Hello to all - this is my first post to the site so I figured I'd make it a worthwhile one that you could enjoy some trials and tribulations from a fellow Kitfox owner and the never-ending carb issues I've encountered with this plane.

For those unfamiliar with the KF4 and the 912-engine mount, the mount requires that the intake manifolds be swapped from one side to the other, thereby moving the cabs themselves further from the CL (centerline) of the engine. The result of this for many of us has been a never-ending saga of leaking carbs due to any vibrations produced by the engine being magnified by the fact that the carbs were moved further away from the engines CL. In all of the reading I've been doing while trying to rectify this issue (October 2021 is when this began on my engine) it has been stated at infinitum that the likely culprit it the vibrations causing the floats in the bowls to be unable to keep the needle seated and thus an overfilling condition occurs in the bowls. This problem is then exacerbated by the fact that the engine now has excess fuel entering the intakes, causing a rich condition, which lowers idle, introduces more vibration.... you get where I'm headed with this.

So.... here we are in April of 2022. The engine has been pulled from the plane twice, full carb rebuilds done on both carbs, polished needle seats in carbs, new ignition units (different story for another time), new stator, new spark plugs and cables, new rubbers all the way around the engine (it was off the plane so why not), 1" crossover welded into intake manifolds, new air filters, etc etc etc and FINALLY the plane actually runs smooth as butter.... for about 35 seconds after startup - and then slowly the engine rpm begins to drop, and the vibrations set in. The interesting part is if I increase throttle to anywhere around 2800 or above, smooth as butter again. This is indicating to me a rich condition at idle that is resolved as the carbs transition away from the idle circuit and into the needle jet circuit within the carbs. If I shut the fuel off and immediately pull the carbs, I can wipe my finger on the inside of the manifold tubes and feel wet fuel - so the rich condition is still presenting itself even after getting rid of the overflowing issue that is so often the sign of the carbs needing attention.

I'm hoping someone has run into this issue before as well and has an answer that I'm not seeing - it's getting sad that this plane hasn't seen the sky since damn near October all because I don't trust the low-end stumble that comes in anywhere below about 2400 RPM.

I do not have any answers for you.. However I am interested in finding out any info that you may come across.. I am currently assembling a KitFox Model 5 for a customer that had an on ground engine fire due to the carburetor falling off the intake manifold due to the low idle shake you explained. Have a new engine for it in a box.

Brian.

Do you have room for a Edge performance fuel injection system?

Do I have the room, yes - would I throw an edge performance EFI system at a early 1990’s 80 hp 912ul - not a chance. For what you gain from the system vs what it costs, I’ll go with a nytro engine before I went with EFI on this rotax.

That said, yesterday was an eventful day. Pulled the exhaust and welded in an o2 sensor bung so that I could monitor air/fuel ratios as the plane idled and warmed up. After installing her second set of new needles (I’ll try to post pics of the old ones under a microscope) as well as new float needle armatures (dunno the actual name) the carbs went back together for what I hope to be the final time. Hopped in the plane, sparked her up, and low and behold were idling at 1700 rpm, no shake at all, and no more surging. The float arms are currently set at 12 mm from the base of the carb (stock setting is 10.5mm but that pours fuel like a champion). Today I’m hoping to get this starter solenoid replaced then fingers crossed she gets a final button up.

The RANS S7 also uses swapped intakes, and I've never had any problems like you describe, just saying.

https://www.google.com/search?q=leaking ... e&ie=UTF-8

Then consider yourself lucky. You seem to be one of the few that hasn’t had this issue YET. Just saying.

In other news, Bane (kitfox has earned that name) started up yesterday and idled with minimal fluctuation. Air/fuel readings were between 12.8 and 14.1 and I’ll be working to tune those last surges out today. But it went from +/- 400 rpm swings to about 80 rpm. The needles that were pulled from the carbs and placed under a scope showed hairline cracks in the needle tips. I’ll try to get an image through the scope lens and post it but previous attempts to this proved futile. Now the last fight is the mid range is reading at a ratio of about 22.1:1 which is NOT GOOD - this has me thinking the needle jet is likely misadjusted or dirty. Fingers crossed today is it!

You sound more educated on the issue that anyone else here, barring a 912 expert chiming in.

