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Another Rough Engine Thread

1974av8r

Member
Hello All,

My RV-12 (912-ULS) has a little over 100 hours on it now, and I am loving it. However, there is a fairly decent vibration from 1800 rpm up to 2600 rpm. I have been to the Rotax inspection class, along with the LSRI class. Definitely not claiming to be an expert by any stretch, but I do have a good understanding of the systems.

The roughness in that range made me immediately think of carb synch, sinking floats, clogged idle circuit, or debris in the enrichening circuit.

So, here is what I have tried so far (with a test run up to 5,000 rpm after reaching operating temperature between pretty much every one of these steps).....

- Changed the screen on the gascolator.

- Carb synch (with gauges) down to the point of perfection. After reading another post, I fabricated “assist” extension springs to aid the Van’s torsion (latest version) springs in keeping the throttle cables tight. What a difference it made on making a carb synch actually enjoyable. They are spot on at idle and 3,500 rpm.

- Weighed the floats, and both pair (less than a year old) are under 7 grams.

- Removed and cleaned the needles, needle jets, and main jets.

- Removed, cleaned, and inspected the float valve. Checked the arms for correct dimension and ensured they are not binding. Tested with the fuel pump on and both working perfectly.

- Removed, cleaned, and thoroughly inspected (with a 10X magnifying glass) the idle jets. They are absolutely pristine...including the small tube running down the middle of them.

- Removed and cleaned the idle mixture screw.

- Cleaned the carburetors with carb cleaner, strands of wire, and compressed air. All passages including the air passage from the front leading to the idle jet, the passage leading from the idle mixture screw to the hole at the base of the butterfly, and the two small transition passages which exit near the same location. I can spray carb cleaner into the idle jet port (with everything removed) while holding my thumb over the idle mixture port, and a nice unobstructed spray will come out of the three small holes under the butterfly, and the brass fitting at the front (the air intake for the idle circuit)....on both carburetors

- Removed and cleaned the enrichening carburetor, including holding the two plates open slightly against the spring to ensure nothing was lodged in between them.

- Replaced the rubbers and gaskets on both carburetors, and re-assembled them.

- Reinstalled the carburetors and did another carb synch.

- Cleaned and re-oiled the air filters.

- Engaged enrichening circuit about 1/8” while at 2000 rpm and things got incredibly smooth.

- Phoned a friend (Dean Vogal) went though everything I have done, and he said to experiment (a very small amount) with the idle mixture screws. I did.....between 1.25 turns out from the seated position up to 1.75 turns out. I could see the effects on EGT at 2000 rpm, but the roughness is still there.

- Moved on to ignition. Replaced the spark plugs (gapped to .025”) and reinstalled with the $100 an ounce paste from Rotax per the manual.

- Checked the ohms on the coils, spark plug caps, trigger coils, and all tests listed in the Heavy manual.

- Did a differential compression test, per the book, and all four cylinders held 78 to 79 psi.

- Removed all the spark plugs to read them, and saw very typical results... #1 and #2 have more soot (and I understand why), while #3 and #4 are more tan.

HOWEVER....the top plug on #4 honestly does not look like it is firing. It is ridiculously clean. The bottom plug on #4 has a tan insulator with soot around the base, but the top plug is still silver.

- Rechecked the coil feeding the bottom plug of cylinders #3 and #4 and the cumulative ohms from spark plug cap to spark plug cap was 15.9k, which is right on the money.

The engine is smooth above 3000 rpm and smooth for a very quick check (gear box, I know) down at 1700 rpm. However, t is still rough from 1800 up to 2600.

I am confident people are going to say its from the float bowls overfilling due to the vibration, but I can run it at 5,000 rpm, reduce the throttle assertively (not aggressively), and it will be vibrating on its way down through those rpm. I would not think the float bowls would fill to the point of flooding in that short amount of time.

Someone else will say, “have you done a dynamic balance on the prop?”. The answer is no, I have not. It is on the list, but the vibration is bad enough at those rpm that I don’t believe the equipment would function correctly. I have a three blade Sensenich with ridiculously light blades, and I do not believe this is causing ALL of the vibration. I am definitely planning on doing it once this is resolved.

Gentlemen, I am asking for suggestions.

I am going to get some fresh 91 octane from WaWa tomorrow so we can rule that out too (unless it fixes it!).

Please tell me your thoughts about the clean plug as well. Ignition checks are pretty much standard +/- 150 rpm on either side and the roughness increases similarly regardless of which circuit is off.

