Look at the intake of the throttle body there is a plastic plate that looks like a choke but that is not it's function, but if it doesn't open it will cause the fuel to air mixture to go full rich and the engine will not start, I have had four 20kw Generac's with this plate sticking shut it would start once in a while but most times the plate would not open causing the the engine to starve for air, would not even fire, if you find that this plate is sticking then try to use a lube made for plastics silicone is one that works ok but there are others even armorall will work, one I sprayed Pam on it.
fuel problems can be tested by just taking a propane torch and opening the valve on the torch just enough to hear gas flowing out of it then put it into the throttle intake and hit the manual start, if the engine starts or even tries to fire you know you have a fuel problem, check for fuel flow after the regulator/gas shutoff valve, and even at the throttle body to make sure you have gas flow all the way to the throttle body, if you use a 15"wc gage to watch the pressure when starting the engine it can tell allot about the supply system, anytime it drops below 5"wc (3.5"wc on newer units) you have a supply problem that is starving the engine also make sure you test the pressure once you have it running at full load to make sure the gas supply is adequate for full load run (I have a 15kw heater box with blower out of a air handler which I can add a couple of other loads for larger generators for this).
Another problem is if it was set to run on propane at the previous location, almost all Generac regulators can be set for NG or propane by changing the ports you use on the regulator or other means (modal dependent), if it is set to run on propane it will not run when using NG as the fuel to air mixture will be way too low, look in the manual for changing it to NG, propane has more BTU's per cubit foot than NG has, so the fuel is cut back quite a bit by the regulator and the throttle body will get far less fuel.
If it didn't try to fire when you tested it with the propane torch then using any spark plug put into one of the plug wires and hold against a grounded part (using insulated pliers I hope), try to make sure you have spark if no spark check the wiring to the coils, make sure you are getting the pulse from the engine control board, of course the oil will shut down the output to the coils as will over temp over speed, some are latching and some are self reseting when restarting but other problems can be such as the crank shaft position censor (most V-Twin air cooled are a flywheel censor) or even the electronics on the board make sure all connections to the main control board got put back on correctly when you replaced it.
Also make sure you installed the gas line flex hose that comes with the generator straight not bent, do not pipe directly to the generator, if they didn't get the flex hose that is supposed to be with the generator get one from a dealer or Generac, also you have to size the gas pipe for the distance and 90? elbows, the manual has all the calculations on sizing the pipe, even though the pipe at the generator is a 1" many times you can end up with a 1?" or larger from the gas meter, make sure the tap at the gas meter is also the correct size not smaller then the calculated pipe size, it doesn't take much restriction to restrict 5"wc of pressure to cause a no start condition, I have had a couple of no start generators cause by the plumbers not installing a large enough pipe to over come the restriction of a long run, always check the manual for the modal you have, I have some of the manuals downloaded in PDF files, so post the model # and I might have it if you need one, also you can get them off Generac's web site by clicking on this link:
17kw manuals.
One last point that many who don't install generators on a regular bases is they forget to check the gas meter to make sure it has enough capacity for all the gas loads of the house, (most only focus on the electric load), nothing like having a backup generator run just fine in the summer but as soon as winter hits and your on generator backup then the furnace tries to fire up the generator stalls when you need it the most, most smaller older houses with older gas meters may only have a 75-150 CFH rated gas regulator/meter, if I remember right you will need at least a 250CFH for a 17kw newer ones are 261CFH, around here all newer gas meters are 250 CFH, I have found that even many plumbers do not know how to correctly size a gas system for a generator, so I have always done it just to take the problems out of the way and just tell the plumber what to install, had a few bend the flex hose 90? which is a big no-no as over time the inner walls of the hose can collapse and you will have a no-start condition, about pulled my hair out on that one.
