Voltage and frequency may or may not be related in your generator. Id guess not. Youre seeing low voltage (is this at the generator or at the load?) because of inefficiencies in the generator itself. If you are straying from 60Hz (some droop, especially under high transient load is acceptable), it is because your engine is not governed properly to stay at the right speed to ensure 60Hz output from the generator.
So if youre off of 60Hz, it may be because of the setpoint of the governor and how it is operating to ensure constant speed and proper fueling under load changes. Some of these things are just a cheap spring and a flimsy arm which may not do the job well, thus the comment of checking it.
If youre seeing 113V at the loads, under load, then Id say not to worry about it that much (especially because if you take it too high you may be at over 120V under light load, which isnt what you want). If youre seeing 113V at the generator, you may actually be below 110V at the loads, which would be out of spec for some of them potentially.
But again, you need to check voltages lightly and heavily loaded, etc. It may be that your generator just cant really support the load that you are presenting properly.
IMO, the best thing to do is go to HD and buy a kill-a-watt meter, and plug it in at one of your loads or at the end of an extension cord. Measure frequency and voltage. Try to put some motor loads of relevance on there so that your PF is down at about 0.8 to get real insight into performance.
