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Engine back firing

P

PVS

Member
Got me stumped. After cleaning plugs and re installed them ran the engine everything was fine. Flew it one hour, stop for gas, started then did my run up before take off. 1700 rpm check my cs prop, ok. Switched of right mag drop off 75 rpms , then the left drop of 125 rpm at started back firing, turned back on rechecked the right again a 75 rpm drop then after a second it back fired. Repeated process once again same results. Ran the engine on both Mags to 2400 no back firing back to 1700 no issue with both mags but does occure on one mag. Any suggestion were to start and what sequence to start. Thank you in advance for any help.
 
Since you "just cleaned the plugs*, I would check to make sure you didn't cross the wires.
 
First of all did you do this mag check full rich or leaned aggressively? Do you have an engine monitor with data logging.

Could be mag issues, or more likely plug issues. How old are the plugs, and when were they gapped and resistance tested last?
 
Similar problem on my O-360...

Misfiring on one mag, combustion in exhaust pipe, aggressive leaning usually cleared it. A friend with a very, very good eye looked at the plugs and said "I think this may be the problem." All plugs looked the same to me, but I changed that one. Problem went away.

YMMV.
 
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Maybe check to see that you didn't accidentally get some anti-seize on one of the plugs. I did that once and caused problems.
 
here is my thoughts. take off and climb out over the airport. then do another mag check and see if the mags are ok. go from there.
 
You might check to see if after cleaning and flying, a small piece of carbon didn?t flake loose and lodge between the electrodes on one plug. Have seen this happen before. Just a guess but-------------

Larry
 
And, is it truly a backfire (combustion coming out the intake), or just a miss? If it's just missing while running on one mag, I'd also vote for fouled/defective plug or harness issue, as 1st line of attack.
 
Big thanks to all

Will try all that has been suggested. The plug wires are set in a manner I believe would be difficult to get wrong, however I would say it could never happen. I"ll check. Remove plugs and really inspect them closely. Thank you again to all for the advise.
 
do you have 4 into one exhaust or four individual pipes, if 4 into one i would agree that one plug is not firing and alowed the unburnt mixture into mixed colony of exhaust and the others that were firing lit off the unburnt mixture in the exhaust.
 
If you have an EGT, you can isolate which is the "cold" cylinder. Swap the plugs top to bottom and see if the problem moves to the other mag - indicating therefore the plugs (and which of them specifically) are the issue.

Dan
 
I just had the same thing happen on our plane. It turned out to be a piece of lead that fouled a plug. Here I an easy way to isolate the bad plug.1. On a cold engine mark each exhaust pipe with a crayon.2.run the engine for about 30 seconds.3. Check which pipe still has the crayon mark and is still cool to the touch.4. Follow the plug wire that leads to the misfiring mag. 5. Remove and inspect that plug. I'll bet it's fouled. If you have an engine analyzer just look for the cold cylinder.
 
What engine? Carb or injected? I got a serious drop on one mag with backfiring, went through plugs, wires, mags overhauled, even inj servo overhauled. It was a little piece of **** in one of the injectors. Look for the simple stuff first.

RT
 
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