Okay, so without pictures, we're very poorly illuminated on what you've really got... part numbers are frequently of very limited use on stuff like this, because they're not part of the Allis engine, they could have been sourced and re-labelled entirely different amidst the supply chain.

But congrats on acquiring a wonderful and svelte piece of industrial fortitude!

Just as an internal combustion engine has Fuel, Air, Compression, source of ignition, and timing in order to run...

ANY generator needs certain things to generate electricity.

Without seeing it, I can surmise byou your description that yours has a very large frame alternator, probably 1200 or 1800 RPM, and as such, it is wound either 4-pole (1800) or 6 pole (1200) inside.  It is probably 3-phase, with TWO sets of windings per pole, and layered out through 12 wires at the interconnect box, to provide several different outputs, to include 230/460v three-phase, or (in double-delta configuration) 120-240 single... perhaps 208/125Y, 277/480Y, etc.

In order for the main alternator to provide this output, it needs a spinning magnetic field.  Engine provides the spinning, and the rotor (in the middle) has big electromagnet windings to provide magnetism.  In order to provide magnetism, this big electromagnet gets DC power referred to as 'excitation'.

The rotor gets it's DC power excitation through slip rings on the rotor shaft, and there's brushes riding on thes slip rings... at least one per ring, frequently there's two or more, all connected together to distribute the load as the rings slide by.

The DC excitation current comes from an Exciter, which is nothing more than a DC generator, also either mounted to, or belt-driven from, the main engine/generator shaft.  As the shaft spins, the DC exciter generates DC power, which is fed through the slip rings, into the main alternator's rotor.

Output VOLTAGE and CURRENT is entirely dependant upon rotor excitation, in order to generate output.

It takes a fair bit of current to power the rotor in a big generator... anywhere from an amp or two, up to 30 or more, depending on the generator's design. The exciter is operationally identical in basic plan as a old-time tractor generator... it has a field, and an output winding.  The output winding goes to the main gen's slip rings.  The exciter's field, however, requires much LESS current, and when it is varied by a REGULATOR, can raise or lower the output of the excitation current, which in turn, raises and lowers the main generator's field intensity, thus, raising or lowering the main generator output.

The REGULATOR on modern generators modulates exciter field current with solid state parts.  YOUR generator PROBABLY does it with a mechanical regulator, and it's a pretty clever little device... usually it uses a bunch of resistors arranged in a ladder, and the ladder is laying at an angle inside the box.  There's a roller that can roll up and down the ladder... and this roller shorts-across the resistors as it rolls from one end to the other.  The roller at the high end of the ramp makes the resistor circuit yield maximum resistance, while roller at the low end makes it minimum resistance.   This resistance is placed in series with the exciter's field, thus, the roller's position controls how much excitation current is generated and sent to the main alternator field.

Here's the cool part:  The ROLLER... is ferrous... and there's an AC electromagnet on the high side of the ramp.  That electromagnet is connected to the AC OUTPUT of the main generator... so as the main generator's output goes high, the electromagnet pulls the roller UP the ramp, reducing field current, driving the main alternator's AC output DOWN.  When the main alternator's AC output sags low, the roller falls down the ramp, passing more current to the exciter field, which increases excitation to the main alternator's field, pushing AC output back up.

I work on plenty of generators, and I've revived GOBS of machines that have the 'Runs but won't generate' issue.  One can jump right in and 'flash' the field, but I've found that for most (especially those who don't know how and why generators work) this usually leads to failure and damage.

Do this instead:

Get really good pictures of the machine, and photo the ID tags of the engine, the main alternator, the exciter.

Remove the electrical box covers, and look for wiring diagrams... if you find any, PHOTO them.  Photo the wires, look for wire tag numbers.

Remove the access covers for the brushes and exciter ends.  You'll see slip rings, you'll probably see a centrifugal switch or two on the main shaft.  Photo all these.

Post your photos.

