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Carburetor Issues

Over the last few months I have been noticing a small amount of dirty fuel/oil dripping out of the nose gear faring onto the nose wheel pant. Nothing really significant and I chalked it up to little leaks, dribbles and dirt on a 5 year old O-360 that I fly behind 300+ hours every year. Nothing else to note in terms of performance anomalies (fuel consumption, power, engine performance in general) except that I seemed to be having to diddle with mixture more and more in the last few months to get nice stable performance at altitude.

Yesterday after a 1 hour flight I pulled up to the hangar and was shutting down. Low and behold the mixture cutoff was fully pulled but it just would not shut down. Would slow way down and then burp and cough and rev up a little more and keep coughing. Seemed like it was definitely not "cutting off". Shut the dual PMAGs off and after a little dieseling it quit.

Pulled the cowl and noticed a lot of fuel sitting in the FAB. (Wow! Where was this fuel coming from? No external leaks anywhere around the carb.) Took off the airfilter thinking the fuel sitting in there was the cause of the continued running as it vaporized. Fired up without the FAB and everything seemed OK. Did a run up and all good. , But, then it started running rough at idle and I had to lean quite a bit for the engine to run smooth. I am at sea level and it was 75 degrees. Odd. Like it was way too rich for some new reason. Tried a shut down and similar story. Shutdown the dual PMAGs and finally got it to quit.

After shut down I noticed a fair amount of fuel dripping off the back of the FAB mounting plate. It was not obvious to me where it was coming from but I think it was internally coming out of the carb...like a float was stuck down and the residual pressure in the engine fuel pump was forcing fuel out...but the mixture was supposedly cutoff? This must have been what has been getting worse over time.

I bought my Lyc O-360 from Van's and it came with an Avstar 10-4164-1 carb. Compared to the carb that I had on my 1972 C172M the Avstar is built like a tank. But something is not right.

I have decided to replace the carb before contemplating any flying. Really get frustrated when I am AOG. I am suspicious of damaged floats and/or mixture valve but I am not sure. Carbs are one thing I have never gotten deep into understanding. Can anybody give me a more definitive answer based on what I have written here? Up until now this carb has 1,260+ trouble free hours on it.
 
Most likely suspect is a needle/seat problem. I bit of dirt or wear in the needle / seat interface causes the bowl to overfill. However, if memory serves me well, I believe the overflow hole would dump fuel outside the carb. You would also notice the engine to be very rich and rough running at full mixture (red knob fully in). A saturated float will exhibit the same symptoms. It would only run optimally when manually leaned.

Another possibility is loose screws that hold the top of the carb to the base. I would expect some external leakage as well, though.

finally, junk in the accelerator pump circuit. If there is debris in the check ball interface, I think it would allow internal fuel leakage. I would have to review the drawings again to determine if this is truly a possibility, but I think it is.

Last possibility is a crack in main body casting. It is a high vibration area and the casting is not the strongest.

Any way, the carb needs to come off for some diagnostics.

Larry
 
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Appreciate the inputs Larry. I have inspected the body very closely and don't see any signs of a crack. As I said the Avstar unit is built like a tank.

Do you have any preferences for either Avstar or Marvel-Schebler carbs based on your experience?
 
Larry mentioned this, but verify the carb bowl screws are tight. They can loosen and result in erratic behavior as you have described and fuel will leak out through the bowl/carb junction. Having the airbox mass supported by the carb can promote the screws loosening.
 
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Appreciate the inputs Larry. I have inspected the body very closely and don't see any signs of a crack. As I said the Avstar unit is built like a tank.

Do you have any preferences for either Avstar or Marvel-Schebler carbs based on your experience?

Sorry, have no experience with the avstar, but I wouldn't call teh MS carbs a high end, innovative design.

There are many walls in the internal guts of the carb casting. The fact that you found no external cracks does not eliminate the possibility of a casting crack causing your problem. It is high on the list for your symptoms (internal leakage). You will find the internal walls much thinner than the exterior walls and may not be "tank" like. In my experience, the carb area on a lycoming experiences a serious amount of vibration and the added leverage of the FAB intensifies the effect. Numerous folks see 063 2024 sheets crack at the FAB. That is not easy to do and takes a good amount of violent shaking.

Good like finding your issue.

