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When good Carburetors go bad.

Art_N412SB

Member
Those of us with E-LSA RV-12's sometimes do our maintenance "on condition". I would like to share with you some of the symptoms that we experienced that lead us to believe it was time get our carburetors overhauled.

The following is a lessons learned email that I sent to my co-owners.

Hopefully there may be something in here that may be a benefit to you.


Dear friends,
Jeff and I flew our aircraft yesterday post carburetor rebuild. Lockwood Aviation did the overhaul for just under $600 for both carburetors. I have to tell you that the difference is night and day.
Prior to the overhaul we were noticing some symptoms. For the last year we had some vibration between 2800 and 3800 RPM. Most noticeable when descending and slowing down. Starting was becoming an issue as well.
When I first started flying our aircraft starting cold with full choke was good. Soon after start the choke would no longer be needed. I feel the choke is not the right name for the control. The air is not choked off or restricted in any way. Rather the fuel air mixture is enriched for starting cold. I would call it a fuel enrichment control.
In the last year or so I noticed that the choke started to have a sweet spot. In other words pulling the choke control full out the engine would bang around and fire up really rough. I started using half choke for starting. Then latter yet I would using one quarter choke. Full choke was actually flooding the engine.
Another symptom was that the first mag check would be 320 RPM drop, but the second mag check would be OK at 100 to 120 mag drop. The bad drop was mostly on system A. I think the plugs were getting carboned up during taxi and needed to be burned off before we got a good mag check. By the way the RV-12 POH refers to this check as a “Mag” check. So I don’t feel bad about calling it a mag check. I know it is a ignition system check.
Then as things got really bad the engine was hesitating while advancing above 2500 RPM through 3800 rpm and being quite rough. This was when we decided to get the carbs overhauled.
When I talked with Joe at Lockwood Aviation, he said it was the idle jet getting gummed up. Also we had some heavy floats. Keith and I had weighed the floats prior to shipping them out, but we did that with my wife’s kitchen scales. The scales were not really calibrated. So we got to get better scales before next time. But, even if they were not overweight we would still have to change them out for new ones anyway. You see they were Bing manufactured floats and we needed the newer Rotax manufactured floats. You can check this by the Rotax floats havening an “R” stamped on the floats. The Bing Carburetors have a “B”.
So all that combined, we were actually flooding the engine. Prior to sending out the carbs we pulled the spark plugs and found that all 4 plugs on the front two cylinders were very black. The four plugs on the two aft cylinders were clean. Joe at Lockwood explained that since the carburetors are located on the back end of the engine the shape and distance of the intake manifold actually causes a different fuel air ratio to the front two cylinders. I suppose that when the idle jet gets gummed up and the floats get heavy the mixture becomes richer for all cylinders but very rich for the front two cylinders.
All of these symptoms have disappeared. The engine is running great. I have less than 150 hours flying the Rotax 912. It has a different feel from the Continental and Lycoming engines that I have flown in the past. Those engines have a smother feel at Idle RPM. The Rotax feels like it bangs around a little more than the other engines. I think this is due to the smaller higher compression cylinders. Not bad, just different.

I hope you found something of use here.
 
200 hour rebuild

Rotax manuals recommend overhauling the carbs every 200 hours which I understand not everyone adheres to in E-LSA world. After taking the Rotax courses at Lockwood I realized how many small parts could wear/fail in those Bing carbs and have adhered to the 200 limit from day one. Yes, $600 every couple of years seems like a lot, but stories like this reinforce the value of preventative maintenance.
 
A clarification.......

There is no requirement for a carb. "overhaul" every 200 hrs.

There is a prescribed "Inspection"
It is basically an inspect and replace any parts that do not meet the wear or test requirements.
It is very specific and is covered in the Heavy Maintenance Manual.

If all of the parts check out, it is simply a disassembly, inspect, and reassembly. I highly recommend that RV-12 owners follow the recommended inspection interval.

There are some parts that are more prone to wear than others (float valve arms is one that comes to mind), but part replacement is not a sure thing every time they are disassembled.
I highly recommend maintaining proper carb synch and dynamic prop balance as a measure to reduce some of the wear that can occur on internal carb. components.
 
A clarification.......

There is no requirement for a carb. "overhaul" every 200 hrs.

