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912 carb rebuild recommendations needed

18 posts in this topic

Posted

I've been chasing after a rough idle below 3000 rpms or so.  I've traced it down to the idle circuit on the pilot side carb.  The air screw makes no difference in the idle quality on that side which leads me to believe something in the idle circuit on that side is gunked up.

I'm no stranger to taking apart carbs and rebuilding them and I've done similar Bing carbs for a motorcycle.

My first thought is to take them apart and to clean and reassemble them but now I'm wondering if I should just send them out somewhere to get professionally rebuilt.  After 20 years maybe it's best to replace all the O rings and needle valves?

Is there a recommended rebuild service I can send them to or is that a waste of money and should I just clean and reassemble them myself?

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Posted (edited)

The rebuild is very simple but the kit is very pricey.  Look and see if the enricher is assembled correctly, the shaft has a dot on the end that you can see at a glance .  The dots both need to be facing forward if I remember correctly which I may not so check it out for correct position.  I think that I would pull it apart and have a look at the diaphragm and clean the passages.  Also take a close look at the socket for cracks.  Also weigh the floats for the correct weight.  There were some bad floats that came out about a year ago that caused me months of misery before I finally found out what was going on.  You can check for leaks at the socket and manifold with spray carb cleaner.  Just double checked, the dot  does need to be forward at about 4 o'clock for the right side and 8 o'clock for the pilot side carb when viewed from their respective sides.  Good hunting in your quest.

Paul

Ps Bing USA in Kansas has parts and a manual available and I think they do repairs also but I would call to make sure.

Edited by wypaul
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Posted

I didn't think the socket or manifold was leaking because it ran great once it passed the idle circuit.  A socket or manifold leak would manifest at all times.

I pulled the carbs off this morning and completely dis-assembled the one that was giving me problems.  These carbs are very basic.  I was surprised.  Right off the bat I found the idle pickup jet plugged and the O-rings are disintegrating into dust.  I think a chunk of the O-ring was plugging the jet.

I'm going to rebuild the carbs myself.  These are a piece of cake compared to some others I've done.

My serial numbers aren't in the range of the ones with float problems.

I've found these rebuild kits online.  Now I'm just trying to decide whether to go all out and get the premium rebuild kit or just the basic one.

http://store.leadingedgeairfoils.com/index.php?cPath=702

 

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Posted

Most of the rebuild can be done with a universal o-ring Kit.  The basic LEAF kits are good, too. Unless the floats are junk, don't replace them.  

The dot on the enrichener should be on the AIR FILTER side of the carb. 

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Posted

I replaced the carb sockets and have cleaned the carbs each year with carb cleaner and compressed air as preventative maintenance. I did not rebuild the carbs. Primarily because I never found debris in the bowls or experienced any rough running issues and Hobbs was under the Rotax maintenance schedule of 200 hrs for a carb rebuild. I am doing the Rotax 5-year rubber replacement requirement for SLSA's on my 912uls now and even though it doesn't include carb o-rings, I am replacing them. You might just buy the diaphragms, I found o-ring cross #'s somewhere and the Harbor Freight (GASP!) Nitrile O-ring kit is your friend. If I read my notes correctly and the carb o-rings are the same, you will need (2) each 8x2, 5x1.5, 2.5x1.5 and (4) each 3.2x108, 6x1.9. Some folks claim you need to use Viton rather than Nitrile o-rings from HF but I don't fit in that camp. Here is a reprint that explains why:

  saber-cycle.com

 

Nitrile (Buna-N) vs Viton® (FKM) O-Rings for a Gold Wing Carburetor

 

We’ve been getting questions asking if our carb kits o-rings are made of Viton®, Dupont’s registered trademark for their fluorocarbon (FKM). The answer is no because we’re simply not convinced of the need to use this expensive material and pass the costs on to our customers.  While both Nitrile and Viton® are in the class of elastomers, there is a recent trend towards using Viton® in the automotive industry due to its alleged superiority to Nitrile relating to heat tolerance, compression, permeation, as well as its defense against biofuels particularly ethanol.

Unfortunately, we find contradicting data on the comparisons of Nitile and Viton® ethanol compatibility leading us to believe the promotion of Viton® superiority is marketing propaganda. Does anyone remember the R12 refrigerant (Freon®) scare when R134A came on the market? Owners of pre-1994 cars were scared into converting their R12 systems to handle the new ozone safe R134A Freon to avoid corrosion of their rubber and plastic components. History proved the scare was unfounded.

Similarly, what of the scare about the effects of Nitrile exposure to ethanol, which is nothing more than grain or ethyl alcohol? Besides being intoxicating alcohol that dates back centuries, ethanol has a long history of being the fuel for heat and light. Its use in automotive combustion engines is nothing new. The original 1908 Model T Ford could be modified to run on either gasoline or ethanol because Henry Ford was a supporter of homegrown renewable fuels.

