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RSA-5 Failure Mode?

Toobuilder

Well Known Member
I need some help! Flew my buddy's -8 for 1.7 today and had a great time exept for the last 3 minutes... After flying all over the countryside and doing landings at multiple airports, it was only after I entered the pattern at my home base that trouble began. As I pulled the power back to idle, the engine went very rough and lost significant power. It acted very much like my Hiperbipe does every time I run a tank dry (normal ops on a long cross country). So I swithched tanks (both had plenty of gas) and hit the boost pump (good pressure) all to no avail. Did a quick mag check on downwind and pulled the throttle to idle (as normal) for a power off pattern to land. At idle, it SEEMED to carry a bit more thrust than normal, so I'm thinking I'll need to kill it on short final with the mixture or ignition. Seems ok after all so I land the thing no problem. Once stopped on the runway, does not want to idle, but at anything off the idle stop, it runs up to 2000 RPM with lots of popping out the exhaust and roughness. Cylinder #3 EGT is almost nothing, so it has given up.

Once at the hangar my brain is trying to comprehend the problem so I start checking stuff that I know I can verify:

Comperession
Fuel flow and injector nozzle pattern check
Ignition timing
Intake leaks
Throttle butterfly loose/induction blockage
Plugs

With all these good, I feel it's safe to run the engine on the ground without much chance of doing any damage to the really expensive stuff. With the tail tied down, I run the engine and it is still running the same (of course), but I can monitor EGT and so forth. Long story short, it is running very, very fat, with lots of smoke, popping, sputtering and roughness. The mixture will change the behavior somewhat, but at best, it barely runs at part throttle, let alone idle. It drops a cylinder or two due to plug fouling, and they come back with new plugs, so it seems fairly obvious to me that the servo has failed somehow and there is simply a super rich mixture.

Now I am an A&P, but I don't have much experience troubleshooting Bendix injection other than flying behind the same system on my Hiperbipe for hundreds of troublefree hours. Am I missing anything? Can I replace a simple diaphragm and be on my way, or am I looking at an overhaul? In theory, this unit only has 388 hours on it, but it was overhauled before 1988, then not flown until 2003. I suppose there are some rubber parts that didn?t like sitting for all those years without use.

Any help is much appreciated!
 
Thanks for that Andy, but I'm super rich, not lean. Fuel flow and nozzle pattern is good. I'll check it anyway when I get home.

One check that I have NOT done yet is to verify the center body seal is leaking (servo outlet capped off, run boost pump with servo full rich/WOT, look for fuel dripping from the 4 impact tubes). Will do that tonight.
 
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OK,

Centerbody leak check done with no discrepancy noted, and inlet screen is clean as can be.

Don, will be calling you in the AM.
 
Mystery solved...

#1 induction tube failed at the sump. The brass bushing that is silver soldered to the tube "sucked" itself into the intake plennum, causing a massive induction leak.

...Never seen that one before, and neither has the Lycoming tech rep.

Thanks Don for the troubleshooting tips, the servo performed perfectly.
 
Been there; done that.

Michael,
I've had that same experience with the Bendix (now Avstar) on my Skybolt; pretty much exactly as you described. Cutting to the chase I had the system rebuilt. Mine was yellow tagged in 1975 with the -4 rev. They are up to -11 now and the overhaul cost is about $1200.
I did every single check in the book (intake leaks, spark plug change, etc) - except one key check. There is one thing you did not mention and that is cleaning your injector nozzles. A major overhaul shop advised me to take them out and clean them as a partially plugged nozzle could give the described indications. I did that while the RSA was in the overhaul shop. That of course is a no-no in that I'll never know if it was the problem or the RSA.
As an aside I could get the engine to run smoothly as I leaned it nearly to idle cut-off.
I don't have a clue what exactly was wrong. AVSTAR overhauled it and it works perfectly.
I don't feel to bad as the RSA needed an overhaul anyway - 35 years is pushing it just a tad. My point is; clean the nozzles then decide if you need an overhaul. BTW the folks at Airflow Performance can do the work also.
Don
 
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