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  #1  
Old 01-06-2019, 10:40 AM
stevengrasley stevengrasley is offline
 
Join Date: Oct 2011
Location: Marathon, FL
Posts: 42
Default 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.
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RV-7A, N783RV
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Lyc YO-360-A1A rebuilt 2/2021
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  #2  
Old 01-06-2019, 12:50 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,721
Default

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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Last edited by lr172 : 01-06-2019 at 12:58 PM.
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  #3  
Old 01-06-2019, 02:02 PM
stevengrasley stevengrasley is offline
 
Join Date: Oct 2011
Location: Marathon, FL
Posts: 42
Default

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?
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RV-7A, N783RV
Flying since 10/1/2013 Tach: 1,700+ hours
Lyc YO-360-A1A rebuilt 2/2021
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  #4  
Old 01-06-2019, 03:50 PM
Sam Buchanan's Avatar
Sam Buchanan Sam Buchanan is offline
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Join Date: Dec 2004
Location: North Alabama
Posts: 4,466
Default

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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Last edited by Sam Buchanan : 01-06-2019 at 04:00 PM.
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  #5  
Old 01-07-2019, 12:20 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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Quote:
Originally Posted by stevengrasley View Post
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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Last edited by lr172 : 01-07-2019 at 12:28 AM.
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  #6  
Old 01-07-2019, 07:46 AM
stevengrasley stevengrasley is offline
 
Join Date: Oct 2011
Location: Marathon, FL
Posts: 42
Default

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.
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RV-7A, N783RV
Flying since 10/1/2013 Tach: 1,700+ hours
Lyc YO-360-A1A rebuilt 2/2021
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  #7  
Old 01-07-2019, 07:49 AM
stevengrasley stevengrasley is offline
 
Join Date: Oct 2011
Location: Marathon, FL
Posts: 42
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Quote:
Originally Posted by Sam Buchanan View Post
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.
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RV-7A, N783RV
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Lyc YO-360-A1A rebuilt 2/2021
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  #8  
Old 01-07-2019, 08:27 PM
DennisRhodes DennisRhodes is offline
 
Join Date: Jul 2007
Location: Taylorsville, Ga
Posts: 886
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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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Last edited by DennisRhodes : 01-08-2019 at 08:29 AM. Reason: typo
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  #9  
Old 01-07-2019, 08:39 PM
1001001's Avatar
1001001 1001001 is offline
 
Join Date: Dec 2014
Location: Just Minutes from KBVI!
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I don?t know if the avstar carbs are constructed like the MS carbs but...

http://www.vansairforce.com/communit...d.php?t=153617
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  #10  
Old 01-07-2019, 09:08 PM
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Planecrazy232 Planecrazy232 is offline
 
Join Date: Aug 2015
Location: Cape Coral, FL
Posts: 435
Default 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.
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