  • Ethanol was used to fuel cars well into the 1920s and 1930s as several efforts were made to sustain a U.S. ethanol program. Standard Oil marketed a 25-percent ethanol by volume gasoline in the 1920s in the Baltimore area. Ford and others continued to promote the use of ethanol, and by 1938 an alcohol plant in Atchison, Kansas, was producing 18 million gallons of ethanol a year, supplying more than 2,000 service stations in the Midwest.4 By the 1940s, however, efforts to sustain the U.S. ethanol program had failed. After World War II, there was little interest in the use of agricultural crops to produce liquid fuels. Fuels from petroleum and natural gas became available in large quantities at low cost, eliminating the economic incentives for production of liquid fuels from crops. Federal officials quickly lost interest in alcohol fuel production, and many of the wartime distilleries were dismantled. Others were converted to beverage alcohol plants. “Outlook for Biomass Ethanol Production and Demand” (PDF) by Joseph DiPardo. United States Department of Energy

Ethanol used over a century ago apparently posed no particular problem to the early engines all of which used nitrile gaskets and o-rings. Otherwise, there would’ve been a wide search for a better replacement material. The gasoline sold in our area has 10% ethanol. We placed nitrile o-rings in 40% ethanol and have seen no measureable difference in the ring over a week’s time.

Heat Tolerance

What about the need for increased heat tolerance in carburetor o-rings? The fuel of a spark-ignition engine is mixed with air within its flammable limits and heated above its flash point before ignition by the spark plug. A chemical’s flash point is the lowest temperature where fluid evaporates to form a combustible concentration of gas. The flash point of ethanol or ethyl alcohol is 55 F and gasoline or petrol is –45 F. Nitrile’s temperature ranges as low as -70 F and up to 275 F. Viton®’s range is to –40 F to 450 F. I found data that stated 75 Duro Viton® only ranges down to –15 F which is far from gasoline’s flashpoint of –45F.

If higher heat tolerance is due to concern about the effects of engine overheating, safeguarding your carburetor o-rings should be the least of your worries. How about a cracked or blown head gasket allowing seepage of coolant into the cylinders and warping the heads? A burst radiator and hoses? A burned piston or engine seizure?  And worst of all, your engine catching fire if a carburetor happens to be flooded at that time.

Should your engine ever catch fire, keep in mind that Viton® puts off hydrogen fluoride gas when it burns which turns into hydrofluoric acid (HF) in the presence of water. Breathing in hydrogen fluoride can cause severe lung damage. When approaching a toasted engine, always use adequate protection against handling HF because of the possibility of Viton® being used in other automotive engine gaskets and seals.

Elastomer compression set is the measurement of how fast it returns to its original shape when compression is removed. 100% compression is bad. The lower the number, the less likely the elastomer will “set” and lose sealing pressure under compression. The compression set of fluorocarbons (Viton®) is 11%, which is greater than Nitrile at 4% but both are considered to be in the very low range when you compare against 100%.

Permeation is the rate a gas or liquid can be transmitted through the elastomer. Nitrile has a rating of 8 while fluorocarbon is rated 12.7, which is not a significant difference and not of any consequence by the use of either material.

Conclusion

Viton® is an excellent product in situations requiring resistance to a broader range of chemicals that apparently are not found in every day gasoline (petrol). Higher heat tolerances are necessary in turbine and jet engines but not the liquid-cooled combustion engine of the Gold Wing. Customers who bought our carb kits over the past decade know this is a non-issue.

How do you know you truly have Viton® o-rings instead of Nitrile? Both Viton® and Nitrile have a poor resistance to esters, ethers, ketones, amines and brake fluid but Viton® excels in its resistance to halogenated solvents most of which are deadly. One easy to acquire and common such solvent is chloroform.

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Posted

Thanks Doug.  I bit the bullet and bought a Rotax rebuild kit.

The smoking gun problem was on the pilot side carb.  It looked like the idle air needle was removed and when it was the o-ring stayed in the carb and when the needle was put back in it mashed the o-ring up into the orifice and plugged everything up.  Other carbs I've worked on don't have O-rings on the idle air needle.  As I got looking at the other O-rings I found that they would easily come apart if you dragged your fingernail over them.  I probably could have just cleaned out the idle circuit on that one carb and replaced the O-ring but it seems like a good time for a rebuild.

These carbs are surprisingly simple and it will give me peace of mind that they'll be rebuilt.

I'm really close.  I just had the condition inspection done and it only took me all of 5 mins to go through the squawk list he came up with.

Pretty soon I can start shopping for some amphib floats.

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Posted

Last question.

The throttle butterfly has the bolts peened over and I had to file them down to remove them so I could replace the o-ring on the throttle shaft. 

I consider the method of peening over the throttle butterfly bolts to be somewhat archaic.

On the new bolts would it be acceptable to use Loctite instead?  I've always used Loctite on other carbs I've done without issue.

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Posted

This would probably be okay to use a fuel proof loctite.  

I would have highly counseled against disassembling the throttle plate unless internal damage was sure.

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Posted

I don't like to disassemble them if I don't have to but I had to get to the o-ring in there.  I'm glad I did because the o-ring just crumbled off the shaft on both carbs.

It's not difficult to remove the plate and shaft.  The problem I always see is when people try to remove the peened over bolts without filing them down.  Then the threads get stripped and the shaft has to get replaced.

