Strange ROTAX/BING float bowl issue

[FastEddieB]

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:

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 I see as 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:

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.

 

[GrassStripFlyBoy]

The float with pin unsecured may have been sort of working, or at least more or less remaining on it's side of bowl, who knows.  I'd not trust pushing the pin back into the bowl, I recommend buying the new version bowls that have a blind boss and won't leak.  If you're not buying a pair, ensure you purchase the correct hand needed.

[FastEddieB]

Thanks! I wasn’t aware of newly designed float bowls. Most Conservative Action would be to go ahead and replace them both. Do you have the revised part # and a source?

[GrassStripFlyBoy]

10 minutes ago, FastEddieB said:

I’m in the middle of an annual condition inspection

I just finished mine this afternoon, about two weeks of part time work.  Long list of little things, and a dozen scrapes & bloody knuckles latter, I've convinced any mechanic working on CT's is earning every dollar they charge.  

[GrassStripFlyBoy]

This is off Leading Edge Airfoils, I believe they can be found elsewhere for less:

[Anticept]

2 hours ago, GrassStripFlyBoy said:

I just finished mine this afternoon, about two weeks of part time work.  Long list of little things, and a dozen scrapes & bloody knuckles latter, I've convinced any mechanic working on CT's is earning every dollar they charge.  

I have a joke I tell people.

Fuel is just for appeasement, but what the airplanes really run off is blood. They break when they have not been satisified recently.

[Anticept]

3 hours ago, FastEddieB said:

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:

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 I see as 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:

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.

 

If you do ever reinstall the pins, I believe you have to use red loctite. Don't quote me on that though, I've not had to do this yet, but there's a reference somewhere!

[Roger Lee]

Do use red Loctite and press it back in place. Be careful not to bend it.

[FastEddieB]

1 hour ago, Roger Lee said:

Do use red Loctite and press it back in place. Be careful not to bend it.

I’ll confirm it bends really easily - though it’s easy to bend straight again.

I’m ordering a pair of the new style float bowls in the morning. Can you confirm, 963 230 and 963 232 are the newer and current design?

[FlyingMonkey]

Good catch!  To check floats after you affix the pin, I'd fill the bowl with fuel and then run the floats all the way up and down.  This will make sure a bend pin does not cause a float to bind up in the bowl.  I had that happen and it literally POURED fuel all down my firewall.  I'm super lucky I didn't have a monstrous engine fire.

So far my only scary problems with the Rotax engine were two big fuel leaks caused by the floats and pins -- one a pin that came out like yours before I thought to epoxy them, and the second the jammed float.  It's a weak aspect of the design IMO.

[FastEddieB]

Thanks for the heads up. 
 

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:

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, due to the placing of the starter system pickup, they’re not.

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.

[Tom Baker]

Besides having the blind holes, the pins are also longer. The design is so the pins are blocked from coming out when the bowl is installed. The only problem is that it prevents the gasket surface from being lapped to help it seal better.

[FlyingMonkey]

9 hours ago, Tom Baker said:

Besides having the blind holes, the pins are also longer. The design is so the pins are blocked from coming out when the bowl is installed. The only problem is that it prevents the gasket surface from being lapped to help it seal better.

Yeah, that's a doubled edged sword.  It also makes pins getting knocked around during removal and install more or a concern.  You might still be able to lap the bowl on the edge of a table, but you'd have to do it very lightly and take great care to evenly hit the entire rim.

[Anticept]

Honestly if it were an issue preventing lapping, I would also file down a pin JUST ENOUGH to let it seat.

It's a stretch to even call it filing, since lapping is a step in between grinding and polishing.

[FlyingMonkey]

1 hour ago, Anticept said:

Honestly if it were an issue preventing lapping, I would also file down a pin JUST ENOUGH to let it seat.

It's a stretch to even call it filing, since lapping is a step in between grinding and polishing.

Good point, there's no safety issue with grinding down the pins.  Even at their highest the floats can't come off the top of the pins at the old version height.

[Skunkworks85]

1 hour ago, FlyingMonkey said:

Good point, there's no safety issue with grinding down the pins.  Even at their highest the floats can't come off the top of the pins at the old version height.

