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Strange ROTAX/BING float bowl issue


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Whenever I have the bowls off I use an inspection mirror to view the main and midrange jets for obstruction and to verify the gaskets are properly seated. I weighed the floats. Everything looked good, which is why the leakage was a disappointing surprise.

Do you have a set interval for replacing the gaskets, or is the 5 year carb overhaul often enough.

I’ll let you know what I find when I get back to it, probably next week.

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10 hours ago, cdarza said:

ive been using the newer, more expensive black rubber gaskets and i find them quite difficult to "seat" properly.   Kinda like a hit and miss.  Just did a carb bowl check last week and had to reseat the gasket a couple times to get no leak.   

Many years ago I bought some black rubber gaskets. I found them too thick to easily get the spring clip seated, so I never used them. I wish I had known about Real Gaskets, but going with official ROTAX parts has its benefits as well.

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First of all, kudos to Lockwood - they got the right float bowl and the new gaskets to me in a couple days in spite of Thanksgiving. Both arrived yesterday. 

As such, I set to work last night. First, I removed the left (in my plane the 1/3 side) float bowl, which was already the new style. Nothing appeared off, the fuel in the bowl seemed at about the right level. I verified the floats were floating at the level of the pins, I swapped gaskets and reassembled. Powered up the electric fuel pump and voila! No leak at all. Yay!

On the 2/4 (right) side, the old style float bowl was in fact filled to the brim. As I went to dump the floats out, the problem became obvious:



In my plane it’s difficult to maneuver the float bowls back in place due to limited room available. As mentioned upthread, the float pins bend easily, and I apparently slightly bent one slightly during the installation process. That caused the float to stick against the side of the bowl, causing the overflowing.

Anyway, reassembled with the new bowl and gasket in place, no leaking with the electric fuel pump on. I’m going to let it warm up a bit - it’s 27°f right now - and hopefully do a ground runup and return-to-service test flight later today. I’ll let you know how it goes.

Also, I’ll follow up with a couple hints I found helped with the float bowl removal/replacement process.


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Those hints:

1) Grease was recommended to hold the gaskets in place. Some may remember I had a bad experience trying to use a gasket sealant, which did not respond well to fuel. In any case, the grease matters. I first tried white lithium grease, then wheel bearing grease. Both were too thin to have much effect. I had some more generic grease in a grease gun, and it was just tacky enough to hold the gaskets in place.

2) Loosening the carb spigot clamps and rotating the carbs slightly greatly increases accessibility to the bowl area. Removing the spring clip temporarily also helps a lot.




As always, open to any suggestions to make this recurring chore more palatable!

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Popping carbs out of socket is my default process, the CT drip pans don't afford adequate clearance and to your point about bending a post and struggling to seat gaskets.  It's simple enough to loosen the clamp screw, remove dampening spring, a little twist & pull - and then you have all the room you need for not jacking up anything.  

I also employ the "Tom Trick" of using a cresent wrench for flipping the bail.  That is the only use for an adjustable wrench on an airplane, but a damn fine one.

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Just did a runup.

Good news: started right up, and after warming up ran it up to almost 5,000 rpm and everything was smooth. After shutting down I don’t see any leaks.

Bad news: still VERY rough with choke application. Smooths right out when I turn the choke off. Then, reapplying it slowly the rpm increases smoothly for the first bit of travel, but then starts to run as if one bank of cylinders is cutting out. The choke mechanisms seem to be moving smoothly and together from OFF to ON. Remember, it was showing this behavior shortly before my annual.

I have rebuilt BINGs, albeit on motorcycles*, so I know what lies under the choke cover. Neither intake jet appeared clogged. I’m thinking my next step is to pull those covers and take a look for obstructions or anything else amiss. And maybe try blowing air into the jet to make sure the passageway is clear.

Has anyone come across this kind of behavior, and if so, what was the fix?

Note: I know it’s not a real choke, but it’s easier than saying “starting circuit” each time. Excellent description here:

*I took this photo when rebuilding the carbs on my R100GS to remind me which way the disc went on reassembly:

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First Q: Are you only using the enrichment circuit for starting and the first dozen or two seconds? It's designed to dump HUGE amounts of fuel into the system for starting, because the fuel has a tendency to collect immediately on the intake and cylinder walls. It's not calibrated for anything else after that.


The enrichment circuit is basically a bypass to the standard carb functionality. It takes inlet air from before the piston, and the exit is in the side just after the throttle plate. The fuel mixing occurs within the enrichment housing, and the entire thing is vacuum driven.

Since your carbs are the standard arrangement with the chose facing outwards from centerline, you should see stamped letters on the shaft when you take them off.

L stamped goes on the RIGHT carb.

R stamped goes on the LEFT carb.

On the outside of the housing, on the shaft, is a little dot to one side. That points towards the CARB intake, which is also towards the little U shape of the choke housing.

I had these on backwards once and yes, you get lots of weird things happening if you can even get the engine to start.

If the above did not fix the issue:

The vertical tube that goes into the carb bowl in a corner feeds the enrichment mechanism. Ensure that is clear. Also, inside that corner, is a tiny little jet at the bottom that is very often overlooked and barely even mentioned in the manuals. Takes a small flat head screwdriver to remove. Take it out and ensure it is clear as well.