Did some Googling and found this priceless document from Rainbow aviation. I'm sure you've done a lot of reading but if you haven't read this description of how the 64 works, read it ASAP.

I think you're barking up the right tree by investigating the needle jets since you say the vibration is in the midrange. Did you say the carbs were rebuilt? What exactly did they replace during rebuild?

Who did the jet needle setup? Are they depth-set with E-clips?

Have you had your prop balanced to eliminate the possibility of an outside vibration somehow influencing the carbs? Seems like a remote possibility but stranger things have happened since many components are only operated with vacuum or dampened with springs.

The throttle cables all in good condition? Splitter working smoothly?

Bench tested the spark plugs? I had a situation once where the plugs worked ok at high RPM but would shit out at lower RPM. Turns out one plug was broken.

We had an extremely annoying vibration in our 912. Went through carbs, ignition boxes, prop balance ad nauseam. Very frustrating. As it turns out there is an issue in the gearbox around 2016 maybe? Ronnie or Morris Smith at SMLA sorted it out…Rotax is hiding from it as the Austrians are the most perfect beings to sanctify the earth with the imprint of their feet and would never make a mistake like that.

It’s worth a look. Ronnie is the man when it comes to Rotax.

AvDES_LLC wrote:https://www.google.com/search?q=leaking+rotax+carb&rlz=1CDGOYI_enUS924US924&oq=leaking+rotax+carb&aqs=chrome..69i57j0i546l3.6363j1j7&hl=en-US&sourceid=chrome-mobile&ie=UTF-8

Then consider yourself lucky. You seem to be one of the few that hasn’t had this issue YET. Just saying.

In other news, Bane (kitfox has earned that name) started up yesterday and idled with minimal fluctuation. Air/fuel readings were between 12.8 and 14.1 and I’ll be working to tune those last surges out today. But it went from +/- 400 rpm swings to about 80 rpm. The needles that were pulled from the carbs and placed under a scope showed hairline cracks in the needle tips. I’ll try to get an image through the scope lens and post it but previous attempts to this proved futile. Now the last fight is the mid range is reading at a ratio of about 22.1:1 which is NOT GOOD - this has me thinking the needle jet is likely misadjusted or dirty. Fingers crossed today is it!

I was just pointing out that the swapped manifold thing is fairly common, and maybe/probably not the root cause. I haven't had it, yet, in 2860 hours, or maybe I had it and didn't know it! The best thing I ever did was swap my throttle out for a MacFarlane Vernier, dual cable type. That and a Zipper BigBore, and a 1" crossover, other than that it's been super reliable and trouble free.

Zzz wrote:You sound more educated on the issue that anyone else here, barring a 912 expert chiming in.

Did some Googling and found this priceless document from Rainbow aviation. I'm sure you've done a lot of reading but if you haven't read this description of how the 64 works, read it ASAP.

I think you're barking up the right tree by investigating the needle jets since you say the vibration is in the midrange. Did you say the carbs were rebuilt? What exactly did they replace during rebuild?

Who did the jet needle setup? Are they depth-set with E-clips?

Have you had your prop balanced to eliminate the possibility of an outside vibration somehow influencing the carbs? Seems like a remote possibility but stranger things have happened since many components are only operated with vacuum or dampened with springs.

The throttle cables all in good condition? Splitter working smoothly?

Bench tested the spark plugs? I had a situation once where the plugs worked ok at high RPM but would shit out at lower RPM. Turns out one plug was broken.

Thank you for that link. I have not yet seen this and will continue to read. In regards to the carb rebuild - new needles, polished the needle seat (non-replaceable), the new Blue floats offered by mcfarlain I believe?, new float armatures, pulled all jets, copper strand wire through jets, carb spray through all openings, moved needle from position 2 to position 3 (from top down), new air filters, all new fuel lines from gascolator forward, new filter, etc etc.

What’s very interesting is I purchased the plane from a my home airport and went up in it with the previous owner - had little to no issues. Then we pushed the plane aside after purchase to finish his other kitfox up. When we came back to this one, we immediately pulled the engine to do the engine mount replacement before I took it over, and boom - never ran right since we reinstalled the engine. This is driving me up a wall since I know the engine ran fine and had no hesitations, to mow chasing this rabbit since October of 2021.