There is a prescribed "Inspection" It is basically an inspect and replace any parts that do not meet the wear or test requirements. It is very specific and is covered in the Heavy Maintenance Manual. If all of the parts check out, it is simply a disassembly, inspect, and reassembly. I highly recommend that RV-12 owners follow the recommended inspection interval.

If I may add an opinion....

The Bing CV is a very simple carb, and it's been around a long time....like since the 70's on BMW bikes. Sure, everyone should know their limitations, but gosh, a superpower is not required to do inspections or parts replacements.
 
What Scott says in above post is correct. Vibration on 9-series engine is rough on carbs particularly because carbs are mounted at extremes from engine centerline. Out by cylinder heads things really jump around.
 
I’m a big proponent of “if it ain’t broke don’t fix it”. I had my carbs overhauled at 500 hours and they swapped my choke assemblies. I had to disassemble and reassemble the carbs. Now at 1100 hours still going strong. I don’t intend to do anything more than bowl inspections unless I notice an issue.

Honestly I read about so many issues folks are having and I look at my little 12 that just runs with no issues. It’s a great plane.
 
I’m a big proponent of “if it ain’t broke don’t fix it”.

I know there are lots of proponents of this philosophy (mostly based on I don't want to spend the money), however, aviation safety/reliability is based on required inspections and repair being performed BEFORE failure occurs based on service history.

I think when you venture outside the normally accepted guidelines for inspections those choices should made very carefully, and be based on fact/solid component history data, rather than on "it ain't broke".

If you carry passengers you have a serious responsibility to them, or maybe you should let them know that you don't believe in accepted aircraft maintenance procedures and prefer to make your own rules, but last time you flew the aircraft "nothing was broken so we should be ok".
 
A clarification.......

There is no requirement for a carb. "overhaul" every 200 hrs.

There is a prescribed "Inspection"
It is basically an inspect and replace any parts that do not meet the wear or test requirements.
It is very specific and is covered in the Heavy Maintenance Manual.

If all of the parts check out, it is simply a disassembly, inspect, and reassembly. I highly recommend that RV-12 owners follow the recommended inspection interval.

There are some parts that are more prone to wear than others (float valve arms is one that comes to mind), but part replacement is not a sure thing every time they are disassembled.
I highly recommend maintaining proper carb synch and dynamic prop balance as a measure to reduce some of the wear that can occur on internal carb. components.


The following components and systems must be replaced every 5 years:

• Venting hose of the carburetors
• Diaphragm on both carburetors
• Carburetor sockets
• All rubber hoses of the cooling system
• All rubber hoses of the fuel system
See SI-912-022, latest issue.
• All rubber hoses of the lubrication system which are part of the engine supply volume and if they are not in the maintenance schedule of aircraft manufacturer
• Connecting hose of the air intake system
• Venting hose of the fuel pump
• V-belt

https://www.rotax-owner.com/pdf/MML_912_Series_ED4.pdf


Keep in mind the recommended 'overhaul' of these carbs is 200/5 yrs from the more experienced folks out there who deal with 912 engines for a living. The wide variance in gas quality I am sure has something to do with it.

Mine only had 85 hrs. Night and day. My floats were R and weighed fine before i sent them off.
 
• V-belt

Shoot, I did not do my V-belt replacement.

I have 680 hrs / 6 years on mine. Starts easy, runs like a top. No carb overhaul, only Marvel Schebler floats, bowl gasket, and inspect. Coolant hoses were replaced with silicone last year along with carb sockets to the JBM ones. My oil and gas are Teflon.

still looking for the v-belt..................
 
I know there are lots of proponents of this philosophy (mostly based on I don't want to spend the money), however, aviation safety/reliability is based on required inspections and repair being performed BEFORE failure occurs based on service history.

I think when you venture outside the normally accepted guidelines for inspections those choices should made very carefully, and be based on fact/solid component history data, rather than on "it ain't broke".

If you carry passengers you have a serious responsibility to them, or maybe you should let them know that you don't believe in accepted aircraft maintenance procedures and prefer to make your own rules, but last time you flew the aircraft "nothing was broken so we should be ok".

Our planes are required to placard the word EXPERIMENTAL where the passengers can see it. That means that it was built by someone, not necessarily factory trained and inspected by someone like you that did not completely dis-assemble it to make sure everything was done correctly.

There are some things that have to be done on the plane (by law) for it to be airworthy, the rest is at pilot discretion. If you do not understand machines you should have a mechanic do your work as prescribed by the mfg. The Mfg set time replacements many times based a on litigious concerns and worst conditions.

