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  #1  
Old 04-19-2021, 10:56 PM
Art_N412SB's Avatar
Art_N412SB Art_N412SB is offline
 
Join Date: Sep 2017
Location: Gig Harbor
Posts: 38
Default When good Carburetors go bad.

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 dont 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 wifes 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.
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Pacific Northwest
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  #2  
Old 04-22-2021, 10:36 AM
PilotBrent PilotBrent is offline
 
Join Date: Dec 2011
Location: Hackettstown, NJ
Posts: 469
Default 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.
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  #3  
Old 04-22-2021, 03:18 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Default

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.
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  #4  
Old 04-22-2021, 03:45 PM
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DanH DanH is online now
 
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Location: 08A
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Quote:
Originally Posted by rvbuilder2002 View Post
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.
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  #5  
Old 04-22-2021, 03:50 PM
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Piper J3 Piper J3 is offline
 
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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.
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  #6  
Old 04-22-2021, 09:45 PM
RFSchaller RFSchaller is offline
 
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Im a big proponent of if it aint broke dont 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 dont 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. Its a great plane.
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  #7  
Old 04-23-2021, 07:48 AM
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Walt Walt is online now
 
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Quote:
Originally Posted by RFSchaller View Post
Im a big proponent of if it aint broke dont 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".
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  #8  
Old 04-23-2021, 08:23 AM
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cactusman cactusman is offline
 
Join Date: Oct 2012
Posts: 467
Default

Quote:
Originally Posted by rvbuilder2002 View Post
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.
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  #9  
Old 04-23-2021, 09:36 AM
seagull seagull is offline
 
Join Date: Dec 2019
Location: San Bernardino
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Default

Quote:
Originally Posted by cactusman View Post
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..................
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  #10  
Old 04-23-2021, 09:46 AM
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cactusman cactusman is offline
 
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Smile

Quote:
Originally Posted by seagull View Post
Shoot, I did not do my V-belt replacement.
still looking for the v-belt..................


Mee tooooo! lol
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