ago. with a press, it was a fairly easy job.

[SkyPirate]

There are a couple you tube videos out there that show the process, doesnt look that hard to do

[68niou1]

fantastic. when i left the airport last night I saw you going out the gate with your airplane in tow. I take it you went up to deer park with it. did you get a chance to fly it yet. I know a few years ago they complained of a rough engine and it kept the airplane from flying. glad you found the problem. what parts did they change out, very curious.

Flew it this morning- spent about two hours in it running around the patch with no issues. Gonna spend a couple of days up in Deer Park til I get used to it then fly it back to my hanger at Felts. Felt great to get up in the air again- with all the issues I was beginning to question my purchase:D

[68niou1]

I have the parts in hand to do mine now too. I hope it works as well.

As a side note, what prop do you have? I have a 3-blade 68" warp with tapered nickel leading edges. 450 hours tt on the motor.

I will check it out and let you know- I have no idea other than it is a warp 3 blade non nickel leading edge. I know it pulls 5200 on climb out so I think its set about perfect from what limited knowledge I have gained-

[Av8r3400]

Here is a couple of photos of the rebuild process.

[Av8r3400]

The bellvue washers are 5th from the right in the last photo.

The old ones are .2015 and .2030" thick. The new ones are .2145 and .2205" thick.

Quite a difference. :eek:


Skypirate: Can you give a link to the videos you found? I couldn't find any...

[SkyPirate]

ok will find them here is one C box ..,..will find B and E ..

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=5&cad=rja&ved=0CEoQtwIwBA&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DK76 PuHObzE4&ei=HoE6Ur2sK4Xc9ATk-YCwAw&usg=AFQjCNEIGdHfjJWSYVdBaqtP_3b_9p8myA&bvm=bv.52288139,d.eWU

B box
http://www.youtube.com/watch?feature=player_detailpage&v=B_pn4XGnaHU

[Av8r3400]

Okay. I did see that one. I couldn't find any for the 912 gearbox. I thought that's what you had found.

[SkyPirate]

still looking I know it's there..ok found one but it requires subscription..

http://www.youtube.com/watch?feature=player_detailpage&v=_vs7x-cfH-U

http://www.youtube.com/watch?feature=player_detailpage&v=_q-WxnWF7uk

both above links require subscription


it's not the one I found previously,..still looking

[Av8r3400]

I did the whole reassemble in reverse order thing today. I installed the new "spring discs" into the assembly replacing the partially collapsed old ones. There really wasn't anything to photograph with that, so...

I did take a couple of shots of the press tool used to disassemble the gear box shaft assembly.

1st photo: This tool is intended for the "B" Rotax 2-stroke box. This is the same tool they use at LEAF, on the 912/914 boxes too. They told me they modified it to fit the larger box. I didn't want to modify the tool (it isn't mine) so I made some shims to help it fit the larger gear better.

2nd photo: Here is what the tool looks like by itself on the gear.

3rd photo: Here is with the shims to spread out the force on the gear a little more.

You put the assembly in the press, supporting the prop flange and then press on this bridge until the retainer clips release. Easy peasy.

[Av8r3400]

My last hope didn't help. The vibration is just as bad as ever. :mad:

[68niou1]

My last hope didn't help. The vibration is just as bad as ever. :mad:

Stupid question- have you replaced your engine mount rubber isolators recently? Just a thought-

[Av8r3400]

No I have not. Something to consider I guess...

[68niou1]

Was yours really smooth before? Because at anything below 2500 mine isn't really smooth, and from what everyone is telling me that is normal on the 80UL. While I don't have very much time in mine yet- I did spend quite a bit in Paul Leadabrand's 914 model 7- and that is night and day compared to mine. His is smooth from any rpm, and I have been told a lot of that is the engine mount from the model 4 to the model 7- :confused:

[Av8r3400]

I bought the plane with 72 hours TT. It ran turbine smooth at all rpm until last year at about 400 hours TT.

It started as an intermittent "rough running" range in the rpm band. At the low power, approach rpm range, 2500-3000. Just a little rough and it would come and go. It has progressively gotten worse over the last ~40 hours of run time. Through out that time I have reset, then disassembled and cleaned then totally rebuilt the carbs. Each time balancing them with our chapter's electronic balancer. I even balanced the prop with a DynaVibe unit.

