Are you figuring from the 14kW that it must be an NG or LPG unit, or are you just guessing?
Putting a restriction on the air intake could be causing increased manifold vacuum and drawing more gas through an undersized supply line.
Whereas on a gasoline or diesel unit it might just be preventing over speed by messing up the fuel/air mixture. (A diesel would not have a throttle plate in the first place, right?)
Nope no guessing, Generac doesn't make a 14kw portable which is the only series that they have that uses gasoline, the only 14kw is the Guardian series which is NG/propane.
And the symptoms the OP described if the generator is not shutting down on over speed is exactly as I found when the generator is starving for fuel (NG)
As far as diesel goes, they only offer a diesel in their industral series and they only make a 10kw then jump to a 15kw, up to a 600kw they don't offer a 14kw diesel.
The 14kw is a newer series they came out with when they came out with the Nexus computer controller system, prior to this it was a 15kw that on NG was derated to 14kw
over the years I have installed and repaired hundreds of Generac's but when they got greedy and changed the ability to order parts and closed their tech support to only their contractors who spent all the money to go through their training seminar and purchase $$$ in stock of parts they require that you may never use, I quit pushing them, Ive had a few of my customers get ripped off by their so called trained tech's and I wasn't very happy, had one try to tell my customer I was over loading their generator because the breaker handles added up to more then the generator could handle, after some nasty calls to Generac they finely called my customer and apologized and admitted their tech was wrong, but still since I can not get repair parts from then or use their tech support (which I very rarely needed) I just tell my customers that I will install them but I can't offer any support for them.
The problem as described by the OP is kind of vague as he didn't really say it was posting a overspeed error every time it shut down, but he did say it has given that error which led me to believe it wasn't the problem at this time, I have had a few customers think they were running way to high of an RPM because at 3600 rmps they do sound like they are screaming, so I did guess that maybe this is what the OP was thinking, as a unit with undersized NG lines or a undersized NG regulator can react just like this, it will start try to go to full speed then die, push the choke on or hold your hand over the throttle body opening and it will try to run but not at the 3600 rpms it needs to, what happens is the NG regulator fills the lines up until it get to the .5psi or 5 inch pounds of pressure, but when the generator bleeds off the pressure when starting is drains the line and the regulator wont allow enough fuel to keep the line at the required .5 psi required by the generator which wont open the solenoid gas valve, putting your hand across the throttle intake allows the vacuum to pull the solenoid open so it allows it to run but at a reduced RPM level and if you tried to load it it would just die, had one that the house didn't have a large enough gas service to feed both the generator and furnace, the generator would start and run just fine until the furnace kicked on and the generator would just die, the gas company changed their regulator and it worked just fine, this is why it is very important to follow the instructions in the manual on sizing the gas line and gas service.
If it is showing the over speed fault on the computer every time is shuts down then there are other problems such as the computer has lost RPM communication with the from the speed sensor, I had this once that was caused by field mice that got into the generator and chewed up some wires, I repaired the damaged wires and put moth balls in it and never had another problem as moth balls drive away most critters like that.
But again arm chair trouble shooting is not the best way to find a problem and it could very easily be something else, so who knows.