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FlyingMonkey

Carbs Rebuilt

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Hey all, just wanted to share my carb rebuild experience.  I did the work myself (ELSA airplane, I hold the repairman cert for this airplane), after a ton of reading and viewing countless YouTube videos and the three rebuild videos from rotax-owner.com (these seemed the best and what I used as I stepped through the rebuild).  I bought the rebuild kit from CPS/Aircraft Spruce.

Overall I'd say the rebuild procedure is "easy".  There are a couple of small gotchas, but Roger had warned me about them previously and the rotax-owner videos mention them too.   If you can turn a screwdriver you can rebuild a Bing carb.  Disconnecting the carbs from the engine is almost as much work as rebuilding them.  There is nothing intimidating about this work.  It took me about three hours to do the rebuilds, working slowing and double-checking everything.  I worked in parallel, doing each step on both carbs which I kept on separate work mats to avoid mixing parts or other errors.  IIRC Roger said it takes him less than an hour.  

My airplane has 604 hours, and I don't know if these carbs have ever been rebuilt...they certainly have not been since I bought the airplane four years and 490hrs ago.  I have been pushing this off, because the engine has been running great and I tend to think "if it ain't broke don't fix it".  I have periodically inspected the carbs and taken to top cover off to look at the diaphragm. 

Given the long service life before the rebuild, I'd consider the condition of the parts in the carb to be outstanding.  The diaphragm looked new, the jets looked new.  There was a very slight wear ring around the conical viton valve seat, but nothing major.  The needle looked new.  One of my float bowls (the one in the picture is new, it was the other one) had some surface corrosion inside, which surprised me since I've never seen a drop of water in my fuel.  I scrubbed the corrosion out with a Scotchbrite pad and rubbed some Corrosion-X into the metal.  There was also surface corrosion on the throttle and choke arms, just from humid Georgia summers I guess.  I did the same scrub and Corrosion-X treatment on them.  Honestly this carb could have probably soldiered on for another 300 hours with no issues, it really did look that good inside.  But I replaced the parts that were in the kit and threw the old parts in tupperware as emergency spares.

Here's one of the carbs apart on my work table:

BEFtzf6.jpg

And here are the reassembled cards, ready to install:

nOJ4dLD.jpg

 

I will reinstall the carbs today, but I will have to rebalance and test run before knowing if everything is working properly.  I am finishing up my annual, so that will happen after all that work is done.  I'll let ya'll know if there are any issues, but I expect everything will be working fine.

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Andy, I know the use of rebuilding is a common term, but even though your airplane is ELSA and you have a repairman certificate. You can not legally rebuild the carbs. As an A&P mechanic with Inspection Authorization, I do annual recurrent training. One of the things that has been drilled into my mind over the years is that making a logbook entry as a mechanic, stating that I have rebuilt something can lead to a violation. The proper term would be overhauled or inspected and repaired. Here is the pertinent regulation.

§43.3   Persons authorized to perform maintenance, preventive maintenance, rebuilding, and alterations.

(j) A manufacturer may—

(1) Rebuild or alter any aircraft, aircraft engine, propeller, or appliance manufactured by him under a type or production certificate;

I agree that the inspection and repair does not require any supernatural skill set, and is very straight forward.

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25 minutes ago, Tom Baker said:

Andy, I know the use of rebuilding is a common term, but even though your airplane is ELSA and you have a repairman certificate. You can not legally rebuild the carbs. As an A&P mechanic with Inspection Authorization, I do annual recurrent training. One of the things that has been drilled into my mind over the years is that making a logbook entry as a mechanic, stating that I have rebuilt something can lead to a violation. The proper term would be overhauled or inspected and repaired. Here is the pertinent regulation.

§43.3   Persons authorized to perform maintenance, preventive maintenance, rebuilding, and alterations.

(j) A manufacturer may—

(1) Rebuild or alter any aircraft, aircraft engine, propeller, or appliance manufactured by him under a type or production certificate;

I agree that the inspection and repair does not require any supernatural skill set, and is very straight forward.