Doing the spring discs in the gearbox was the last item on the list. Now I start over with a new list... :rolleyes:

Monday I'll be calling LEAF and Lockwood looking for more ideas.

I'll gladly accept any other ideas on this problem. Don't be shy. :o

[JimS]

Larry,
Probably a stupid question but have you changed plugs lately?

JimS

[Av8r3400]

Plugs have ~20 hours on them.

I had a suggestion of maybe looking at the plug wires, though...

[Av8r3400]

Does Kitfox still have/produce the engine mount rubber isolators, pn 44024.000? (Per S/B 54A)

I tried calling today, but they must have have been sleeping off the fly-in this past weekend...

[kmach]

Does Kitfox still have/produce the engine mount rubber isolators, pn 44024.000? (Per S/B 54A)

I tried calling today, but they must have have been sleeping off the fly-in this past weekend...

Hi , They do. I got part # 44024 and # 44023 from them in the spring. :)

[68niou1]

I just ordered those today myself. I have been trying to get rid of the vibration/rough running and this is next on my list. Just to recap here is what I have done-

Rebuilt carbs, new needles with Viton tip, new floats, etc.
Rebuilt gearbox.
Synced many, many, many, many times. Bought the throttle adjuster from Highwing LLC that has been helpful.
Changed plugs

Anything I have forgotten? Thing runs glass smooth from 2500 up- pulls 5200 on climbout, just rough running at 2500 down. Very annoying on base to final:mad:

[Av8r3400]

Okay. I'll continue my saga here instead of spreading to the other "prop balance" thread.

I worked on my plane through the week to this morning. I removed the gearbox again and re-shimmed it per the instructions in the "heavy maintenance" manual I downloaded from Rotax. Per this procedure it took an additional .2mm (.008") shim to properly align the parts. During the work I took some additional photos of the "spauling" I noticed on the dog hub and the mating portion of the driven gear. (Photos below)

This week I also modified my engine mounts per Kitfox service bulletin 54A (http://www.kitfoxaircraft.com/support/service_bulletins/sb54.htm). By shortening the sleeves inside the rubber isolators (which were in very good condition). My sleeves were 1.25 (1-1/4)" long. I shortened them to the recommended .930" long. This tightened the motor mount considerably.

I thought that for sure the motor mount modifications would have solved my problem. It didn't. :mad:

I test ran the engine and noticed a significant improvement. So much so that I decided a test flight was in order. After reassembling the cowl on the plane I started it and taxied out. During the taxi I noticed the motor was still somewhat rough in the 2200-2800 rpm band, not as much as previously, but still not "fixed". Higher and lower throttle settings were smooth as can be.

Take off and climb were just as good as ever with a 1200 fpm climb and good power. Turning down wind I throttled back to 4000 rpm with no ill effects. Nice and smooth. Just prior to turning base, I throttled back to 3000 rpm and the motor began to shake, just as badly as before. I turned direct for the runway and pulled power to idle knowing I had the runway made already. Smoother, but still rough I landed and taxied to the ramp.


Here are my conclusions: The issue causing the vibrations is still there. (Obviously :rolleyes:) The engine mount modifications helped the problem by not allowing the vibrations to "snowball" out of control during ground testing. As an engineer the wear/spauling I found on the parts in the gearbox, I would not have normally considered very bad. Speaking with several experts I found that this may not be the case. I think that this "damage" to the gearbox parts is not a problem under power, but when "windmilling" or at a very low power setting, it causes this vibration to start.

I have ordered a new gearset and dog hub from Rotax. I will (hopefully) have these parts from Austria by the end of the month. To some additional hope, the new gearset is a different design than the original set I have now. I will install these new parts and report the results then.

If this last step is unsuccessful, I am left with no other option than taking the whole plane (or maybe just the motor) to a close Rotax shop (LEAF) for repair as I am completely out of ideas.