Larry
 
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Thanks for the insight. As I said in my first post I am not deeply knowledgable of carbs. I am keenly aware of the plate cracks as I had them. I went thicker when I made a new plate and I added some L angle on the left side to increase stiffness just in case. Has worked perfectly since. Also, after rebuilding the carb heat hinge 3 times from destruction due to vibration I redesigned it with baffle material which to me is a much better design. I'll try to load some pics of my solutions in the next day or two (sitting on a commercial flight for a couple days away on biz.)

I ordered a rebuilt carb so I can get back in the air and then I plan to tear mine down and see what I learn and do a rebuild in the process. I'll let you know what I find.
 
Larry mentioned this, but verify the carb bowl screws are tight. They can loosen and result in erratic behavior as you have described and fuel will leak out through the bowl/carb junction. Having the airbox mass supported by the carb can promote the screws loosening.

Already checked this. All good. But I will check again to be sure. If this were loose I would think I would see leakage from around the bowl not from inside the throat which is where the leak appears to be coming from.
 
I had similiar carb prob where run was noticeably rich and would not shut down with mixture ctl. I found all four hex head screws which had tab locks in place but lose and leaking fuel at the carb top to bowl joint. Removed , repaired, rebuilt acc pump and seems to be back to normal now. Was a MA 4SPA
 
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Send to a professional

I recently sent my carb to D & G Supply http://www.dgsupply.com/ and had them rebuild it. Good people and easy to deal with. If a carb has been in service for 10 years most of the manufactures recommend a rebuild regardless of hours. Mine came back looking like a brand new carb. If I didn't take pictures of it before I sent it I would never believe it was the same one. Engine runs better than ever. It's the heart of the engine- don't mess with it.
 
Already checked this. All good. But I will check again to be sure. If this were loose I would think I would see leakage from around the bowl not from inside the throat which is where the leak appears to be coming from.

A loose top will leak fuel from the bowl into the throat, as well as leaking externally. The bowl and throat share a wall.

Larry
 
Echo Anthony

I have used D&G in the past and always found their support over the phone, or when I sent them a carb, very good. I believe I found some support documents on their website to help with troubleshooting in the past. Good luck with yours. It sounds like you are on the correct path.
 
Reading the OP it appears there is a change happening in the carb, from scanning the Service Bulletins for Marvel, Kelley, Avstar etc. I would guess a sinking float condition is likely.
Another possible condition may be happening, remote, but maybe... I had an Avstar carburetor give out on me on our way back from OSH one year. The engine exhibited similar lean idle cut out symptoms, some fuel in the FAB etc, but generally engine was running reasonably, till a simple mixture adjustment at 8000 ft nearly killed the engine. After that point it was everything we could do to keep the engine running, resulting in a forced landing. What was finally determined after the carb was changed was the Mixture Metering Sleeve in the bottom of the float bowl came loose, literally leaning to shutting off the fuel flow. The offending part is illustrated in the link below:

http://republicseabee.com/files/marvel schebler overhaul new.pdf
Figure 2-20. Carburetor Body and Bowl Assembly – Exploded View
23. Mixture metering sleeve

So definitely ground the aircraft till the issue is identified & fixed...
 
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Well folks, I was running into a 3-4 week lead time for a replacement Avstar unit so I opted to get a Marvel-Schebler OH'ed unit. I had plans in less time than that which required I get back in the air. I was expecting some challenges with the switch in vendor and found some configuration differences with the bowl on the MS carb. Suffice it to say that the Avstar unit is a much more clean lines design. The result was that I needed to fashion a new FAB top plate to account for the differences in the bowl base. Four hours later problem solved and now I have base plates for either carb.

I put the new carb on and everything runs perfect. Test flight and pulled the cowl for a full reinspection and all appears good after a second test flight. All my numbers are where I would expect. So I turned to the faulty Avstar.

First, for all of you worried about loose screws and such, this thing was tight. Nothing loose. Nothing tweaked anywhere. I was tight to the bottom of the enging and the bowl was tight to the carb body. BUT, when I was loosening the bowl screws I heard an internal "clank" after a turn or two on about the 5th screw. I did not jossle the carb when this occurred. I was simply loosening the bowl screw when I heard this noise. Upon opening the carb I found absolutely nothing unusual. The metal floats were in beautiful condition. The accelerator pump leather, spring, etc. all fine. The mixture sleeve and valve was all in fine shape. I pulled the needle valve and found no dirt or fouling and it looked like the rubber tip was literally brand new. There are no cracks in the body anywhere. About the only thing I can suspect is that the floats were stuck. But why was the leaking a progressive problem and then all of the sudden it developed a pronounced more significant failure?