I’ve gotten it down to just the right carb acting funny now. I need like the needle seat has some burr in it that is causing a leak by as it’s the only one that no matter what I do there seems to always be excess fuel in the bowl at any adjustment shy of 14.5mm. If I do 14.5mm, the plane idles GREAT, but when you go to full power run up, you get 5200 rpm for about 5 seconds then the engine falls on its face - carb fuel bowl not refilling fast enough to maintain 5200 rpm. Today I’m going to set that needle seat up with a bit of high grit polish to make sure no burrs or chatter marks exist on that carb, then try to take her up.

courierguy wrote:

AvDES_LLC wrote:https://www.google.com/search?q=leaking+rotax+carb&rlz=1CDGOYI_enUS924US924&oq=leaking+rotax+carb&aqs=chrome..69i57j0i546l3.6363j1j7&hl=en-US&sourceid=chrome-mobile&ie=UTF-8

Then consider yourself lucky. You seem to be one of the few that hasn’t had this issue YET. Just saying.

In other news, Bane (kitfox has earned that name) started up yesterday and idled with minimal fluctuation. Air/fuel readings were between 12.8 and 14.1 and I’ll be working to tune those last surges out today. But it went from +/- 400 rpm swings to about 80 rpm. The needles that were pulled from the carbs and placed under a scope showed hairline cracks in the needle tips. I’ll try to get an image through the scope lens and post it but previous attempts to this proved futile. Now the last fight is the mid range is reading at a ratio of about 22.1:1 which is NOT GOOD - this has me thinking the needle jet is likely misadjusted or dirty. Fingers crossed today is it!

I was just pointing out that the swapped manifold thing is fairly common, and maybe/probably not the root cause. I haven't had it, yet, in 2860 hours, or maybe I had it and didn't know it! The best thing I ever did was swap my throttle out for a MacFarlane Vernier, dual cable type. That and a Zipper BigBore, and a 1" crossover, other than that it's been super reliable and trouble free.

No problem at all - re-reading that last post it did kinda sound a bit a**holeish in the response. Apologies - I needed a snickers.

I’ve done the 1 inch cross over as well as have that dual vernier cable from macfarlane - love that thing. I will say however that a throttle knob in the location provided by kitfox in the model 4 is a NIGHTMARE. Hits my knee every time I fly and I feel like I don’t have full right turn capability due to that damn knob - I’ll address that later.

I don’t want it taken as me stating that the carb intake swap is the cause as much as I want it know that it exacerbates the cause. When an engine vibrates, there is the centerline by which to hose vibrations take place - think a +5 -5 sine wave - our center would be the zero point. When we take the intakes and away sides, the swap pushes the carbs further away from that center line and more toward the peaks of the sine wave thereby causing a greater impact AT the carb as a result of these vibrations. Tinkering IN the carb to solve an issue that likely in some level has to with location and less with internals - good way to open a rabbit hole that chases its own tail.

I’m hoping this thread becomes one where we’re able to finally collect and share some real world data on this issue that drives our decisions as opposed to throwing money parts and time at the problem hoping to solve it. We’ve all played that game before and it never goes well - not to mention it’s NOT how we should approach any problems - be them in life or with airplanes.

Today comes with interesting and exciting updates.

Biggest one is Bane is running and idling PERFECTLY at 1700 rpm with an air / fuel ratio of 12.8 - 12.9 : 1.

Coming into the mid range there’s still a bit too much fuel being delivered - around 3500 rpm the ratio is working out to around 11.3:1 - honestly may consider leaving this alone as we spend so little time in this range and I don’t feel it to be too large of an issue as rich is better than lean.

Lastly we come into wide open throttle and the ratios currently working out to be about 13.3 - 13.4:1 which is too lean for my comfort. Interestingly I was able to gain an additional 200 rpm during ground run up with these adjustments so I’ll be getting another degree into the prop before calling it final.

Final adjustments as follows:
1) 5/8 turn out from seated position on idle air screw
2) 12.5 mm measurement from bowl top to float armature
3) needle at second clip position from top
4) current main jet is 158. I’ll be polishing this thing to see if maybe corrosion is restricting that last bit of fuel I need at wide open, if not, 159 main jet is likely needed

This has been a learning experience for sure, but undoubtedly has also been a reminder of the past. Tune your carbs to your engines. Tooling wears during production, anomalies in base material are present - there will always be differences and no two things are ever identical. This includes engines and carbs. The idea of having a one size fits all setting for carbs is a pipe dream and these past months of testing as well as the data collected over the past weeks proves this.

