BrianMartin
I have a 14 KW SN 7891023 that just started backfiring on startup and while running. 

I performed a valve lash adjustment - Cyl 2 was slightly out of spec - no change after adjustment.

NG pressure 6.5" WC static and running. Approx 15-20' of 3/4" black pipe reduced to 1/2" at the flex line. Line has not been changed since install approx 9 years ago.

Fuel plenum does not appear to be corroded.

Stepper motor working properly, and bringing the unit back to 3600rpm after each "hiccup".

Air filter is clean.

I did notice that the unit will start and run on cylinder 1 alone, but had trouble starting on cylinder 2 alone. I didn't have a lot of time so I didn't continue with a check of the coils, but at this point they are my #1 suspect.
Would a bad coil cause a small backfire at start up and while running??

Plugs, filters and oil changed a couple months ago. I will re-check plug gap clearance on my return visit.

Wondering if anyone else had a common culprit for me to check while im on site...

Thanks

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78buckshot
If you are positive that the valve lash is correct then the ignition mags would be the next suspect. Fuel system sounds good, you would not normally see a backfire issue unless the fuel was way out of spec but with these units it's pretty hard to have it that far out.
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grsthegreat
Di you have an inline spark tester to verify spark? Also how are the plugs. Im wondering if an intermittent or errant spark when its not calling for one could cause this.  
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Peddler
Replace the coils, I often see units with bad coils starrt better with one wire removed,  It is a timing issue and or sympathetic firing any way new coils will solve your problem. 
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BrianMartin
Thanks for the responses.

78buckshot, I'll check valve lash again before I go any deeper. 

grsthegreat, I do and will check the spark on my next visit.

Peddler, I'm leaning that way myself. It seemed to run smoother on one cylinder, im sure I have a weak coil.
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Jim Z
FWIW, I had a 16hp Generac that had backfiring thru the muffler. Did the valves. Cleaned the carb.  Replaced the mags. Still had the issue.  Went back through the carb more carefully this time, using tip-cleaners to confirm holes were open. Removed the welch plugs hiding the low-speed mixture, backed them out 1/2. Problem solved.
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BrianMartin
I replaced the coils today and it cleared the problem. Thanks guys!
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78buckshot
Replacing the coils is time consuming but it takes care of the issue, thanks for getting back to us.
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