Wish me luck. :o

[SkyPirate]

good luck Larry ,..hope it cures your problem,..since I now own a 912UL I'm anxious for you ,..I can add a little experience with gears,....when I was racing I ran 6:20 gears in the rear on the short tracks,..this allowed for allot of torgue to be applied to the rear tires,..we are talking allot more HP turning it too,..but every 5 or 6 races we'd have to re adjust the lash ,..the reason, low RPM/ torque chatter in the rear end,..when the gears chatter like that it creates heat,..heat can cause cracks,..and what looks like pitting and blue'd marks on the gears,.just like what your photo shows

[Av8r3400]

The photos don't show it well, but the coloring is not blue, but a copper sort of tone. I don't think they are getting quite hot enough to blue. But, I don't doubt there is some damage there causing some issues. Also, the procedure to assemble these parts calls for nickel anti-seize to be applied to the dog ears and to the spring discs. I don't know if this will make a difference.

The new design gear set does change the angle of the dog ears. This is most likely an improvement on the old design. Call it a revision "B" if you will.

As a side note there are two gear sizes available for the 912. A 22mm wide and 27mm wide set. My motor is of early 90s vintage and the crankshaft can not accept the 27mm set, only the 22mm. My other motor, in the project plane, is from the later 90s and came with the 27mm set (making a simple gearbox swap impossible! :mad:).

I'm still holding out hope for the new design gear set...

[War Eagle]

Okay.


Wish me luck. :o

Oh man! It hurts me to look at this.

Nothing from Rotax is cheap so I hope this fixes the problem and that you don't have to mortgage the house.

By all means: Good Luck.

[Danzer1]

Same can happen with rapid power adjustments or operating to long at to low an rpm - can cause similar issues as overspeeding as the engine is lugging at low rpm.

While you've got it apart, waiting for the parts, I'd suggest checking the crankshaft for true and runout - the specs are in the heavy manual you downloaded. If out of range you likely have an internal issue that means sending it out.

If withing tolerance, reassemble, being mindful of the clearances in the manual and I would then would check the prop hub for true and runout.

If the crankshaft is okay, and the prop hub isn't, your problem would likely still lie in the gearbox. If both are okay, you could have (or had) a prop issue.

You mentioned balancing with a DynaVibe - were there any unusual readings? How far out was it before adjustment? and how close did it actually balance?

I don't necessarily agree that the problem lies in the windmilling, as most vibration/rough issues are more noticable at low rpm - that doesn't mean they aren't there at higher rpm, their just harder to detect. Also the spalling is not consistent around the gears, on all the flats and all the valleys, which suggests something is out of alignment, balance or true.

The issue could have been caused by the out of spec spring discs and/or an out of balance prop and now replacing the gear set might be the last step in fixing the issue (hopefully). I don't think the problem is with the gear set itself, but rather is the result of what the problem was or is.

But if either the crankshaft or prop hub are out of spec - I wouldn't bother gong any further until the cause is found and resolved.

Good Luck,
Greg

[kmach]

Hi all,

Av8r3400 ,did you measure the friction torque before and after the gearbox rebuild ?

I am very interested in the before and after numbers .

[Av8r3400]

I do not have a clutch in my gearbox, so there is no friction torque measurement.

[kmach]

I missed that , looking at your signature its says both engines 80 hp. :o

[Av8r3400]

Gearbox parts arrived today.

High of -5º tomorrow and not much better for the next few days. I won't be putting this project together for a bit…

[68niou1]

Weenie. Sounds like a excuse to me.:D

[Av8r3400]

Today's peak temperature… (-35º tonight) :mad:

C'mon over and put it together for me. I'll buy you lunch.

[Av8r3400]

Just another note: Further conversations with LEAF, Lockwood and CPS techs have lead me to believe that this is not a completely unheard of issue with the 1-4 Kitfox models with the oil tank in the rear. (Needing the reversed intakes) Sometimes the engine mount modification (SB 54A (http://www.kitfoxaircraft.com/support/service_bulletins/sb54.htm)) fixes the issue. Sometimes it's the gearbox rebuild. And sometimes it requires the addition of the clutch.

The guys at LEAF had this issue with a IV and finally resolved it by adding the over-run clutch. I'm hoping I don't need to go that far…

[HighWing]

Larry,
Interesting because I have not heard of it talked about over the years. One thing you might check. I was reading some historical stuff on one of the other forums and a guy with a rough running engine traced it to backward facing slides in the carburetors. they look the very similar front to back, but are not the same. If they are installed with the front facing back, it will result in roughness due to poor fuel mixture. Have you rebuilt your carburetors lately?

[Av8r3400]

I've had the carbs apart three (3) times through this ordeal. I have gotten very intimately familiar with them and their nuances. I agree, it would be very easy to get the slides turned around.