For more instant updates - I tend to post throughout the day as I’m working - follow us on Insta-spacey AvDES.llc

Good to hear!
One thing I've always wondered, is maybe what seems like a rough running engine to one pilot, may not be to another. In my case, since I have no idea what my fuel/air ratio is, among other things, I just fly along blissfully ignorant. I know some that obsess over the differing EGT's and go to great lengths to correct them, another thing I have only a vague concern about. My main thing is knowing I can go out to the hangar, hit the starter, and go flying, I compare it to any of the Toyota rigs I drive, it's just been super reliable and trouble free. If it ain't broke, I don't fix it, probably not the best attitude but so far so good. I like reading accounts like yours to get more educated on what I should already know! I have said before, that I was dragged kicking and screaming into the Rotax camp (my first 7 had a Subaru conversion), due to my perception of it being too complicated and too expensive. Now I am one of their biggest proponents. I have ignored most of the factory recommendations along the way, I never wait for the temps to reach 120 before takeoff is one example (I use 85), I never "replace all rubber components every 600 hours", I don't use their oil filters, I go 600 hours between plug changes, I don't balance my carbs more often than every few years, and so on. I do religiously change the oil at 50 hours, in all else I am a slacker, and the little sucker STILL has delivered the most trouble free flying for me in the last 14 years that I have ever experienced. The only reason I changed out my engine after reaching past it's TBO, (2300 hrs TT) was I was feeling guilty!

courierguy wrote:Good to hear!
One thing I've always wondered, is maybe what seems like a rough running engine to one pilot, may not be to another. In my case, since I have no idea what my fuel/air ratio is, among other things, I just fly along blissfully ignorant. I know some that obsess over the differing EGT's and go to great lengths to correct them, another thing I have only a vague concern about. My main thing is knowing I can go out to the hangar, hit the starter, and go flying, I compare it to any of the Toyota rigs I drive, it's just been super reliable and trouble free. If it ain't broke, I don't fix it, probably not the best attitude but so far so good. I like reading accounts like yours to get more educated on what I should already know! I have said before, that I was dragged kicking and screaming into the Rotax camp (my first 7 had a Subaru conversion), due to my perception of it being too complicated and too expensive. Now I am one of their biggest proponents. I have ignored most of the factory recommendations along the way, I never wait for the temps to reach 120 before takeoff is one example (I use 85), I never "replace all rubber components every 600 hours", I don't use their oil filters, I go 600 hours between plug changes, I don't balance my carbs more often than every few years, and so on. I do religiously change the oil at 50 hours, in all else I am a slacker, and the little sucker STILL has delivered the most trouble free flying for me in the last 14 years that I have ever experienced. The only reason I changed out my engine after reaching past it's TBO, (2300 hrs TT) was I was feeling guilty!

This engine has by far been the largest pain in my ass in history of engines to date. I have a history of engine building and a bit of tuning knowledge, and nothing has fought me quite like this rotax. That said, it was also due to my stupidity in listening to things said online and following the masses as opposed to doing it right - I’ll take the hit on that as a reminder to slow down and use your knowledge, not what the forum warriors keep perpetuating. For future knowledge, an engines air fuel ratio (in my opinion) provides the most wholesome data about how the engine is operating with a single sensor. An engine has best power at a ratio between 12.8 and 13.1 parts air to 1 part fuel. Anything higher than 13.1 is indicating a lean condition in the engine ( should also correlate with excessively high EGT) whereas anything below 12.8 is heading into the rush world ( cooler egts) - when you get down into the 8:1 area, you’re so rich you can expect soot and likely fouled plugs, Huge reduction in power, and a nasty chugging / vibration in the engine - these are all common symptoms expressed by many owners when they are making base to final and pulling power out. This was supported with the data collected that at idle we can achieve 12.8 and wide open around 13.4 with stock jets, but the mid range dropped will into the low 11 / high 10 range telling us that the needle jet is likely too large. Again this is a stock jet so I’d imagine this condition to be present on many engines.

That said, I was also half considering picking up a Jubaru engine to see how it did. I’ve never flown behind one or ever turned a wrench on one, but the gent in the hangar across the way has one in his plane and that thing is so damn smooth it’s downright impressive.