Also, I know the clean plug must be firing (unless it is dieseling) because the #4 cylinder has the EGT probe. The bottom plug is on circuit B, and when circuit A is turned off, the EGT rises. I would assume a 3-1/2 cylinder engine (meaning one plug not firing) going to a 3 cylinder engine (turning off the other plug in that cylinder) would cause more than a little rumble.

Thank you in advance.
 
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Very well presented / detailed report but it is missing one important detail (for me anyway).

Has it always been like this? If not was it a progressive development (seemed to slowly get worse over time? Or did you just suddenly notice it during one particular flight?
 
Scott, I am honored and thankful for you responding to this. I’m not much of a “poster”, but I read these threads religiously.... and I’ve noticed things seem to get fixed when you chime in.

The vibration has been there since day one I believe.

I did the first flight in the airplane after I built it, and I think the excitement probably masked the vibration for the first few hours.

Eventually, I felt like it just didn’t seem right at those lower throttle settings, and I asked around on the airport for some help. A local “Rotax guy” came and looked at it, did a carb synch, looked at and sprayed carb cleaner on the jets, and said it’s “good to go”. He said it was the aluminum airframe that was exaggerating the vibrations from the boxer engine, and I believed him.

After going to the class in Sebring, reading countless blogs, and then actually physically listening to other 912’s at the airport....I have learned they can be incredibly smooth from 1800 rpm up to 5500 rpm when all systems are go.

I had such high hopes after everything I have done to the airplane over the last week that I am probably a little hypersensitive to the vibration now.

Regardless, to answer your question......I believe it’s been there since the first run, and it has not gotten any better or worse (unless the “choke” is activated ever so slightly, and it smooths out as aforementioned).
 
#1 Do a "mag check" at the RPM where you have the most vibration. Does one side drop more than the other? How much is the difference?

#2 how are you hooking up your carb balancer? Do you disconnect the cross over tube and plug into the rubber hoses or pinch off the cross over tube and connect to the manifold ports?
 
Scott, I am honored and thankful for you responding to this. I’m not much of a “poster”, but I read these threads religiously.... and I’ve noticed things seem to get fixed when you chime in.

The vibration has been there since day one I believe.

I did the first flight in the airplane after I built it, and I think the excitement probably masked the vibration for the first few hours.

Eventually, I felt like it just didn’t seem right at those lower throttle settings, and I asked around on the airport for some help. A local “Rotax guy” came and looked at it, did a carb synch, looked at and sprayed carb cleaner on the jets, and said it’s “good to go”. He said it was the aluminum airframe that was exaggerating the vibrations from the boxer engine, and I believed him.

After going to the class in Sebring, reading countless blogs, and then actually physically listening to other 912’s at the airport....I have learned they can be incredibly smooth from 1800 rpm up to 5500 rpm when all systems are go.

I had such high hopes after everything I have done to the airplane over the last week that I am probably a little hypersensitive to the vibration now.

Regardless, to answer your question......I believe it’s been there since the first run, and it has not gotten any better or worse (unless the “choke” is activated ever so slightly, and it smooths out as aforementioned).

If I am understanding everything correctly, it sounds like it runs smooth at a minimum idle and above about 2500 RPM and in the rough range it smooths out if you pull the enrichment control.

To me this rules out an induction leak because that would have the most influence at the lowest throttle setting. Particularly if the EGT between the two sides is very similar.

I am inclined to lean towards it possibly being a manufacturing problem with one of the carbs. I have seen indication of there being some variability between one carb and another to where a given engine may run just ok with one set of carb's but then they were swapped for a different set and it ran fantastic through the entire throttle range.

It would be good to rule out it being an induction leak before heading into that troubleshooting path though. With the carbs removed from the rubber mounting flanges (no need to disconnect any of the cables or fuel line) you can connect the outlet of a shop vac to the carb mount and then soap/water check all of the manifold connections for leaks.

If that is good, I suggest investigating a way to install a different set of carbs as a test. Not sure but I think I have heard in the past that Lockwood has a test set that can be borrowed with payment of a deposit? Perhaps Dean could help with that in some way?

BTW, when doing testing / adjusting, do not fear running the engine below 1800 for short periods of time. A small amount of exposure to those RPM's wont do any harm. It is the extend flight hrs with regular operation in that range that can be harmful.

Confirming smooth operation and good carb synch, from the idle stop (between 1625-1650 RPM at full operating temp) up to about 3000 RPM or so is a good troubleshooting tool that can tell a lot about how the carbs are working.