Then read my next entry.

When you've acquired a new generator of unknown or errant operational condition,  it is important, before starting it up, to remove the covers and look for beech nuts...  sunflower seeds, aluminum foil, plastic bags, chewed up upholstery from an Edsel Wagon...  and of course, dirt, straw, grass clippings, and anything else that mouse dragged in before he urinated all over the copper workings and died.

Clean that crud out.  I START with a gas-powered a leaf blower, then an air-hose and a long tube.  Then I cover up the exhaust, intake, and magneto/distributor, give it a spray bottle visit with a mild degreaser, then salute it clean with a pressure washer from NOT LESS than two feet away.

Yes, the brushes, slip rings, windings, centrifugal switches... everything but sensitive electronics and wiring diagrams.  Get the crap out.  Then fire up the leaf blower, blow the worst of the water out, and again with the air hose.

Don't worry, it's not running, so nothing's energized, and it'll be dry enough to fire up.

Take a spray bottle of your favorite lube, and a spray bottle of electrical contact cleaner in each hand, and a can of 'air' (like what IT guys use to clean cookie crumbs from your keyboard) in your back pocket.  Start with the cleaner, on the brush holders.  Eye protection on (stuff will come by for a visit!) spray each brush holder, and wiggle the brushes.  look for excessive wear, and if they're sticky, work 'em loose.  A little oil AFTER you've cleaned them  will provide protection and lubrication.  Repeat this for any centrifugal switches.

Open the cover of the mechanical regulator.  Photo any diagrams.  A dry toothbrush will sweep out any crud, a spritz of contact cleaner will rinse it out, and the 'can of air' to clear it (don't use an air hose, as they usually have enough water and oil to foul your cleaning job).

Once it's clean, and you've found nothing awry, connect some sort of RESISTIVE test load to the output.  Portable heaters, incandsecent lamps, an old kitchen stove all work great.  Leave them OFF for now, but connect a voltmeter and ammeter to the output.

Ready to start it?  No you're not.

Pick up the phone, order in some Chinese food, and request three extra sets of chopsticks.  Relax, enjoy the MSG, and keep reading:

Leave the generator's output breaker OFF. 

Start the engine.  Use a tachometer to verify that the engine is running at some speed relatively close to the generator's running RPM.  Common output frequency in the US is 60 cycles per second, which for a two-pole ('screamer') is 3600rpm.  REAL generators are four-pole (1800) or six pole (1200) design, and REALLY BIG ones will be even slower, because they're driven by oil-drinking or steam-snorting, fire breathing monsters.

Once it's spinning, look at the meters.  No output?  Take a voltmeter, set it to something appropriate for the exciter's output (some will be 20, 50, mebbie 100vdc), and look for excitation.  No excitation?  Here's where the fun starts:

take out a chopstick.  If they're metal, you ordered from a very ritzy Chinese restaraunt, you have earned the title of 'Food Snob', and deserve to be electrocuted repeatedly.

With the chopstick, tap on each of the centrifugal switches.  Oftentimes, the centrifugal switches' function will be to cut in the excitation ONCE the generator is up to speed (and to shut it down if it falls below design speed!).

Look at your meter...do you see ANY voltage?  You might see a LITTLE (say... 10-20 volts).

DO NOT... DO NOT....   DO NOT  be tempted to grab the throttle linkage and race the engine!!!

The governor of a generator is designed to be very sensitive, and to do so, designers frequently set up the governor's shaft speed to turn SEVERAL TIMES FASTER than the engine, and they run lighter weights, and stiffer springs in the centrifugal mechanism, to make it responsive to the generator's speed range.  I've worked on generators that use a 6:1 gearing on a 1200rpm generator (that's 7200rpm!!) but it's more common to see 3 or 4:1.

RACING THE ENGINE will causes the governor to go WAY BEYOND it's safe speed, and they rarely survive.