If you look closely at the slides and the diaphragms, there are clocking marks that denote the direction of the slide to the diaphragm and then the diaphragm to the body of the carb.

Just to put it down, these are the steps I've taken so far…


Synced Carbs
Disassembled, cleaned, adjusted and re-synced carbs
Disassembled, cleaned, installed rebuild kits, adjusted and re-synced carbs
Serviced engine mounts per #SB 54A
Removed, replaced spring discs, cleaned and re-assembled gearbox
Removed, cleaned, re-shimmed, lubricated and re-assembled gearbox

[Danzer1]

If I remember right, you also rebalanced your prop. After you do the gearbox parts change, it may pay to re-balance the prop again.

Although, I would consider doing it with a Dynavibe GX2 for a $100 or 2 more than the standard. It uses 2 sensors and can balance quicker and with much better accuracy. Also with 2 sensors, it can pinpoint engine related vibration location issues. Like a mis-alligned alternator, etc.

I've been researching them somewhat lately and they are getting great reviews for balance and finding other problems. Link here: http://www.rpxtech.com/DynaVibeGX2.html

Might be worth a shot!

Greg

[Slyfox]

I would like to put some understanding into the mix.

first one is the balancing of the carbs. how are you doing it. where are you pulling the vacuum for the balancer outfit, either vacuum gauges or a device for showing balance.

with this thought remember that you are balancing two engines in one. you have a left and a right side, hence the balance pipe in the middle. if you pull your vacuum for testing off the big ports that the balance pipe runs off of, you have a problem doing so, trust me. reason. there is too much opening. so, Yes you can do this, but you need to remember that you now have a problem. that problem being to much of a good thing. if you use a vacuum gauge you will see right off what I'm talking about, the gauge will bounce all over the place. the fix for this is to install a ball valve and close it off, if you do this you will almost shut it off before it stabilizes. than you can use the gauge and continue with your test.

my answer is to block the tube between sides. how I do this is to remove the tube that the factory has and replace it with rubber hose, as a perminate install. now I have a means of taking a clamping fixture or pliers in the form of locking pliers with a round surface and clamping off the hose.

than I take my balancing device and hook it to the carburetors. there is a fitting with a screw in it right in front of the carb. using this connection, use of a restrictor is not needed and can connect direct. the results is a nice smooth vacuum source. now the reading is trustworthy.

Now if you want to know if the carbs are truly balanced try this and it has proven true for first start and to find if my adjustments slipped on my carbs. you open the throttle all the way and look at the relationship of each carb at the throttle lever to the full open stop. if you really can't tell than pull the throttle back to idle very slightly to give you about a .030 clearance on one side between the arm and the stop at full throttle. now you can compare each side to see if they are basically the same. if not . than your balance is off. you can even just start from scratch and put both arms to the same position when at full throttle, or just off full, for comparison reasons. once you have both carbs opening at the same spot, now you start it and I can almost guarantee it will run good. now go from there.

[68niou1]

So got my parts this week, pulled my gearbox apart and did some comparisons. The dog gear is completely different. It looks like the gears are slightly different as well.

[68niou1]

Here is a side view of the new gear-

[68niou1]

Comparison of the old dog gear vs. new one-

[68niou1]

So the new dog gear and big gear are thicker than the original, so in my case even with the shims removed I still couldn't get the keepers in so I had to machine the lower spacer. Nothing is easy when it comes to Rotax. Got the gearbox back together, replaced the lower drive gear, and ran out of time:mad:
So I won't get a chance to finish putting everything together til after the first of the year-

[Av8r3400]

Thanks for the update and photos, Scott. I'm hoping to get out to the hangar and work on mine over the holidays.

How much heat did you need to use on the big 42mm nut?

How about talking abut the alignment for the crank pin situation?

[68niou1]

No heat. The nut was not hard to get off. My only issue was locking the crankshaft, the manual tells you to pull a plug and find top dead center, then put the lock screw in place. What it doesn't tell you is when you look in the hole and find the space in the crank it's really easy to put the lock screw in the wrong place. If you put the lock pin in and you get any movement at all at the prop it's in the wrong place. You need to look for a slot that looks like a timing mark, not a gap. There will be no movement with the lock pin in the right spot. I also used my special Honda tool (basically a custom 42mm wrench) to take off the 42mm nut however I think a 42mm socket from harbor freight would work just fine.

