Decathlon sputtering when inverted...

Z

Zlin526

New member
Recently, my 1977 150hp CS Decathlon has started a relatively faint sputtering ( afterfiring) during any negative G. It's not enough to register on any instruments, but, it has never done this before. It starts immediately after rolling into inverted, level flight and continues until I roll back upright. It is very similar to the erratic firing experienced after a hot start on a summer day, but, not as loud. Upright, positive G flight is perfectly smooth...

Pre-takeoff Mag checks are normal ( 80-100 rpm total drop - 50 between mags) and timing was checked/plugs cleaned/new air filter during the annual approx. 15 hours ago. The Bendix RSA fuel system was overhauled just prior to my purchase 6 years/ 240 hrs. ago. I've been flying the plane in the same location/temperatures the entire time I've owned it and have only used fresh avgas and never experienced this until recently.

A little bit of troubleshooting I've done so far ( I'm not an A&P, but, I have a pretty good working knowledge) :

1. Fuel pressure is approximately the same inverted as upright - I've also turned the electric boost pump on while inverted with no improvement.

2. I've also tried adjusting the mixture from full rich to peak EGT while inverted (briefly!) with no effect.

3. I've also tried turning off each mag individually while inverted and the problem persists on each mag.

4. Alternate air has no effect, either.

5. I've pulled the cowling off and looked for obvious culprits ( all FWF fuel lines were replaced 24 months ago and no obvious fuel/air leaks and no apparent leaks on the gascolator and electric fuel pump shroud). I also went over the entire engine mount looking for cracks, etc.. that could be opening up under negative G and affecting ignition, fuel delivery, etc..

Has anybody experienced this or can offer some suggestions where to continue?
 
You stated that fuel pressure is approximately the same? How much of a change is “approximately the same”? Barely noticeable gauge flicker? 1 or 2 psi?

You also mentioned that you leaned to peak EGT. Was this done with an engine monitor? If so any noticeable change in CHT/EGT? You said nothing indicated on the instruments, but sometimes the scan is not as good when hanging by the belts.

Is this level inverted flight at -1G?

The only thing I can suggest is push more negative G in an inverted climb. This plus more load on the engine may exacerbate the sputtering, yielding more clues.

Bill
 
I can't add much, but I like Bill's idea of adding more neg Gs to see what happens. Sounds like fuel.
Please let us know when you find the problem.

Bill B
 
I'm happy (and embarrassed) to say I finally found the problem. After pulling the cowling for the second time and finding nothing amiss, I decided to go fly inverted again and try to make a more detailed observation. As I removed the plane from my hangar and looked over the trailing edge of the wing just prior to stepping under, I noticed that the weatherstripping on the upper right wing root, while flat, seemed to be "not in a straight line".... I reached up and lo and behold, the last 24" was detached from the aluminum skin ( there's a little 1/2" channel in the rubber that fits around the inboard skin). The problem was that the forward portion was still attached and when static ( in the hangar) , the detached end lay down flat on top of its correct position, but, unsecured. Apparently, when upright, the loose piece was sucked far enough away from the fabric ( or held down against it) and when I went inverted the airflow caused it to slap rapidly against the fabric, simulating my engine misfire...

I resecured it and went flying this evening, everything is good - except for the quarter-sized paint crack in the otherwise pristine aerothane where the loose end was slapping the fabric... :(
 
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