Other than potentially removing a design poka-yoke, as stated in Toms response, Sure no safety issue....:

 

13 hours ago, Tom Baker said:

Besides having the blind holes, the pins are also longer. The design is so the pins are blocked from coming out when the bowl is installed. The only problem is that it prevents the gasket surface from being lapped to help it seal better.

. 

[Anticept]

That's why i said, if you lap the bowl and the pins wont let it seat anymore, take a tiny bit off the top of the pins so it does. Absolute bare minimum. If an equal amount off the pins is taken as the bowl, then the pins will still contact and be held in place. Talking thousandths of an inch.

[Tom Baker]

3 hours ago, Anticept said:

That's why i said, if you lap the bowl and the pins wont let it seat anymore, take a tiny bit off the top of the pins so it does. Absolute bare minimum. If an equal amount off the pins is taken as the bowl, then the pins will still contact and be held in place. Talking thousandths of an inch.

It is not that you lap the bowl and the pins won't let it seat anymore, it is the pins stick out past the mating surface preventing lapping in the first place. If you remove enough from the pin so you can lap the sealing joint you have removed the safety feature of the pin not having enough room to come out while the bowl is installed.

[mikey70]

I had a pin come loose a couple years ago. I knew some was very wrong when I was burning 8 gph at cruise. I was damn lucky there was no fire, the carbs are right over the exhaust. I drove the pin back in and replaced the floats with the newer ones required on the service bulletin. I weighed the new ones before I put them in 42.6 grains on a digital powder scale. On the last leg of a cross country last week I started burning 5.9 gph instead of 5.0 or 5.1. I knew there was another problem. The bottom of the fuselage was fuel stained, a sure sign of overflow or leaking from the carbs. The offending carb had fuel stains in the drip tray, after removing the bowl the pins looked good but one of the floats was not as high in the fuel as the other. When I weighed them the bad one was 90 grains, more than double its original weight. Im going to replace both bowls and floats but Leading Edge is out of stock.  In the meantime I still have my old float which were never a problem. By the way, the total weight of the two floats in that carb is over 9 grams. So much for the new and improved $150 ones. Too bad we cant use the MS epoxy ones in SLSA. 

[Anticept]

5 hours ago, Tom Baker said:

It is not that you lap the bowl and the pins won't let it seat anymore, it is the pins stick out past the mating surface preventing lapping in the first place. If you remove enough from the pin so you can lap the sealing joint you have removed the safety feature of the pin not having enough room to come out while the bowl is installed.

Ohhh I see where the confusion is!

You can make a lapping surface by milling out a hole in the middle just big enough to allow the movements for lapping if you don't want to remove the pins. Clamp down fences to keep you from moving the bowl too far and hitting a pin.

[Tom Baker]

2 hours ago, mikey70 said:

I had a pin come loose a couple years ago. I knew some was very wrong when I was burning 8 gph at cruise. I was damn lucky there was no fire, the carbs are right over the exhaust. I drove the pin back in and replaced the floats with the newer ones required on the service bulletin. I weighed the new ones before I put them in 42.6 grains on a digital powder scale. On the last leg of a cross country last week I started burning 5.9 gph instead of 5.0 or 5.1. I knew there was another problem. The bottom of the fuselage was fuel stained, a sure sign of overflow or leaking from the carbs. The offending carb had fuel stains in the drip tray, after removing the bowl the pins looked good but one of the floats was not as high in the fuel as the other. When I weighed them the bad one was 90 grains, more than double its original weight. Im going to replace both bowls and floats but Leading Edge is out of stock.  In the meantime I still have my old float which were never a problem. By the way, the total weight of the two floats in that carb is over 9 grams. So much for the new and improved $150 ones. Too bad we cant use the MS epoxy ones in SLSA. 

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.

[FlyingMonkey]

13 hours ago, 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.

I had some heavy floats last year and had to upgrade to the latest.  Fingers crossed.  $300 for four plastic floats! 

[FastEddieB]

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:

 

 

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!

[Anticept]

If your bowl is overflowing, then the needle valve and seat aren't actually sealing during your test

That said, what kind of pressure is your electric pump pushing?

[Roger Lee]

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.