Backfire at low power

danhagan

danhagan

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El Paso, TX
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danhagan
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KDNA Dona Ana NM
Plane was in annual previously and has 3 hours since. At annual, an induction leak was diagnosed and repaired as well as a small oil ine and fuel line "cracks" apparently pretty intact (not leaking - how that works I don't know).

Currently, when reducing power to 1500 RPM to land and ESPECIALLY when going throttle idle, I get backfiring approximately one every 3 seconds ... they're also not real strong. Suggestions?
 
danhagan said:
Plane was in annual previously and has 3 hours since. At annual, an induction leak was diagnosed and repaired as well as a small oil ine and fuel line "cracks" apparently pretty intact (not leaking - how that works I don't know).

Currently, when reducing power to 1500 RPM to land and ESPECIALLY when going throttle idle, I get backfiring approximately one every 3 seconds ... they're also not real strong. Suggestions?
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Adjust your fuel metering valve, it was probably enrichened to deal with the induction leak symptoms, and now it's getting loaded when you reduce throttle.
 
Henning said:
Adjust your fuel metering valve, it was probably enrichened to deal with the induction leak symptoms, and now it's getting loaded when you reduce throttle.
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Thanks Henning will have that looked at ...
 
Check for cracks in the exhaust system.
 
I'd bet the farm Henning is right on the money. Mixture set too rich to compensate for the extra air.

My Cardinal (O-360) does that if I use carb heat and full rich on base/final. Way too rich. On my plane, the mixture screw on a MA-4-5 carb should be set to give a 50 RPM rise just before the engine quits when pulling the mixture all the way out. If you get a 100 RPM or greater rise when doing this, it is probably too rich.
 
I had popping when I reduced throttle for landing and during the rollout. Went on for a long time and mechanic swore there was no leak. Until one day he heard me popping my way into the field and got fed up.

He found a clamp on the induction side with a missing nut on it in a hard to see and, gathering from his language, hard to reach spot. Replaced and tightened it and has not popped one time since.
 
Mechanics couldn't find anything. I had the prop seal repalcement and the two failed EGT probes replacement completed. Guess I'll try to fly again this weekend and see if I can replicate it.
 
danhagan said:
Mechanics couldn't find anything. I had the prop seal repalcement and the two failed EGT probes replacement completed. Guess I'll try to fly again this weekend and see if I can replicate it.
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Did they adjust the fuel servo per manufacturer spec?
 
Henning said:
Did they adjust the fuel servo per manufacturer spec?
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I'll have to check. He indicated he could not get a backfire, but I imagine things are different on the ground (as they aren't changing the mix much).
 
Flew again this past weekend. Did eight laps in the pattern, a few different mix settings on base to final (normal for max power and a couple richer than normal- a couple of power off 180s as well). Couldn't get it to backfire now. Backfiring occurred at KSRR (Ruidoso NM) and on return flight a few weeks ago, but no other flights prior or after have I been able to replicate. Guess I'll keep an eye on it ....
 
Sometime a set of conditions just line up and you get a couple of pops for no significant cause. If the problem is not repeatable, it is likely not much of a problem. Like you said keep an eye on it, now that you are aware, just monitor the situation and deal with it accordingly.
 
Not sayin you're doing this, but

I've seen people drag planes in with power, or carry a god amount of power all the way to just before the threshold, very quickly yank the power to idle and touch down, I've seen that dumb pilot trick cause backfiring on quite a few airplanes.

Not saying you're ham fisting it, but that can cause it as well.
 
James331 said:
Not sayin you're doing this, but

I've seen people drag planes in with power, or carry a god amount of power all the way to just before the threshold, very quickly yank the power to idle and touch down, I've seen that dumb pilot trick cause backfiring on quite a few airplanes.

Not saying you're ham fisting it, but that can cause it as well.
Click to expand...

You don't even have to be particularly ham fisted to do it, a long rich descent at low RPM can set you up as well.
 
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