@BruceM,

You nailed it ! Thanks.
I rewired to 120 volts and all noise and vibration disappeared.
If you look at the pictures above, the 4-prong 240 volt receptacle in the orange box now has two 20A, 120 volt receptacles.
Full load and still quiet. No vibration.

My house transfer switch requires split phase 240, so this generator will be mostly useless as a backup house generator.
Fortunately it powers the greenhouse 120vac electrical + heat loads, so it still has a job.
Hmmmm.... time for another generator build for the house ? 

Now, here's a question for you (and anyone else).
The listeroid/ST-3 will be powering a greenhouse that is not grid connected.
It's electric outlets are not connected to the house, so they are not wired to a source that has ground and neutral bonded.
The ST-3 head has a floating neutral (not bonded to ground)
When powering the greenhouse, the whole system is a floating neutral.

When using the ST heads for home service the floating neutral is desired, but for powering anything other than a house, do we bond the ground with neutral?  or not?
As a test, I made a plug which connects ground to neutral, and plugged it into the generator outlet while running. No observable change.
However, the safety aspect changed. For the better? or for the worse? I don't know.

Here's another (but similar) example:
A listeroid with ST head powering a 120 volt 75 amp (24 volt)  battery charger for a large battery bank (1900 watts). No grid connection at all.
The charger is plugged directly in to a 120 outlet on the 120VAC ST head using standard 20A 120 volt plug with L1, Neutral, and Ground prongs.
Should the ground be connected to the neutral somewhere at the origin of the power ?

cheers

I spoke too soon when I installed my new ST3 head.
When I test ran the unit (unloaded) it was quiet and smooth.
Then I tested it with a balanced 240 volt ac load. Again smooth and quiet.
So I thought all was good.

Well ... today I tested it with a 1200 watt resistive space heater on one leg and only a 75 watt light on the other leg.
The growling and vibration was so bad that I thought something had broken in my system !
Terrible vibration every time I unbalanced the two legs, but just fine when the power is balanced.
A] Is this normal for an ST3 (do they need a balanced load all time) ?
B] Can it be fixed ??

Veggie

Here's some pictures.
The unit is wired for 230 volt and I am loading one leg more than the other when the vibrations and noise occurs.

Bump !!!

Any ideas at all regarding ST head efficiencies?
I'm trying to get an idea if they are much more efficient than a 24 volt Delco industrial DC alternator rated at 55%

@ Hugh Conway,

When you installed your 9/16" spring, did you make any other mods to the governor? or was it just an external spring swap ?

Veggie

Thanks 38AC,
That box you installed is much better that the thin sheet metal unit that comes with the ST heads.
Also, I will take note of the rectifier advice and buy backup.

cheers

You can place the pump on the return side of the system after the water has been cooled by the radiator.
The cooled water coming back to the engine usually does not exceed your max rating limit.
I use a standard Bell & Gosset water circulator on my Listeroid with no issues.

50 pounds of carbon powder ??   
Wow, I had better check my dual tank muffler more often than planned.
My guess is that the powder would have gone into the atmosphere otherwise, eventually settling in the soil and becoming part of the natural carbon storage system.
And you estimate that this was after ~3000 hours ?

Just wondering, what type of cooling system do you use?
Is the engine coolant getting up to 190f during use? If running too cool, maybe a lot of soot production.

veggie