What causes a generator to run fine, but not put out enough voltage? I'm only getting 75 volts at the 120 volt outlets.

With those voltages how can the voltage adjuster be working fine?

Measure the Generator after 10 minutes the first measure but not only to one power outlet with doing a measure with a multimeter its not done so because the generator is fully unloaded then you will see 105 vac
Measure on one outlet and use a paint stipper a 2000 watt type and measure at the same time on the second power outlet with a multimeter on AC you should get more volts out to measure instead of 105 volts then you hear that its generate power and blow more smoke you will be seen a 115 or 120 volts if you increase this generator its possible you get if you are increase the idle power to 120 volt can create an over loading of 200 volt or more over the outlets that can give damage if it the stator electric field generate power outside its level it can blow up your expensive tools and in a short time you think there is not enough power on my tools anymore then your armatures are become bad and the tools are hot in a short time warmt dissipation has blow up armature and nothing smels hot but the tools are blowing up. and you see nothing inside because the tools will not are getting black burned and hot but the to high magnetic pulse burns the weak points inside the armature in a fraction of time.
Beware of that the adjusting is normally doing with a tacho meter and a suggestion of about 3000rpm and not more of the stator what its max depend on model . but adjust by the load power of a working paint stripper it can have enough if you adjust 120 volt by load.

the governor is not set properly, good job catching this issue. also check it under a load, if you still find the voltages too high, I recommend having it serviced.
setting the governor can be tricky.make sure you mark the shaft.

if the rpm's are not right the voltage regulator can't put out more than its putting out

Can you manually adjust the voltage on the generator without load?? If you have 120/240 60hz output on it and you are not overloading the unit (amperage), then it may be as simple as adjusting whats called your gain setting for your voltage regulator. Obviously you are seeing what happens when it is set too low. Trying raising that setting until it becomes unstable then back it off. Unstable meaning voltage starts jumping all over.

check the voltage regulator

Most low voltage conditions on generators are simply due to a low engine rpm. Most modern generators are 2- pole windings. So your engine rpm should theoretically run @ 3600 rpm. which will produce 120/240 volts @ 60 HZ. In actuality You want to set your rpm's @ Approx. 3720rpm no load. About 62.5 Hz. Without getting to technical this will give you your desired 120/240 volts loaded. Hope this helps.

well I have a special flasher unit but I believe you can put 12 volts to the brushes while it's running. That should do it.

Hi, You don't say whether this is under load or not but if not is within the acceptable range, which is +/- 10% . Generally speaking, voltage on a generator is not the issue but the load which it will carry, hence their wattage ratings. Measure your household voltage and you will see that it varies from day to day but usually stays within the limits and your electrical panel is rated in amperage. Hope this answers your question!

no!   It will be ruined if you try that.