IO540 backfiring

JBrown243

JBrown243

Pre-takeoff checklist
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Sep 6, 2012
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201
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San Jose, CA
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Display name:
Jason
I just brought my Comanche into the show because the engine started to what sounded like backfire at higher altitudes. It only is happening after passing 8,000 ft. I just got it back from annual, and have never had this happen before, although the last time I was above 8,000ft was about 4-5 months ago. It did not do it then though and I was at 11500 on that flight.

As soon as I get below roughly 8000ft it stops. I'm curious if this has ever happened to anyone, any ideas? I was thinking maybe it has to do with the plugs the the leads or the mags at higher altitudes (lower o2 density for spark?). That is all I can think of unless its fuel flow.

I've flown several times above 8,000 ft many times in the 3 years i have owned the plane and this is a first for me.
 
Misrigged mixture cable? Timings off?
 
What are the manifold pressure, RPM, and mixture settings? Fuel pressure/flow? Fuel condition? Spark plug, wire harness, mags?
 
Shooting from the hip, I'd look for carbon trails and resultant arcing in the magnetos.
 
Leaky intake runner?
 
SCCutler said:
Shooting from the hip, I'd look for carbon trails and resultant arcing in the magnetos.
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That's what I'd suspect. As the air pressure drops, arcing and crossfiring in the mags becomes more of an issue, and a marginal or dirty mag will let it happen at lower altitudes. High-altitude, turbocharged aircraft have pressurized mags.

Dan
 
Dan Thomas said:
That's what I'd suspect. As the air pressure drops, arcing and crossfiring in the mags becomes more of an issue, and a marginal or dirty mag will let it happen at lower altitudes. High-altitude, turbocharged aircraft have pressurized mags.

Dan
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3rd vote for this. I had a similar issue on my Cherokee 6, o-540. The left mag was arcing internally, but only above ~8000 feet. I could actually hear the popping over the radio. I switched between the mags and there was a huge power drop on the left. I did a precautionary landing, but on the ground I could do a full power run-up and there was no significant difference between the mags.
 
Ted DuPuis said:
What are the manifold pressure, RPM, and mixture settings? Fuel pressure/flow? Fuel condition? Spark plug, wire harness, mags?
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I fly it by the Lycoming for the Fuel & Power Chart IO-540-D
at 8000ft thats 21"MP and 2300RPM gives me about 10-11gph (book says 12-14). fuel is clean and new. plane is just out of annual maybe 5hrs on it since annual.

SCCutler said:
Shooting from the hip, I'd look for carbon trails and resultant arcing in the magnetos.
Click to expand...
Carbon trails off the exhaust?

COFlyBoy said:
3rd vote for this. I had a similar issue on my Cherokee 6, o-540. The left mag was arcing internally, but only above ~8000 feet. I could actually hear the popping over the radio. I switched between the mags and there was a huge power drop on the left. I did a precautionary landing, but on the ground I could do a full power run-up and there was no significant difference between the mags.
Click to expand...
how did you find the problem? My plane is in the shop they are "researching" the problem. anything I can have the shop look for and test for?
 
Carbon trails off the exhaust? [/QUOTE said:
Carbon trails on the distributor block inside the mag would allow the high- voltage impulse to find it's way to the wrong spark plug at the wrong time.
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