I disagree.  My aircraft category is experimental, not light sport.  That's why I was required to remove the Light Sport decals from it and replace them with Experimental placards.  Anybody can perform ANY work on an experimental aircraft, with no ratings whatsoever.  A repairman certificate is required ONLY for the annual condition inspection.  Any other work, other than certification tasks like pitot/static checks, can be performed by anybody.  My wife can legally rebuild the carbs on an experimental airplane.  This airplane is no longer bound by any factory guidance except safety alerts.  

This was gone over in detail in my LSRM course, and the DAR that taught it was adamant that any person can perform any work on any aircraft in the experimental category, except for certification tasks as mentioned and the condition inspection, which requires the repairman certificate. "Major Alterations" like engine or prop changes, might require notifying the FAA and a period of flight testing similar to EA-B phase I testing.    

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30 minutes ago, Tom Baker said:

Andy, I know the use of rebuilding is a common term, but even though your airplane is ELSA and you have a repairman certificate. You can not legally rebuild the carbs. As an A&P mechanic with Inspection Authorization, I do annual recurrent training. One of the things that has been drilled into my mind over the years is that making a logbook entry as a mechanic, stating that I have rebuilt something can lead to a violation. The proper term would be overhauled or inspected and repaired. Here is the pertinent regulation.

§43.3   Persons authorized to perform maintenance, preventive maintenance, rebuilding, and alterations.

(j) A manufacturer may—

(1) Rebuild or alter any aircraft, aircraft engine, propeller, or appliance manufactured by him under a type or production certificate;

I agree that the inspection and repair does not require any supernatural skill set, and is very straight forward.

(c) The holder of a repairman certificate may perform maintenance, preventive maintenance, and alterations as provided in part 65 of this chapter.

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Andy, I think the point I was trying to make didn't come across correctly. Since part 43 does not apply to your airplane using it as a reference was probably not the best choice. You were perfectly within your rights to legally perform the work. My issue is how you described the work. You performed maintenance, inspection,  repair, and maybe even overhaul not rebuilding. The level of work you performed does not meet the FAA's definition of rebuilding. There is a good article in here describing the differences.

https://www.aviationsuppliers.org/ASA/files/ccLibraryFiles/Filename/000000000194/10-1.pdf

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1 hour ago, Tom Baker said:

Andy, I think the point I was trying to make didn't come across correctly. Since part 43 does not apply to your airplane using it as a reference was probably not the best choice. You were perfectly within your rights to legally perform the work. My issue is how you described the work. You performed maintenance, inspection,  repair, and maybe even overhaul not rebuilding. The level of work you performed does not meet the FAA's definition of rebuilding. There is a good article in here describing the differences.

https://www.aviationsuppliers.org/ASA/files/ccLibraryFiles/Filename/000000000194/10-1.pdf

Ah, fair enough.  Well, I did the periodic maintenance on the carb beyond just inspection.  Call it what you will!  :)

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Tom is right, regardless of the aircraft classification, the terms rebuilt and overhauled are reserved for very specific circumstances.

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My local AI buddy told me on LSA airplanes there are no annuals they are condition inspections. Is this true?? He also said one the book is signed off the inspecting A&P is not responsible for anything after the inspection. Is this true??

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Both true, but with a couple hitches.

First, they are called annual condition inspections. It's a minced-word game the FAA plays. They achieve the same thing though, it's a yearly inspection.

As for the A&P being responsible: he obviously hasn't gone to court yet.

  • Anything that happens after the inspection isn't his responsibility. Why would it be? You pay him to check the condition of the aircraft at that snapshot in time.
  • The hitch here is whether or not it was something that really did happen AFTER the inspection, or if it was something they missed. This is the "gotcha". It's incredibly hard to prove the condition didn't exist during the inspection, but it's not hard to convince people that the inspector was somehow negligent.

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Andy, did you remove the (I forget the name) big alloy screw with the wide slot?  I ground a screwdriver to the correct width and then heated the carb body a bit to soften the loctite. Works well.

One thing I didn't do was remove the two screws that hold the butterfly to the cross shaft so that the shaft can be removed (and the o-rings that seal the shaft in the carb body replaced).  As you know, the screws are peened over to prevent them from loosening (and being sucked into the combustion chamber).  Not so easy to remove.  The replacement screws should also be peened, once they are installed.  I suppose it would be possible to build a jig for this procedure so that the shaft didn't bend while peening the screws.  Anybody else do this particular job when, um, refreshing the carburetors?