A well adjusting / operating 912ULS will feel quite smooth from 1600 all the way up through 3500 or so. Some of the carbs run a bit on the rich side from 3500 to about 5000 RPM so being slightly less smooth in that range is not unusual.

The only other thing I can suggest is go ahead with the dyn prop balance before doing anything extreme like swapping carbs.
If off it would likely induce roughness at other RPM's but there could be a strange interaction between the vibration damping of the gear box and the propeller. Be sure to reconfirm that all 3 blades are as close as possible to the same angle value before doing the dyn. balance.
 
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I would also recheck the prop angles using the Tool 0002 and magnetic level. If those angles are a little out it can cause some vibration. Also I concur with the dynamic prop balance. Made a huge difference for me.

I see you replaced the plugs - although the gap is a little tight I think they increased it (more debate on that too on Rotax Owners...i think i use .028) If no 4 is not firing I would carefully check the inside of the rubber spark plug connector and the other side of that wiring carefully.
 

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One last thing to consider - sending your carbs in for 5 year/200 hr rebuild and inspection. Although it sounds like you have done a thorough inspection yourself, if you are pushing the 5 years, a rebuild might be worth the $$$. There are some good videos on it of course, but I paid the $600 for Lockwood to do both at 79 hours TTSN (5 years) - engine ran like new when I got them back.
 
This just might be an “Oh Well…” situation. Rotax 912 is really two separate power-plants that share a common crankshaft. Each power-plant has its own carburetor and it’s a great day if both work in unison. Carbs may not transition from idle jet, needle jet, main jet exactly the same.

As Greg says, synch carbs at off-idle. Need to have good synch at slow speed to minimize wear on gearbox. That being said, don’t operate engine at slow speeds. Even with good synch, slow speeds are bad for gearbox.

To summarize… have good synch at slow speed, don’t sweat mid-range, make sure ignitions check well at 4000 RPM with close to equal smoothness. Cruise speed should be pretty smooth (prop balanced), EGT’s nearly same (R side may be higher because oil tank proximity), and ignition check, in the air at cruise, should show very little rpm drop.
 
These two old RVATORs have some info regarding the causes of rough running engines - One article written by Van himself attributes to exactly your problem - enrichment circuit fixed the problem and a small speck of something in the main jet caused it.
 

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Had a 912 on a Rans S7 that we ran 700+ hours. When we had rough running it nearly always was cured by replacing the sparkplug resistor boot. To replace you un-screw the boot off of the ignition wire trim the end of the wire 3/16", measure how far full insertion in to the boot is and mark the ignition wire to that length with tape. You then screw the new boot on until you are confident you are at or near full insertion.

To find a non or poor firing cylinder: run the engine on the rough mag and shoot the exhaust pipes with an infrared thermometer. Alternatively you can use a wet rag and keep touching the pipes, immediately after shut down, as they cool down; it will become obvious which one is the cool one.
 
I'm in agreement with what Jim said about the carbs transitioning from idle jets to main jets. They have vacumn operated slides that may come off the stop slightly different times. How much time do you really spend at that RPM? I warm my engine up at 2000-2400, do ingnition check at 4000 then go fly cruising at around 5300. I don't know how severe your vibration is but I have noted that mine seems to vibe more at 2000-2700 then smooths out....I have balanced my prop down to .05 IPS, carbs are synced (Additional springs installed) If it were possible for you to compare with someone else's RV12 or 912ULS that may help. I notice my firewall vibrates during warm up, Vans has a kit with angles to stiffen it, that may help with most of my vibration. I have a 2010 model that I didn't build, the newer ones may come with the added angles, they are installed on the RH side of firewall, I'm probably going to install them during my next Condition inspection...good luck with your trouble shooting.
 
UPDATE!!!!!!

First off, thank you all very much for the numerous great tips. To cut to the chase, my issue is resolved....and I found it in an area that had never crossed my mind.

The epoxied “BB” plugs.

I did an induction leak test with the shop vac, and could not produce any bubbles anywhere. I had questioned the crossover tube in my mind numerous times, but after the soapy water test I was confident all hoses and gaskets were sealed properly.

Note....if you do this test yourself, REINSTALL THE RETURN OIL LINE TO THE SUMP BEFORE YOU APPLY AIR PRESSURE TO THE CARB SOCKET WITH THE OUTLET SIDE OF THE SHOP VAC! I always remove the return oil line to facilitate an easier removal of the right carb, and when I applied air pressure with the shop vac, it (obviously in hind sight) did a quite efficient “burping” procedure. I made a real mess. Lesson learned!