I installed the new bolts with the kit but didn't peen them over.  I just tightened them and applied the green penetrating loktite to both sides.  If I ever need to remove the bolts in the future I can just heat them up and remove them.  For me that stuff holds as good as welding.

I've rebuilt carbs in the past this way with no issues but this was my first set of aircraft carbs and I didn't know if this method was acceptable.

Anyway, the carbs are done and they came out perfect.  Now to reinstall them.

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Posted

That's good to know, Greg.  I've always been leery to disassemble the throttle plates...

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Posted

I've never seen or heard of a medium-high strength Loctite bond failing when used within the manufacturer spec recommendations and proper application instructions. These carbs will never reach the product's high limit temps so I think it's fine. And why wouldn't it be, it's exactly what the product is designed to do.

Greg, good to see you doing a thorough inspection and addressing these things. Folks here don't know this sale came about pretty darn quick with items still on the punch list, it is a huge relief to me knowing it landed with someone up to the task. You're going to have a great plane once you put your personal stamp on it!

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Posted

Doug, none of these items should go against your talent as a builder.  These are just maintenance items.  I went into it knowing there were still some little things to be done and that you weren't done completing them.  I'm really happy with the plane and everyone at the airport is very impressed with the build on it and believe me some of these old farts love to pick apart builds.

That being said I got the carbs on it today, set the idle, and balanced them out.  This thing purrs like a kitten now.  Night and day difference.  It runs up and idles super smooth now.

I like doing stuff like this because now I understand it.  Now some of the rotax guys at the airport that were contemplating sending their carbs our for rebuild want me to do it...

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Posted

I like doing stuff like this because now I understand it.

You made the same comment to me when we met and it didn't go unnoticed. Same reason I bought an E-AB kit when I decided to get back into flying. I was on the fence about selling the plane before ticking everything off my list but that same need-to-know hands on attitude is the reason it's in your hangar now. It was important to me was who and how the next owner responded when faced with a potentially new learning curve. Not trying to get all sappy here but my concerns are long gone and I am glad I said yes and the plane stayed close by. If you find something that doesn't look or seem right, bring it up for discussion. Particularly if it is a safety related issue. There are so many ways to skin a cat in the E-AB world and everyone here is always looking to learn, including me. I like validation but can take criticism for the good of the class just as well. Mostly curious if your action items mirror my punch list. Two sets of eyes are always better than one and I think we share the same attitude to get it right without cutting corners. It sounds like you are almost there, is some fall flying in the cards this year?

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Posted

 

It sounds like you are almost there, is some fall flying in the cards this year?

Well I'm itching to see if it will fly but I've still got some things to do before I'm comfortable.  I want my instructor to be comfortable with it too. 

After running it up yesterday I found fuel leaking from the carb vent.  I pulled the float bowl and found that one of the brand new floats I just put in wasn't floating and the valve was being held open.  What kind of crap is this where they charge you a fortune for these foam pieces of garbage they call floats?  I'm going to put the old ones back in because they seemed to be working fine.  I put the new ones in a container of fuel and found that they all float at different levels.  One was obviously bad.  Now I'm wishing I just got the basic o-ring kit.  You would think they would have better quality control over the manufacture of the new floats.

It's definitely a different mentality when you're working on a plane.  I find myself double and triple checking everything I've done.  When wrenching on cars I'm used to having tools laying everywhere and basic chaos whereas now I have to treat the work as if it is surgery.  When I'm done using one tool it gets put back where it belongs and so on.  God forbid I leave a screwdriver sitting on top of the manifold when I'm running up the engine.

I've also found I like to use torque mark paint on just about any bolt I can find.  It gives me a real quick visual during a preflight to see if anything is coming loose.  If the orange dot doesn't look right.  Something is wrong.  Easy.

Its kind of fun to maintain these.  I remember thinking when I was flying the 172 that I had never even had the cowl off this thing to look a the engine.  I was trusting that some mechanic knew what he was doing and didn't forget to tighten something or ad a hose clamp.  I don't know about you but I've personally run into more than one idiot mechanic in my life that I wouldn't even let tie my shoes let along maintain a complex piece of machinery.  There is an added layer of comfort when you understand the process and have personally done the work.

Getting close.

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Posted

Contact who you got the Kit from and tell them that the floats are junk. 

This is a known problem and you should be able to get your money back. 

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Posted

I just called and they were very helpful.  They are sending me a new set of floats at no charge.

Anyone want to buy a slightly used set for a good deal?  Only used once. ^_^

Pretty damn scary that I could have just risked an engine out or an inflight fire due to crappy quality control.

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Posted

Like Larry said, the new floats have been hit or miss on sooo many occasions. You wouldn't be the first that put their old floats back in. Better QC at Rotax for aircraft parts? YA THINK! Needless to say, o-rings only no floats with my 912uls carb rebuild. There are a fair number of folks on the Rotax-Owners.com forum boards that subscribe to the 'if it ain't broke, don't fix it' philosophy. They argue in the case of certain Rotax maintenance 'requirements', you may actually do more harm mucking around with stuff that has proven longevity in excess of the manufacturer's published maintenance schedule.

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Posted

Amen to that, Doug. 

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