Case in point, the -12 nose gear, one fails under extreme conditions, they all need replacement. It is your call, if you don't have students po-go sticking your plane for 1700+ hrs you may decide not to replace it. Should you mention to your passenger that there is a SB on the nose gear but you didn't do it?

Case in point, the mfg recommends changing oil at 100 hrs and plugs at 200 hrs. I do my oil at 50 hrs and plugs at 100, my choice. I do not use 100LL I just feel it is better.

Again, if you don't understand the machine and can't make logical decisions on how the machine has been used and its needs, stick to what the manufacture recommends.
 
The following components and systems must be replaced every 5 years:

• Venting hose of the carburetors
• Diaphragm on both carburetors
• Carburetor sockets
• All rubber hoses of the cooling system
• All rubber hoses of the fuel system
See SI-912-022, latest issue.
• All rubber hoses of the lubrication system which are part of the engine supply volume and if they are not in the maintenance schedule of aircraft manufacturer
• Connecting hose of the air intake system
• Venting hose of the fuel pump
• V-belt

https://www.rotax-owner.com/pdf/MML_912_Series_ED4.pdf


Keep in mind the recommended 'overhaul' of these carbs is 200/5 yrs from the more experienced folks out there who deal with 912 engines for a living. The wide variance in gas quality I am sure has something to do with it.

Mine only had 85 hrs. Night and day. My floats were R and weighed fine before i sent them off.

5 year Rubber component replacement has no connection with the inspection interval of the carbs. Some RV-12 have accumulated 200+ hrs per year, so they end up with 3-4 carb inspections by the time they get to a mandatory parts replacement.

I am very familiar with the Rotax Heavy Maint. doc you linked to. Already mention, the checklist in Section 5-20-00 specifies that an inspection be done every 200 hrs. The inspection is detailed in Section 73-00-00. It walks you though all of the steps and specifies replacing parts that don't meet spec.
In my mind an overhaul of the carb would be automatically replacing all minor parts regardless of condition. This is not what is specified, and it is not necessary, as long as proper inspections are being done on the required intervals (and then of course replacing the rubber parts on the 5 year interval, which in my opinion is not reasonable but it is what it is. That standard puts all aircraft in the same time frame requirement. It is not reasonable to lump an aircraft with an un-cowled engine, that sits outside with potential sun exposure 365 day per year, with an aircraft that is fully cowled and is stored in a hangar all year, but I digress)
 

They have an engine overhaul kit as well, but when we do the engine inspection on each 200 hr interval, we don't disassemble the entire engine and automatically replace all of the parts because it hit 200 hrs :rolleyes:

Some times it does make sense to do a complete overhaul of the carburetors.

One of those times is when you are doing a compete overhaul of the engine. ;)
 
They have an engine overhaul kit as well, but when we do the engine inspection on each 200 hr interval, we don't disassemble the entire engine and automatically replace all of the parts because it hit 200 hrs :rolleyes:

Some times it does make sense to do a complete overhaul of the carburetors.

One of those times is when you are doing a compete overhaul of the engine. ;)

Well I think we all know the later engine is good to 2000 hrs, and then we just ship it Lockwood for an estimate....or a new IS 915 engine maybe....

Since Van's demonstrators/trainers all run on 100LL 100% of the time, I'm not sure you have the same exposure and experience with autogas issues gumming these carbs up like Lockwood and others have. Van himself brought an expert in after some frustration as I recall (RVATOR article he wrote) and the 912 expert found some small amounts of grit inside his main jet.

Just saying that these carbs require (as the OP nobly posted) some extra attention at times. My 2 cents.

PS - 3rd set of floats installed...Marvels on standby.
 
Well I think we all know the later engine is good to 2000 hrs, and then we just ship it Lockwood for an estimate....or a new IS 915 engine maybe....

Since Van's demonstrators/trainers all run on 100LL 100% of the time, I'm not sure you have the same exposure and experience with autogas issues gumming these carbs up like Lockwood and others have. Van himself brought an expert in after some frustration as I recall (RVATOR article he wrote) and the 912 expert found some small amounts of grit inside his main jet.

Just saying that these carbs require (as the OP nobly posted) some extra attention at times. My 2 cents.

PS - 3rd set of floats installed...Marvels on standby.