I bought a Dynavibe at Sun-N-Fun from Steve Sennett (great guy and incredibly knowledgeable), brought it home and got the prop from .12 IPS down to .02 IPS. Noticeable difference at the higher RPM’s, but no change to the roughness between 1800-2800.

Completely disassembled the carbs on the bench again, and for the first time I removed the throttle arms and butterflies (excellent video on Rotax-Owner.com about this procedure) because that was honestly the only thing I had not inspected or replaced on the carbs. They looked fine (carbs have a little over 100 hours on them), but I replaced the O-rings on the throttle arms in hopes of that somehow being the magic bullet, replaced the screws, and carefully “peened” the threads after reassembly.

Fingers crossed, but no joy. Same roughness.

I ventured down the electrical and mechanical path for about a week again...checking trigger coil clearances, triple checking spark plug gaps, checking resistances on coils, plugs, plug caps, leads, etc., trimmed the leads and reinserted into the caps, swapped modules, and finally pulled the valve covers to see if a lifter was soft........absolutely no difference whatsoever after any of it.

I then sent a message of desperation out to Dean Vogel to see if I could “rent” a set of carbs for a few days, and while I was waiting for a response I decided to pull mine off again (they would need to be removed regardless).

Sitting on the bench just staring at them, and comparing the similarities and differences of the two. Not to sound belligerent, but they are such incredibly simple devices (and that goes to show how truly genius the design really is). Seriously though, if the floats close the fuel valve properly, the piston diaphragm and spring are not broken, the needle fits in the needle jet as it is supposed to, and NOTHING is clogged.....there is no logical way they won’t work.

I could not understand how every port, channel, orifice, jet, o-ring, and gasket on them could be perfectly clean (or new), and somehow there was still a problem.

They work perfectly at cold start-up with the throttle completely closed and the starting carbs (“chokes”) engaged, which meant the starting carb fuel jets in the corner of the bowls, the starting carbs themselves, and all connecting passages/tubes were working correctly.

They work perfectly with the throttle at idle and the starting carbs completely disengaged, which meant the idle air jets, idle fuel jets, idle mixture needles, and all connecting passages were functioning properly.

They work perfectly from 3,000 rpm up to 4,800 rpm, which meant the throttle arms, butterflies, pistons, diaphragms, needles, needle jets, and diffusers were all doing their job.

They work perfectly at WOT, which meant the main jets were also fine.

The only place they did not work perfectly was when the transition tubes (the two microscopic holes at the bottom of the carb throats next to the idle mixture needle port, which are covered by the butterfly in the idle position) were exposed before the needle jet was in play.....aka low throttle....aka 1800 - 2800 rpm.

I have cleaned and verified the functionality of those little bas*&^%$ so many times that a part of me still said, “it can’t be”....but everything pointed to them.

So, out of literally exhausting everything else on the carbs, I started studying the OUTSIDE of the carb bodies. Once again, I have so much respect for the designers of these things, because they truly are brilliant. I started studying how the “passages” are machined, and I had unfortunately always kind of glanced over the epoxy covered “bb’s”. I really thought there was no real “action” happening there.

I wanted to kick myself, because I was wrong.

Plenty of light along with my big magnifiers, I found pin holes in the epoxy on the “bb” holes of BOTH carbs. Ironically, the biggest voids were on the holes for the transition tube machining (the “bb” between the idle mixture needle and the primer port).

I applied new epoxy on every “bb” to be found, put everything back together, fired it up, did a carb synch, and the engine is running smoother that it EVER has since the first time it ran!!!!

The whole thing reads like an NTSB report in hindsight.....air was leaking in when the vacuum was the strongest over the transition tubes (low throttle), and it was making the mixture too lean. Cracking the “choke” ever so slightly made it run perfectly because the extra fuel (to mix with the “extra” air) is what it needed to make a proper mixture.

What a crazy path getting to this point, but the amount I have learned about the carburetors over the last few weeks has honestly made it worth it. The respect I have for the design is massive.

Hopefully this was a “one time” event, but I believe it should be added to the list of usual suspects of rough lower rpm (carbs out of synch, heavy floats, clogged idle jet, clogged transition tubes, and clogged idle air tube).