[pyolet]

I posted on your prop vibration thread about this....be sure to check compliance with Kitfox SB #54. Sounds trivial, but I believe a lot of the engine vibration related failures (clutch dogs, fuel pump, ignition modules, carbs...) can be attributed to this. It made a dramatic difference for my install. I've attached a pic of one of the spacers from my KF2 mount....they were all 3/16" too long. FWIW, Woody.

[68niou1]

Appreciate the input- if I forgot to note it already in the thread I have already done SB#54. Larry has as well. Definately appreciate everyone's input as it is all helpful.

[Av8r3400]

I would agree whole heartedly with this. My mounts were not in compliance with this SB. Though I believe my gearbox was too far gone for this alone to cure the problem.

Today I was able to get my new gears and dog ring installed. I ran out of daylight to do a test run. That will have to wait for a few days because we are getting another arctic blast here. :mad:

In the photo below I highlighted the crankshaft visible through a port in the crank case. It shows a notch in the crank that the locking pin goes into allowing the removal of the crank nut holding the drive gear.

The nut is torqued to 147 foot pounds. That's pretty tight, but not the end of the world. Keep in mind that the nut is a reverse thread, too.

[68niou1]

Update- today was the first day I had a chance to finally get everything back together. The weather partially cooperated and my buddy Frank came over and kept an eye on everything outside while I watched inside stuff. Vibration and ratcheting is gone. Runs turbine smooth at all rpm ranges. Did not get to fly it yet as my doors are still off, but currently it appears that my problem is fixed. Hopefully Larry's is too-

[DesertFox4]

Great news Scott. Now we wait for Larry to post equally good news.:)

[Av8r3400]

Aside from the weather kicking my butt, I'm working my way through round two of the bronchitis/pneumonia thing….

I was hoping to get her running this Saturday, it's going to be balmy in the low 20s, but I doubt that's going to happen now.

I did get everything put together last weekend and discovered a leaking fuel pump. :mad: Luckily I have a spare to steal off my Mangy Fox project. Once I get that installed I will definitely post the results.

[DesertFox4]

Aside from the weather kicking my butt, I'm working my way through round two of the bronchitis/pneumonia thing…. :eek: :(

Larry, hope you're feeling better soon.

[Slyfox]

fantastic, way to go. If you want someone to fly along side you give me a call. my fox is always wanting to go for a flight.

[Av8r3400]

I finally got out to the airport to get some work done. Tis project is kind of like watching dominoes fall...

I got the gearbox together only to find a leaking fuel pump. :mad:

Swapped out the pump with the one off my project plane only to find the main valve leaking. Took that apart to replace o-rings and was finally able to run the motor!

It ran well being very cold. Once it warmed up the vibrations are virtually gone now. Yeahhhh!! :)

During the test run I found the supply line between the carbs leaking now... :eek:

Oy vey!

[Dorsal]

Still great news, glad to hear!!

[DesertFox4]

Like that news Larry. You can handle a few small repairs now that the large one is solved.;)

[SkyPirate]

Awesome Larry aka Gear Box Guru :)

[Av8r3400]

Awesome Larry aka Gear Box Guru :)

I have earned that title in my EAA Chapter already… :eek:

I don't know that I am a guru of anything but not being able to fly my plane for the last 5 months! I hope I can remember how...

[Av8r3400]

I'm still struggling with a rough running engine from 2000-3000 rpm.

It is dramatically better since the gearbox rebuild, but it's still not where it should be. At 1800 and lower, the engine is butter-smooth. Above 3000 it smooths out again, totally.

Some "experts" are suggesting that because of the reversal of the intakes on a I-IV 912 instal, the shaking of the carbs will eventually ruin them. I'm not completely buying this.

I'm now into a chicken and egg paradox. Did the carb trouble cause the gearbox trouble or vise versa?? Or is there even carb trouble to begin with… :mad:


Back to the hangar tomorrow to tear things apart again. Maybe I should've bought a Rans… http://www.touringandtenting.com/forums/style_emoticons/default/wall.gif

[Danzer1]

That's a tough one Larry. I do have a few suggestions though.

Is this the same place it was rough and same "kind" of roughness as before the gearbox rebuild?