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Fred, I did exactly what you did, ground a screwdriver and heated the body to loosen the loctite as suggested by Roger Lee.  Worked great.  I didn't do the butterfly either, I inspected the o-ring from the outside and it looked perfect, so the risk didn't seem worth the reward.

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I overhaul lots of carbs.

The screw down inside the carb piston that holds the needle in place is called the fixation screw. Just hold the piston in a thick rag and apply the heat down inside and it does loosen the Loctite. Failure to do this and just use muscle can and will many times twist the screw head off and you have no idea the PITA it is to get out without ruining the piston.

I have found many butterfly screws not to be peened over very much and in fact add a touch of heat to the bar they will just unscrew right out. When you put new ones in the peening process is very easy. I have a 6" piece of 1 3/8" round aluminum rod that I place straight up in a vice. That's a perfect fit  to slide the carb down on top of and the screw heads and bar rest right on top of the flat aluminum top for support. Then take a 6" flat headed drift punch and a hammer and place the punch on top of the screw ends and peen them over some. You don't need to get carried away peening them over. That said mine will not unscrew unless they are ground off on those ends.

You can use a file to file the screw ends down when removing them, but that takes forever. I just use a Dremel tool with either a sanding drum  or a grinding head. Takes them down flush in just a couple of seconds.

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33 minutes ago, Roger Lee said:

I overhaul lots of carbs.

The screw down inside the carb piston that holds the needle in place is called the fixation screw. Just hold the piston in a thick rag and apply the heat down inside and it does loosen the Loctite. Failure to do this and just use muscle can and will many times twist the screw head off and you have no idea the PITA it is to get out without ruining the piston.

Over the years I have had a few twisted off. I have never ruined a piston getting them out after twisting the heads off. It just takes a little longer and involves the use of power tools. The biggest issue for me is it being an inconvenience because of having to wait 2 days to get replacement screws. 

I too now have a special screwdriver bit for screw removal.

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I too have a dedicated screwdriver for the carb fixation screw. It's a huge fat tip that fits snugly in the screw head. Spreads out the load a lot and makes it much easier to remove without damaging.

I had a piston that I ruined trying to remove one before I had this screwdriver. It was not coming out, even when I used heat. The other one was stuck too, but after making that screwdriver it came out no problem.

I should have sent out the piston to an EDM company, probably would have been cheaper.

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If you heat the fixation screw barrel up they all come out easy. You just need a plain straight tip screwdriver. I have never in all these years had any issues with removal if you heat it. I used to use a heat gun all the time, but I switched many years ago to a small butane torch. It works faster, but more important I can direct my heat in a more specific area and not heat up everything like a heat gun does.

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Did all that.

I checked it with a thermometer, it was plenty warm. Yes, I might have made it too hot, except i did this in stages, getting progressively warmer and trying each time. It absolutely would not come out.

The screwdriver that I ground the tip down on fits the screw snugly, and I can put a LOT more force on it without damaging the screw.

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When I say heat it I mean hot. Too hot to touch. This is why I say use a thick rag. This is also why I like my little butane torch. I can direct the heat right on that barrel where the fixation screw sits and getting it that hot expands the metal of the barrel. So this loosens it in two ways.

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If you do strip one out a #3 Craftsman screw extractor does a wonderful job of removing the stripped fixation screw. It is actually easier to remove after it is stripped, but it is not much use afterwards.

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Tried extractors. it tore it up even worse. After i drilled it out (and went too far and damaged the piston), i used a screwdriver to pick off the remains of the thread, and even then they were really difficult to remove. There was a gritty compound that I am not sure if it was corrosion, but it was pretty bad.

I feel like nobody had inspected that thing before.

Now that I keep on top of it, they are quite easy to remove. And I only use a couple drops of 243, it holds them but not really hard.

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Roger, thanks for the information about peening the butterfly screws.  Makes sense to me.

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I used a Harbor Freight heat gun on the low (600F) setting to heat the piston, it took about 20 seconds to get hot enough, it was too hot to touch within ten seconds.

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