There is a difference between Open Circuit Voltage and Arc Voltage. Open Circuit Voltage (OCV) is specified by the NEMA as being 82+/- volts maximum to ensure the welder isn't electrocuted when replacing the electrode in the electrode holder. The Arc Voltage is the voltage drop across the arc while welding.

You can make a simple lad bank by mixing about a pound of salt with five gallons of water in a plastic bucket. Connect each welding lead to a good sized pate that can fit into the plastic bucket. The plates should be suspended from a nonconductive support (broom stick will work) such that the places can be moved closer or further apart. The current and voltage can varied by moving the plates closer or further apart.

Caution: this process disassociates the water into hydrogen and oxygen. The accumulation of the gases can cause an explosion, so do not attempt this inside a building.

Normal arc voltage will vary between 23 to 29 volts depending on the type and diameter of the covered electrode. Low hydrogen covered electrode operate at the lower end of the voltage whereas the cellulose covered electrodes operate at the higher end of the voltage range listed.

The load cell will place a major load on the engine and the generator/alternator. You can easily toast the welding machine if you are not careful and do not monitor the voltage and amperage. Keep in mind the duty cycle of the machine and do not exceed it while testing the machine.  

I do not recommend this test method to the average welder unless he has a good understanding of basic electricity and understands duty cycle. One must recognize the inherent dangers presented when working/generating explosive gases, the potential for explosions and fire, as well as the danger to the welding machine.

Oh, by the way, it will definitely heat the engine up and blow out any carbon accumulations if one does not overheat the welder first.

Best regards - Al

Was armature dirty or dark ?  If so get it cleaned and make sure brushes are seated good.  Pull a brush back and make sure it returns and seats good.  May need new brushes or springs if doesn't.  Mice been in it while sitting?  Worse thing for it and armature and exciter will need cleaning & new brushes if so.  Seat brushes in by dribbling some water or water vinegar mix over armature while running and let it dry off then run it hard welding to get brushes seated in better.  Make sure air cleaner is not plugged up too.  It's going to heat it up some like Al said and that's what you need to do load bank it then or get a box of 3/16" or larger rods and run it hard.