These past months have had me pulling my head out chasing a rough engine that was fully stock and all adjustments made per the rotax manual. Today I’ll be walking into my hangar with my fingers crossed that it may be the last day I spend under that cowling before getting to spend some real time with her in the air.

Hopefully I’ll see some of y’all at Kitfox Factory fly in or maybe HSF2022!!!

Well I had a request from another user on Team Kitfox to make a small video about what all I have done in an effort to solve these idling and vibration issues. The link to the video is below - any and all comments and suggestions welcome!

https://www.youtube.com/watch?v=GxEORpdJp0w&t=30s

I was following along here and on the Kitfox forum (and I don't even have a 912).

Very good information in your video. From my 4 cylinder, 4 carb motorcycle days I can confirm that there is no silver bullet set of jets that make your machine run perfectly. Too many variables involved.

well thank you very much for following along! Im glad to state that today the Kitfox came out of annual with flying colors. I sat down, opened my maintenance log book (currently logging hours toward my A&P so high level details were written in AMT logs regarding all work completed), and my IA proceeded to pick apart every entry since I purchased this plane. When it came to the engine and carb portion of the logs, he closed them, told me to get in the plane, and start it - he wanted to witness these claims.

Lets just say that even the IA was BLOWN AWAY at the consistency in idle, minimal vibrations throughout the throttle range, and a perfect air-fuel mixture throughtout. He signed my logs with a smile and was happy to see that someone was, "taking the time to treat the engine the way it needed to be treated." That gave me a rather large boost in confidence as a gentleman with 40+ years in aviation and his hands in all kinds of planes took the time to make that statement - I think were finally headed down the right track with solving these 912 horror stories we so often see.

AvDES_LLC wrote:well thank you very much for following along! Im glad to state that today the Kitfox came out of annual with flying colors. I sat down, opened my maintenance log book (currently logging hours toward my A&P so high level details were written in AMT logs regarding all work completed), and my IA proceeded to pick apart every entry since I purchased this plane. When it came to the engine and carb portion of the logs, he closed them, told me to get in the plane, and start it - he wanted to witness these claims.

Lets just say that even the IA was BLOWN AWAY at the consistency in idle, minimal vibrations throughout the throttle range, and a perfect air-fuel mixture throughtout. He signed my logs with a smile and was happy to see that someone was, "taking the time to treat the engine the way it needed to be treated." That gave me a rather large boost in confidence as a gentleman with 40+ years in aviation and his hands in all kinds of planes took the time to make that statement - I think were finally headed down the right track with solving these 912 horror stories we so often see.

You are allowed to log hrs worked on an experimental in the states toward your A&P? That's sweet. In Canada it has to be a certified aircraft to count towards out AME.

A1Skinner wrote:

AvDES_LLC wrote:well thank you very much for following along! Im glad to state that today the Kitfox came out of annual with flying colors. I sat down, opened my maintenance log book (currently logging hours toward my A&P so high level details were written in AMT logs regarding all work completed), and my IA proceeded to pick apart every entry since I purchased this plane. When it came to the engine and carb portion of the logs, he closed them, told me to get in the plane, and start it - he wanted to witness these claims.

Lets just say that even the IA was BLOWN AWAY at the consistency in idle, minimal vibrations throughout the throttle range, and a perfect air-fuel mixture throughtout. He signed my logs with a smile and was happy to see that someone was, "taking the time to treat the engine the way it needed to be treated." That gave me a rather large boost in confidence as a gentleman with 40+ years in aviation and his hands in all kinds of planes took the time to make that statement - I think were finally headed down the right track with solving these 912 horror stories we so often see.

You are allowed to log hrs worked on an experimental in the states toward your A&P? That's sweet. In Canada it has to be a certified aircraft to count towards out AME.

That’s crazy. I didn’t know it was certified only for you guys. I was taking with Nat and she blew my mind when telling me y’all don’t have a tailwheel endorsement up there - you just get in and learn to fly. Thought that was pretty cool.

Yup. No TW endorsement. Our high performance/complex rating is also very different. For a CS prop or retract we don't need high performance rating until it's stall speed is over 70 kts. But we don't have the drivers license medical thing. So some plusses, some minuses.