Make sure the o-ring on the shaft in the enrichment circuit is in good condition / replace it. As well as the gasket around the housing. Vac leak here will easily cause issues.

Carb syncing: idle is critical, absolutely critical, even to the enrichment circuit. As a reminder, the enrichment circuit is VACUUM driven, so throttle plate position has everything to do with the effectiveness of the enrichment circuit. Therefore, if your carbs are not syncing very well at the low end, this can cause a whole array of issues including enrichment circuit abnormalities, and you need to figure out why they won't even sync.

The idle stop screws are there for small adjustments to idle and sync, if you really have to drive one in or out, you have a carb problem, find out why.

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Thanks for the detailed response. Hopefully this thread will be useful to link to when others have issues.

My SOP for a cold start is throttle closed and full choke. The engine has a tiny bit of coarseness for a few seconds until the “soft start” un-retards the timing. The engine runs fine and I can ease the choke off fully over a minute or so as it warms up.

The problem here came on suddenly in a couple of starts before my annual. It can’t be the choke discs installed wrong, since the carbs haven’t been seriously messed with since their second overhaul a few years ago and have been working properly.

I’m strongly suspecting something blocked in the fuel circuit. Today I plan on pulling the float bowls (again!) and removing the starter circuit jets and see if there’s anything obvious blocking them. I’m considering taking off the starter disc cover to also look for blockage. This seems unlikely, since the starter jet is so tiny compared to the apertures in the starter disc.

Anyway, this is getting tedious, and again pushing me towards the “hate” end of my 51 year love/hate relationship with BING carbs.

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Sometimes it’s just an obvious screw up.

I pulled one of the new float bowls and looking at the bottom of the carb was not seeing the jet for the starter circuit. I had the impression it was at the bottom of a hollow tube.  Looking back at the diagram from Rainbow Aviation, I was reminded that the jet was actually located in the bowl. I looked and guess what - the bowls don’t ship with the jet installed. D’oh!

So I just transferred the jet from the old bowl to the new one. 


No wonder it wasn’t working right! Hopefully that’s all it was. Does not explain my rough starting on choke before the annual, but in another hour or so after I switch the other jet I’ll be ready for another runup. Fingers crossed!


edited to add: Surprisingly, when I went to move the jet from the old bowl to the new one on the other side (2/4) that bowl did have the jet in place as delivered. Wonder why one did and one didn’t? And no wonder with the choke engaged it felt like it was running on one bank of cylinders!

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To follow up, yesterday afternoon I did a runup and the choke worked perfectly. Threw the cowling back on and did my return-to-service test flight and all was well. Logbooks updated and another Annual Condition Inspection is in the books.

Sometimes I get frustrated at how much of my life is spent maintaining, repairing and troubleshooting things. The plane for sure but also home electrical and plumbing, WiFi and other computer gremlins, cars and motorcycles and a zillion other things. Yes, it’s rewarding when a gremlin is tracked down and squashed, but sometimes I agree with Steve Jobs - things should just work!

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6 hours ago, Mike Koerner said:

The trick may be not to own stuff. Just rent instead. Let someone else worry about maintaining it.

That is definitely more economical for CTs unless you fly a LOT.  When you add in hangar, insurance, fuel, taxes, maintenance, etc you could fly a lot of hours in a rental for that money.  I know personally I could get a substantial "raise" instantly by selling my CT.

But there is something to be said for pride of ownership, and knowing exactly the state of the airplane and all its quirks every time you get in it.

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On 11/25/2021 at 11:58 AM, Roger Lee said:

If the fuel comes out of the air intake and none from the bowl then it's the float armature height that controls the needle valve and float / fuel level. If it's coming down into the drip tray and there is fuel on the bottom of the carb bowl then it's either the bowl gasket needs replacing (the old cork or fiber gasket) or the new rubber one is not properly seated in the carb groove and or the bowl is not properly seated in the groove. 

Please remind us how to adjust the armature height.  I know the tab that's involved, but can't remember how to measure.

EDIT:   IIRC, you have to remove the carb, flip it over, and measure from the flat bowl lip.

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There's a tool for that you can buy from rotax. Most people just set it to be level. I have found that the tool actually is useful for setting it accurately, because the tab can bend in different ways, changing the height even though it's level.

Never found a measurement from the actual flange end.

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It's quite easy. The carb does need to be upside down. Then you need a metric ruler and most are in millimetres. With the carb pointing upside down place the ruler on the outside edge of the carb. Slide the measuring side over to the brass float armature. The height should be 10.5mm from the edge of the carb to the top of the armature. If it's 10 or 11mm it isn't a big shake. If you need to adjust the height then bend the tab the the needle valve is hanging from. Just takes a minute or two to do. Oct. and Nov. were definitely carb overhaul months.

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  • 2 weeks later...
On 11/18/2021 at 7:30 PM, Tom Baker said:

From two years ago I think there is a new, new and improved float. I have not heard of anyone having issues with the latest version.

When I ordered them I asked if they were the new and improved, Leading Edge said yes but I'm afraid they were wrong since these were gaining weight after installation. The SB said you did NOT have to replace if they had a dot but they did not actually say that all new ones had the dot. Expensive mistake.

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