I have contacted Avstar directly. I had ordered a new carb online and asked them to cancel the order since it was going to be another couple of weeks to get the unit. And I told them I want to send my unit back for a rebuild. The nice lady basically yelled at me when I told her I had opened it. When I told her it was installed on an experimental she was a little less pushy but still said that they could not put it on the bench to test it. I get that but I was very curious about what I was going to find. So far, nothing. I'll report with what if anything I learn from Avstar.
 
Sometimes the float pin can wear enough to allow the float to rub against the float bowl. This can cause the float to hang up on the bowl and result in erratic float operation. If this was the case you might have been able to see wear marks on the side of the bowl.

Glad you are safely back in the air. :)
 
The metal floats were in beautiful condition.

The condition of metal floats cannot be assessed visually. When they fail, they do so at a solder joint and allow gas to enter the interior, making them heavy. Take the float off the bowl and shake it to check for the presence of gas inside.

Larry
 
The float axel pin was sloppy as I expected from just about every other carb I have ever torn apart. Is it sloppy enough to allow the float to hit the side of the bowl? Not sure but I am going to look at that much more closely before I send to Avstar.
 
I had the float assembly out and in fact did shake it to see what I would discover. My next thought was the good old submersion test and look for bubbles. I’ll take a closer look. So far not suspicious of a bad float but in general these days nothings surprises me.
 
I had similiar carb prob where run was noticeably rich and would not shut down with mixture ctl. I found all four hex head screws which had tab locks in place but lose and leaking fuel at the carb top to bowl joint. Removed , repaired, rebuilt acc pump and seems to be back to normal now. Was a MA 4SPA

I also just had this exact problem. All four hex head screws were loose. Mixture control would not shut down engine. Tightened screws, resecured tab locks, run the engine runs and shuts down normally as it should. Just now found associated SB 366C that covers this issue.

DATE:
June 2, 2016
MANDATORY
SERVICE BULLETIN
Service Bulletin No. 366C
(Supersedes Service Bulletin No. 366B)
Engineering Aspects are
FAA Approved
SUBJECT:
Carburetor Throttle Body Screw Inspection
MODELS AFFECTED:
All Lycoming engines equipped with Marvel-Schebler, Facet, Precision, or Volare updraft carburetor models MA-3A, MA-3PA, MA-3SPA, MA-4SPA, MA-4-5, MA-4-5AA, and MA-6AA.
TIME OF COMPLIANCE: Within the next 50 hours of engine operation and every 50 hours thereafter
during routine engine maintenance
REASON FOR REVISION: Correct the CAUTION after Step 3.
NOTICE: Incomplete review of all the information in this document can cause errors. Read the entire
Service Bulletin to make sure you have a complete understanding of the requirements.
Instances have been reported of leakage through the gasket between the bowl assembly and throttle body of the carburetor, evident by fuel stains in the area of the leak. Leakage of this type can be accompanied by loose screws that attach the bowl and throttle body.
See Figure 1 for illustrative example of the throttle body attaching screws.
INSTRUCTIONS FOR COMPLIANCE:
1. Examine the carburetor for evidence of fuel stains on the outside of the bowl along the parting line.
2. Complete a check for movement between the throttle body and bowl.
3. Make sure each throttle body-to-bowl attaching screw is secure and not loose by attempting to move the screw and/or locktab washers with your fingers.
A CAUTION: IF ANY ATTACHING SCREW IS LOOSE, DO NOT ATTEMPT TO TURN THE
SCREW WITH A SCREWDRIVER. IF YOU TURN THE SCREW WITH A SCREWDRIVER, IT COULD DAMAGE THE LOCK WASHERS AND INCREASE THE LIKELIHOOD THAT THE SCREWS WILL BECOME LOOSE.
4. If there is evidence of fuel stains, movement between the throttle body and bowl, or the throttle body-to-bowl attaching screws are loose then remove the carburetor in accordance with the applicable engine and airframe service manuals. Send the carburetor to a qualified repair station for troubleshooting and repair. Reinstall the carburetor in accordance with the applicable engine and airframe service manuals.
5. Make an appropriate logbook entry showing compliance with this Service Bulletin.
General Aviation
Manufacturers Association
MO
09
ISSUED
DAY
14
YEAR
73
MO
06
REVISED
DAY
02
YEAR
PAGE NO. REVISION
1 of 2
C
16
©2016 Avco Corporation. All Rights Reserved.
Lycoming Engines is a division of Avco Corporation.
 
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