I don’t know what information you have that would make you say that John. We fly our 12iS demonstrator and our flying club RV 12 ULS exclusively on auto fuel (except when the are away from home base) and we always have.
As for Van’s experience, I’m very familiar with that one. It was immediately after he first started flying the airplane and a carb got contaminated by something that passed through from the system. This does happen on occasion because of contaminants that didn’t get flushed out of the system before the engine was put into service. I don’t think he’s had any problems since that issue was resolved.

By the way, using 100 low lead doesn’t assure no carburetor problems. It actually can cause a whole set of different issues because of the blue dye. That tends to accumulate in the car and after while can make the slides start to stick so the use of 100 low lead doesn’t guarantee less maintenance.
 
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I don’t know what information you have that would make you say that John. We fly our 12iS demonstrator and our flying club RV 12 ULS exclusively on auto fuel (except when the are away from home base) and we always have.
As for Van’s experience, I’m very familiar with that one. It was immediately after he first started flying the airplane and a carb got contaminated by something that passed through from the system. This does happen on occasion because of contaminants that didn’t get flushed out of the system before the engine was put into service. I don’t think he’s had any problems since that issue was resolved.

By the way, using 100 low lead doesn’t assure no carburetor problems. It actually can cause a whole set of different issues because of the blue dye. That tends to accumulate in the car and after while can make the slides start to stick so the use of 100 low lead doesn’t guarantee less maintenance.

Oh that was the comment Mike S told me when we gassed up using 100LL during a training session a long while back. He said they never use autogas because of logistics...but perhaps that was just his training operation or maybe things have changed. I dunno. My mistake.

I think that rubber diaphragm is very susceptible as well. Maybe temp extremes here have created more of an event too. Anything rubber or battery powered seems to have a 3 year life span in AZ. Can almost set your clock to it....in fact most locals do - get the 3 year battery warranty and take it back to Costco on 2 years and 364 days.
 
,,, I'm not sure you have the same exposure and experience with autogas issues gumming these carbs up like Lockwood and others have.

John, as previously noted, Bing CV's have been hanging on motorcycles about 50 years, maybe more. They've seen every kind of gas in every country in the world. Care and feeding is not much harder than a pet rock.
 
quote "Prior to sending out the carbs we pulled the spark plugs and found that all 4 plugs on the front two cylinders were very black. The four plugs on the two aft cylinders were clean. Joe at Lockwood explained that since the carburetors are located on the back end of the engine the shape and distance of the intake manifold actually causes a different fuel air ratio to the front two cylinders. I suppose that when the idle jet gets gummed up and the floats get heavy the mixture becomes richer for all cylinders but very rich for the front two cylinders."

This paragraph was in the first post of this thread. I have this same problem but have a hard time wrapping my head around the cause of the front plugs running so rich. My front plugs were black and the back plugs were so clean they don't even have any carbon on them. I don't see where 2" more intake manifold causes a richer mixture. Once the carbs have mixed the fuel/air that's it, that's the mixture that's in the manifold.

I just sold a C195 in Dec and it's hard to find anything with a more uneven intake system. You don't get one set of cylinders running richer than the others. i'm just an old A&P that's been working on cars, planes, motorcycles, tractors........so bear with me.

Ron
 
I would bet if you ran the engine at cruise (in the air under load with good airflow) then and shut it off, come in and land / pull the plugs you would see the plugs all a light tan, no front only rich plugs.

The engine does not get even cooling front to back at low RPM on the ground plus the intake doesn't flow to the front as well at the lower RPM's. You probably spent a few minutes at the low RPM on approach, taxing, parking right before pulling the plugs, that low RPM time is what dirtied up the front plugs.
 
I do have a long taxis to and from the runway so there is a lot of low RPM time.
BUT, the heads are water cooled so all the head temps should be the same. Cyl temps should be close since they are all under the plenum getting whatever cooling air there is.
Plus I still contend that the mixture is the same after the carbs. no matter how long the intake manifold is. Are we saying that #3 and #4 are sucking all the air out of the mixture and leaving #1 and #2 with mostly fuel? The carbs determine the fuel/air ratio and that's what is in the manifold.
You may be dead on but it still doesn't make sense to me.
Thanks for trying to educate me.
Ron
 
Thanks,. I just went to the web page and read some of the post about the same problem. Looks like it's common after long taxis.
I guess it's time to accept that explanation in my thick head. Thank you, everyone, for your help.
Ron
 
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