Gently tap on each of the brushes.  Look for sparkies... big and little, and LISTEN TO THE ENGINE.  When you hear it start to labor... even just a little grunt... that's your signal.  The EXCITER needs a LITTLE crankshaft horsepower in order to raise the main alternator's field... that GRUNT means it is trying to build a field.

So if you get no 'grunt', don't flash the field YET.

Tap on the voltage regulator... look for a change on your meter.

Basically, ANYTHING you do that results in a change on your meter, is a clue leading you to the problem, and if it responds to tapping, tap it MORE. 

If you get NO response to tapping... go get a BIG MAGNET.

If you can't find one big one, get a BUNCH of smaller ones (I salvage mine from car speakers and microwave ovens, but PLEASE read up on how to safely remove them, and dispose of the Klystron tube core properly, some still have beryllium).

Take that magnet, and stick it to the SIDE of the exciter.

Watch your meter!  Does it come up any?  did the engine grunt a little?  If not, move the magnet around to a different spot on the exciter case... an inch or two at a time.

Resume 'tapping' on parts.  Tap on the ones that yield the most response.

EVENTUALLY, you'll get a meter indication that jumps up a fair amount... like... it was 11v, now it's 60.

Time to throw the switch.  Set your resistive load to something modest.  I like to start with a 50w incandescent lamp (not an LED or flourescent... you want a good old filament in there).  It'll glow.  Let it glow, go take a potty break while it does. If it doesn't glow, and the meter drops, increase the engine RPM JUST A WEE BIT, and see if it comes up.  If not, shut the lamp off and let it run longer, add more magnet, etc.,

After a while, it'll glow a little stronger, watch that meter, and do some sport-tapping with a chopstick.  By the way, it's not unusual for a chopstick to get a little frizzled on the end from tapping, or getting it in somewhere closer than that it should have been.  Chopsticks are good for this.

As it starts glowing more, shift to more load... and each time, let it run for a good 20-30 minutes.

What's happening here, is that the generator's EXCITER has been 'sleeping' for a long time... it has no natural magnetism, so when it's spun, it generates no effective DC output.  A generator, when used regularly, will have RESIDUAL MAGNETISM... magnetism that is inherent as a result of it being used frequently, and when you start spinning the rotor, you get an 'inital start' off this magnetism.

A tiny bit of corrosion or dirt in connections in the regulator aren't typically a problem for an exciter's 20-200vDC range, but when it's starting up with very little residual magnetism, the intensity is not enough to create excitation output voltage high enough to break through what little resistance exists in the circuit.

ADDING a magnet to the outside helps bootstrap the exciter... it gives it an artificial magnetic field to wake up with.

After a little while, you'll have a generator that will generate full output VOLTAGE.

Start adding RESISITIVE load... no electric motors, no transformers or ballast-lamps.  Resistive... toasters, milkhouse heaters, heat lamps, electric stoves, toaster ovens, even a hair dryer.

IF you see voltage DROP... back off the load.  If you see it at spec, add more load... keep ramping it up little-at-a-time 'till you have a fair chunk of output... for smaller generators, take 'em to FULL LOAD... make them grunt.  On really big'ns, try to get to at least a half load.

Run them at that load for several hours, and repeat it tomorrow, thursday, and sat**day. If you want it to be really good, solid, reliable, and you don't have frequent outages, put it on a load for at least a half hour, once a month. 

With that exercise regimen, not only will you always have a good build of residual magnetism, the brushes and slip rings, regulator and centrifugal switches will stay free, the bearing grease will stay properly distributed (I didn't mention before, but generators that have been sitting a long time in one position, tend to have their bearing grease all at the BOTTOM of the bearing... and usually, it's got the consistency of GRANITE...).

It also keeps the engine in good health-  water pump seals, crankshaft seals, magneto/distributor, valve springs and stems all like regular exercise.

This has been a few years back.

BTW. A 6 cyl Allis  Buda diesel was in command.