Scott and Joe both hit the nail on the head regarding an induction leak, but I never thought to look ON the carburetors (obviously excluding gaskets and o-rings).

I think I feel like Wile E. Coyote would have felt if he somehow ever could have actually caught the Roadrunner........”What am I supposed to do now?”

Go fly. :)

Thank you all again, and consider this one solved.
 
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Richard,

I’m trying to figure out what “bb”s you are talking about. My engine runs a little rough when I pull back the power for descent, so I’d like to look at my carbs for this. Are the bb’s on the outside or inside of the carbs? Looking at a carburetor diagram hasn’t made it obvious to me.

Thanks,
Rich
 
Balls are pressed into cross-drilled holes in the casting...
-
 

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The carburetors are cast, and the small passages (some with a diameter of down around .020” from what I understand) are machined after the casting is.....cast.

The machining (drilling) originates from numerous points on the exterior of the carburetor body, and the byproduct of fabricating those precise passages is a hole at every location where the drill penetrated the casting.

A “ball” or “BB” is then pressed into the casting to plug the now unneeded hole, and covered with epoxy to hold the “BB” in place....which will prevent air and/or fuel from entering or escaping. As I said before, the design is absolutely brilliant.

The only possible improvement, in my extremely humble opinion (which I am sure has been discussed or thought of and nixed), would be soldering or spot welding over the “BB’s”:

In hindsight, I have thought endlessly about the possibility of the carburetor cleaner I used jeopardizing the integrity of the epoxy. The fact that the engine was rough in those RPM from the first run makes me want to say “no”, but I have learned a lot in my +/- 10 carburetor strip downs over the last few weeks.

The O-Rings are definitely NOT compatible with carburetor cleaner, and Rotax makes a very clear point of stating this fact in their literature and videos. I also did some very unscientific testing and found that the carb cleaner WILL start to degrade the epoxy after being in contact with it for enough time.

Regardless, great lessons learned, and when Rotax says to use ONLY fuel and compressed air to clean the carbs....I am a believer.
 
:D:D:D:D

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Hey Jim, you have described the rough running conditions of my engine to a tee. I have been fighting this thing for months. The rough engine causes flooding of the carbs, then fouls the plugs and makes identifying the cause almost impossible. My only clue is that I have high EGT on the left bank at various power settings The EGTs at idle are very close, but other power settings will show up to 200 degrees higher on the left bank. I'm hoping to investigate the BBs sealant in the next few days. Any suggestions for the best epoxie to use?
Thanks, so much for posting all of the details. Maybe, just maybe I can again enjoy a smooth running engine.......Tom
 
This is a great post. Thank you for sharing the details! Since others may stumble across this post while troubleshooting a similar problem, let me share what happened with my engine.

I spent months troubleshooting a slight roughness / higher RPM drop on the "A" ignition circuit. "B" ignition circuit ran great, the engine seemed to run fine at full power with both ignition circuits operating. But because I could only duplicate the problem when running on "A" ignition, it obviously had to be an ignition issue, right?

Wrong.

I had swapped in known good ignition modules, reworked spark plug wires, checked trigger coil gap, checked ignition circuit wiring continuity along with the specified resistance readings of the coils, replaced the spark plugs (twice) and pretty much swapped everything else between ignition systems. No luck.

Then I had the opportunity to speak to Dean from Lockwood while at SnF. He patiently listened to my troubleshooting steps. About 3 steps in he suggested I had a carb problem. When asked if I had done a "choke check", I politely replied with my own question: "what's a choke check"? He proceeded to suggest that, while doing my ignition check at 4000 RPM on the "A" circuit, I should pull on the choke. If the engine ran more smoothly or more roughly (in other words, any change other than a different RPM), then I likely had a carb problem. I performed the check and the engine roughness increased.

Hmmm...

Previously, I had a clogged idle jet on the left carb. I had disassembled and cleaned it. At the same time I checked the right carb for debris (found none). The idle problem was fixed, but I still had the slight roughness and about a 250 RPM drop on "A". The engine is right at 5 years and approximately 160 hrs TT, so at Dean's suggestion both carbs went to Lockwood for the 200hr/5 year rework. I reinstalled both and balanced the carbs yesterday. My engine is back to running perfectly, and it flew like a champ this morning!

Unfortunately, there was no teardown report done on the carbs - so I don't know the exact root cause of my problem. I'm suspecting it was a slight clog somewhere in the left carburetor.

I hope this helps someone else who might be out there chasing their tail like I was!
 
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