Is it rough when cold and all the way through full warmup to operating temperature? Does it get better or worse or no change as it gets to full temp?

What do your plugs look like? (I'd start with reading the plugs) - that can give you a pretty good clue if it's carb related and then go from there. If carb related in mid range only, most likely sources are - clip height, out of round or bent needle, out of round or partially clogged needle jet, slide not moving freely and/or proper fuel height in the bowl.

Have you rebalanced the gearbox/prop combo after the rebuild? Changing to the new gears and removing and reinstalling everything can cause out of phase, only noticeable at certain rpm's. I'd definitely want to rebalance after the work you've done - especially so as to not burn up the new gears.

You have an advantage now if you balance with a Dynavibe II (or equal) as it is (1) much more accurate (2) is much quicker and more precise (3) uses 2 sensors - one on the front of the engine - essentially to balance the gearbox/prop combo and another on the rear of the engine that compares with the sensor on the front. This combination can then point to other sources of misalignment or out of balance condition and location - such as a misaligned alternator or flywheel, etc. This could then solve your issue or point you in a new direction as to the source of the problem. In any case, you would then know that your newly reinstalled prop/gearbox is balanced as close as humanly possible and not the problem.

Good Luck

[Av8r3400]

We only have the single sensor Dynavibe and I did refresh the balance with it. (Not much change) I have also re-synced the carbs to the point of near perfection.

The vibration is different than what it was before. It is a "rough running" rather than "vibration" action. It is also no where near as sever as it was before the rebuild. It actually gets worse as the motor warms up.

My plan at the moment is to pull the slides out of the carbs and inspect them for wear and damage per the Rotax "experts" recommendation. I have the carbs off my project plane in hand, too if I decide to rob some parts from them...

I like your spark plug inspection idea too, that will happen first (easiest).

[Danzer1]

Sounds like a carb issue so far. Getting worse as it warms up generally means a rich issue. If it got better as it warmed up that would lead to a lean issue. So, still could be needle clip height, needle trueness/wear, needle jet out of round (letting to much fuel through) or float bowl fuel height (lower = leaner).

I'm guessing this is being done on the ground (static)? If so, you might see what happens (if anything) if you prop the tail up to level flight attitude. That might give you a clue if it's a float height issue. Just one of the reasons I've never been a fan of carbs on aircraft - fuel delivery changes with every attitude change.

You mentioned Rans - can't wait to see the new S20 Raven they are building with the 180hp UL engine - I hear it's getting close! I'm also hearing all good things about the 912 version now flying. Hmmm and only a 5 hour drive from here! Okay back on topic.

Post some pics of the plugs (ends, side and angled down into the insulator) if you'd like another viewpoint!

[av8rps]

We only have the single sensor Dynavibe....(snip) ...I like your spark plug inspection idea too, that will happen first (easiest).

Larry,

Not sure if this will help your situation or not, but I had a similar situation with my 912ul in 2007. It started running really rough when I had the plane at the Seaplane Base at OSH. During the week I had countless friends and every Rotax "Expert" I could find try and help me figure out what was wrong. But after a whole week of messing with it, the problem still persisted.

So after the convention I dug into it, and eventually found the problem. I narrowed it down by looking at my spark plugs, only to find one lower plug that was not burning as clean as the others (it was obviously firing, but was sooted up considerably more than the others). I traced back the wiring to the coils and modules, and finally...I found it!

A wire coming out of one of the igntion modules had a broken wire between the module and the multi wire connector.

On early 912ul's the wires coming out of the ignition modules are so short that the weight of the connector shaking during engine operation fatigues the wires to the point they can break. In my case just one wire was broken which controlled that spark plug. And even worse is that it would occasionally make contact, making the condition very intermittent (and hard to find :(). I repaired the wire by braiding the wires back together as good as I could with the short wiring, applying a very light solder job over the braiding, and then sealed it up with heat shrink tubing. I know it is generally viewed as a no-no to use solder on aircraft, but my only other option I could come up with was to buy a big buck ($800??) ignition module. Repairing it solved my problem, but one day I may have to replace that module.

Apparently Rotax is aware of this problem as all the new 912's use longer wires from the box to the connector, which makes it much easier to secure the connectors without fatiguing the wires.

