Strange ROTAX/BING float bowl issue

FastEddieB

FastEddieB

Touchdown! Greaser!
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Lenoir City, TN/Mineral Bluff, GA
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Fast Eddie B
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I’m in the middle of an annual condition inspection on my Sky Arrow with a ROTAX 912ULS.

One thing I do is pull the float bowls, looking for debris, examining the jets and weighing the floats. When I freed the first float bowl, this is what I found:

51680321320_9eb754e4c4_z.jpg


I’m wondering if it has been like this for a while, and how it would affect running. The plane ran fine, though I have noticed a lot of vibration after starting with the “choke” on.

The pins the floats ride on are press fit into holes that aren’t blind which seems like a design flaw. Hence, if one comes adrift fuel will leak out of the hole. Years ago someone suggested puttIng epoxy over the holes just in case. I had done so, and the hole left from the missing pin still had the epoxy in place, though the other hole didn’t:

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I tapped the errant pin back in and re-epoxied both holes. Has anyone else seen this as a recurring problem? I’m wondering if lightly tapping the pins to seat them each annual might be a good idea.
 
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How often do you pull the carb bowls? Running Bing carbs on two strokes (many years ago) I would pull the bowls for just about every preflight (those were Bing 54 carbs but the float/bowl setup appears the same).

Never seen what happened to you. Interesting to say the least ...
 
Have you tried to contact a dealer to see if there is an update to the bowl?
 
FastEddieB said:
Usually just annually, but crud in the float bowl is one of the first things you look at with rough running.
Click to expand...

True. With the two strokes we were mixing oil & auto fuel together. That seemed to attract crud quite often, hence the reason I checked them so often.
 
Capt. Geoffrey Thorpe said:
The main jet stuff should prevent`the float from tipping over.
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The recent rough starts I mentioned were VERY rough, as if running initially on one many of cylinders when the “choke” was on. Choke is in quotes because the BING’s use a separate starting circuit for cold starts. Maybe the errant float was interfering with that starter jet?
 
FastEddieB said:
Maybe the errant float was interfering with that starter jet?
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Don't think so - the starting circuit sucks fuel from the brass tube hanging down from the carburetor body into the float bowl - the float couldn't block it. Video here

I would expect problems to be with the fuel level in the bowl.
 
Capt. Geoffrey Thorpe said:
Don't think so - the starting circuit sucks fuel from the brass tube hanging down from the carburetor body into the float bowl - the float couldn't block it. Video here
Click to expand...

Thanks for the video. I was misremembering which jet did what.
 
As a followup…

I ordered the revised float bowls from Lockwood on Monday, and they arrived today. Kudos to them for rapid shipping.

Unfortunately, only one of the float bowls was actually the most current version. You can see them here:

51686202182_c0a418f656_z.jpg


Fortunately, the one on the left is the one I really needed. You can see the new ones have a blind hole for the pins to seat in, fixing what I see as a design defect. You can see below though they seem symmetrical, but due to the placing of the starter system pickup, they’re not.

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Anyway, Lockwood said to send the old style one back. They don’t have a current one in stock, but will send one as soon as it comes in. Not a huge deal, since we’re discussing the one I didn’t really need, that I was just replacing proactively.
 
Tuesday night was a little depressing, but first a quiz:

What is 51 years?

Answer: The amount of time Fast Eddie has been futzing around, on and off, with BING carbs!

First was in 1970 on a BMW R75 picked up in Munich as part of BMW's overseas delivery program:

10037791246_bf53ea99a4_z.jpg


And then later on a 1986 R80RT and a 1990 R100GS/PD. And since 2007 on my Sky Arrow.

Over the years I've had a love/hate relationship with BING carbs. They're an engineering masterpiece, but with flaws. I think nearly every airhead BMW owner has had at least one gas soaked boot due to float bowls overflowing. My problems over the years have been with the floats and float bowls primarily, but not exclusively. Non-injected ROTAX owners generally know the first place to look for fueling problems is the floats and float bowls.

Which brings us to Tuesday night. Annual checklist complete, everything was back together except the cowling, and I had planned to do a final runup Wednesday morning followed by a return-to-service test flight. But one thing I like to do if I've had the float bowls off is to run the electric fuel pump and check for leakage. Seemed OK at first, but shortly thereafter I smelled gas and looked to see the outside of both float bowls damp with gas. Drat!

Anyway, seems like Lockwood has the newer style float bowl for my 2/4 bank side carb enroute to arrive tomorrow. I figured I'd go ahead and replace the gaskets as well (I have the neoprene green ones, which replaced the original cork ones, which have apparently been superseded by black ones) and called to order 4 so as to have spares. Until I found out they're something like $21 each (!). So I have 2 on the way and will investigate the leaking once I have everything in hand.

This is about the 11th annual condition inspection I've done, so I've had the float bowls off and on repeatedly with only rare problems - usually the float bowl not quite seating properly. I'm also having trouble envisioning why, if the needle and seat are sealing properly, the fuel would ever make it high enough in the bowl to overflow in the first place. It will be the middle of next week before I can mess with it any more, but I'll let you know what I find. Good news is its now looking like a December signoff, so this year I've gained two months!
 
Good outcome of preventative maintenance. I suspect the through holes were a product of the casting process where the engineers were worried about a blind hole, with it's inherent mold pin issues and material flow issues.
 
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:

51706941451_9e18cfac13_z.jpg


51706941501_84ae282c1a_z.jpg


In my plane it’s difficult to maneuver the float bowls back in place due to limited room available. 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.
 
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 does removing the fuel line.

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51706152212_e7a189d6dc_z.jpg



As always, open to any suggestions to make this recurring chore more palatable!
 
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FastEddieB said:
Tell me more. Links?
Click to expand...
You can get them at most aviation supply vendors. EZ Turn (the former Fuel Lube) is a thick paste that keeps things in place during assembly and is used primarily for fuel applications. Just a little goes a long way. The Assembly Fluid is used mainly for orings but has it place in other area as well and is not as a paste but a thick liquid. Both are sticky and can get everywhere so use with caution. ;)
https://www.unitederie.com/other-products-and-services.html
https://www.ultracheminc.com/products/industrial-specialty-oils/assembly-fluid-1/
 
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:

https://electricmotorglider.com/wp-content/uploads/2017/10/CV-Carb-Part-2-web.pdf

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

14698595595_13f58b3528_n.jpg
 
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FastEddieB said:
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.
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Given that the starting circuit passes both air and fuel, if the fuel part is obstructed, you will add air and lean out the mixture on one bank as you pull the "choke".
 
Regarding the "design defect", it's generally considered good practice to press pins into through bored holes so they can be pressed back out again if necessary to replace them. In this case I guess it's a case of unintended consequences.
 
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.

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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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Reminds me of when I used to race vintage motorcycles, Yamaha two strokes. The RD350 and RD400s both had 28mm Mikuni carburetors, and many parts interchanged. But there were a year or two of high-top Mikuni's. And if you put a low-top carburetor slide in a high top carburetor body, the slide could potentially raise up above the pin the carburetor body that prevented them from rotating. Which means the slide would stick open. I found that out at the end of the straight, when I went to close the throttle and the bike just kept revving... Good times.
 
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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