Diffculty starting Onan 2500 genset

There are so many things it could be... A very long process of elimination but you need to start somewhere. First, start from the beginning.. at the tank. Look at the end of the fuel line for any obstruction is it getting fuel to the regulator? Next if the line has any dip in it, it can collect junk and the oily stinky stuff that may be blocking the hose to the regulator. It is like tar, one day clear one day plugged. Remove the hose to the regulator and make sure it is clear. Next the regulator itself. How old is it? Is it an auto switch? Remove the line on the out side that goes to the generator, is it getting fuel through the regulator? Trace the line to the Generator. Remove the line from the generator and while you have it disconnected from the regulator and the generator, blow compressed air through it and make sure it is clear. Now for the generator itself. How old is it and what model is it? When did you replace the air filter if ever? Did you check the filter cover? It has to be properly seated or it will not run. Does it have a breather hose that is just inside the cover? Check if it is plugged with insects. If it does have the hose, put a screened filter on it. Check the oil level? Oil cut off switch could be an issue. Have you changed the plug? Do it just because. Check the plug wire, it can ground out if rubbing on the case, it is a very tight fit. This is just a start. There are so many different ones. A model number would help a lot as then we would know at least what and where to start if it is even the generator at all. Again start with the lines and regulator first...regulators are cheap, replace it regardless, they can fail at anytime.