On a last note, I also helped a friend fix a brand new 912S that ran rough right out of the box. Again, he contacted every Rotax expert he could, but nothing helped. So one day we were looking it over together and I noticed that he had moved the wire coming off the stator to one side to give it more clearance. We moved the wire back to where it was originally, and amazingly we fixed the problem. Apparently that wire is very sensitive to where it is located on the back of the engine and/or how the shielding is grounded. This may have been just a fluke since normally we aren't moving stator wire locations on our engines, but it might be worth looking at in the case you have damaged shielding, broken wire, etc.

These are just a couple ideas I had to try and help you out, as I'm sure this is killing you knowing the busy summer flying season is almost here.

Paul Seehafer

[Av8r3400]

Thanks, Paul. I did have one "ugly" plug when I changed them this weekend. Lower, cylinder #2.

Another lead I can follow [sic]. I do have spare ignition modules on the mangy fox I can "borrow".

[av8rps]

That's unique, same cylinder as mine.

Yes, it would make sense to try a module off of Mangy Fox - that is just plug-and-play. Childs play compared to all the other stuff you've gone through.

But hey, isn't it nice to have a whole parts plane laying around just in case you need something? ;)

Paul

[Av8r3400]

I am now cautiously optimistic that I have finally fixed the issue.

It was carberation. The giveaway was the carbon buildup on the plugs on one side. This lead me to believe that carb was running rich.

Long story short, I took off the carbs and went through them again. I really didn't find anything that was a glaring problem, but who knows…

I did also replace the carb boots with JBM Industries (http://jbmindustries.com/912socket.html) version. Again, I really couldn't see a problem with the old ones, but who knows…

It's a bit frustrating when you "fix" something and don't really know what you fixed.

Never the less, she's running much better and the problem area, 2000-3000 rpm is much, much better.

[Av8r3400]

As of today I've put on almost 5 hours and the motor is still running well, throughout the rev band.

I'm still not sure what the magic bullet was, but it was definitely in the carbs.

The JBM Industries Carb sockets are a welcomed addition. Well made, better than the Rotax OEM, and half the price.

One of the other things I did was to reverse sides on the carbs, putting the left carb on the right side and vise versa. This put the throttle arms and enricher linkages on the outside of the carbs when mounted on the engine. Re-routing the cables and re-plumbing the fuel lines was of course also needed. Needless to say thins makes tuning and working on the carbs infinitely easier.

I also learned that removing and installing the carbs is far easier if you remove the entire intake manifold and carb as an assembly. Installing the boot to the manifold and the carb to the boot is far easier with this assembly off the engine. For instance keeping the torque on the two mounting bolts, from the boot to the manifold, even is not even possible without removing the assembly from the engine.

Now, if we can only come up with a better source for carb parts. These are basically the same carbs used on older BMW R (http://brook.reams.me/bmw-motorcyle-rebuilds/1973-bmw-r755-rebuild-project/bmw-r75-5-bing-carburetor-rebuild/#Parts_List) series motorcycles…

Another link (http://www.bingcarburetor.com/bmw/bmw.html)...

[Av8r3400]

Here is a comparison to of the JBM versus the OEM carb boots.

[Dorsal]

Glad to hear she is running well, been a long time coming.

[Av8r3400]

Thanks, Fin. Yes, it has been a long time coming. :o

[68niou1]

Good to hear!! There's hope!!!:D:D

[Dave S]

Larry,

Great to get things running right again....

Curious....what model/source is the gascolator nounted on the lower part of the cowl on the right side?

Thanks,

Dave S

[Av8r3400]

Sorry, Dave, I don't know.

I'll make a point to look at it closer next time I have the cowl off. (Hopefully a long time.)

I did get one from ACS for my project plane that was a nice little one like that...

[Dave S]

Larry,

Thanks for the reply...but leave that cowl on as long as possible!...You have suffered enough.

Also like to thank you for posting the comparison on the carb sockets....I am up for replacing them this summer and based on your report the non rotax ones look like the ticket.

Sincerely,

Dave S

[Av8r3400]

[engineer's hat = on]

The manufacture of the JBM sockets is better mainly due to the sealing surface being more consistent and having less rubber on it. This allows the socket to be installed flatter, more parallel to the manifold face. I think this gives a better seal on that face.

Also by installing the socket, then the carb to the manifold "on the bench", then putting the assembly on the engine, this allows for a more precise assembly of these parts.

[/engineer hat]