UUMM, an older Lincoln portable? Gas engine? Runs normally at 1550 rpms? With the age of the unit it could be that it isn't running at peak performance. My SAE 300 with a six cylinder Continental ran 3/32, 1/8, and 5/32 6010 just fine.
But, the operating parameters were definitely different than the newer units with higher rpm units.
You might have the machine put on a load band to see how it is operating.
He Is In Control, Have a Great Day, Brent