Peddler
I have recently had two interesting service calls on Nexis controlled units both about 5-6 years old.  The first, a 14 KW, had the choke plate bent up at about a 30 degree angle and had faulted on overcrank.  It started on the first crank cycle before I discovered the bent choke plate and ran fine.  This unit has about 519 hours on it and I have adjusted the valves in the past and they were within specs. when I checked them again.  At first I thought someone had been messing with the choke but then decided it must have been a backfire and decided to replace the coils even though it starts and runs fine while I'm there.  

The second is a 20KW which again had faulted on overcrank.  This one would not start and when I took the air cleaner cover off I found the choke plate nearly vertical.  As I started to address the choke I noticed light down in the venturi and on closer examination realized the entire back of the plastic throttle body was blown out.  Looking under the airbox from the generator end I could see both butterflies and the crankcase breather nipple was broken off as well.  Again I checked the valves and they were in spec as this unit only had about 85 hours.  While checking the valve clearance I noticed #1 cylinder valve train had milky oil all over the components as where #2 was clean so I assume #1 wasn't making any heat for some time.  Again I have scheduled a coil change with a new airbox. 

Thought it interesting that I had these two in one week and they are both about the same age though much more time on the one unit.  
Quote
78buckshot
I have not run across that issue however I have had a couple with blown out mufflers, are yours running natural or LP? I guess I would do the coils and then check the fuel pressure - maybe too high?, or the solenoid not seating and letting fuel bleed through.
Quote
Peddler
The 14 KW is natural 7" wc and the 20KW with the more severe damage is LP11" wc.
Quote
78buckshot
It seems a little premature for magneto issues at 5 -6 years but with 500 hours on the 14kw you might as well do them. I am leaning towards fuel leaking into the intake tract while the units are sitting still, especially the LP but can be the same thing with the natural gas unit.
Quote
10kray
I do not know if this info is helpful to your problems but here it is, we,(our Hunting Club), had an 10K generac, LP 11"wc and we were experiencing the same issues with backfiring upon startup and those fault overcrank reports. We upon inspection found the same problems with our butterflies, they were bent badly and needed replacing. Our repair/maintenance person found that the grounding cable in the unit wasn't any good and installed a more robust cable and it resolved the backfiring upon startup problem. That unit ran and our club put 2300 hours on that unit before retiring it for a new Eco-Gen. Hope this is helpful and maybe answers  or fuels some thoughts about why this may be happening. 10KRay 
Quote