joinery45 --
There are a couple other things you should check on your new-to-you Starrett 98: First that the level vial is firmly attached to its carrier tube, and second that the vial is rotationally aligned to the base.
The outside diameter of the glass vial is slightly smaller than the inside diameter of the carrier. Traditionally the vial is fixed to the carrier with small plaster plugs cast over both ends of the vial, but some newer instruments use Room Temperature Vulcanizing (RTV) rubber instead of plaster. These materials are very stable when fully cured, are at least slightly adhesive, and don't shrink enough to overcome their adhesive properties when setting.
Be that as it may, I've found several instruments with a loose vial. A couple of times the vial has been loose enough to make an audible rattle if the instrument is vigorously shaken, although on an instrument like the Starrett 98 will make enough noise to obscure the vial rattle unless the rotating cover is held in place against an end cap while shaking the instrument.
Assuming that you have a good ear and a very quiet room, you should hear nothing except maybe fluid swishing when vigorously shaking the instrument up by your ear.
The other way to find a loose vial is to feel for it. Press your finger against the vial near an edge of the so-you-can-see-the-bubble cutout in the vial carrier and try to push the vial north, south, east, west, and down looking for any hint of movement independent of the carrier.
Level vials are very sensitive to thermal distortion, and in the time it's taken you to press the vial this way and that the heat from your finger has distorted the vial. Pick the instrument up, shake it for a few seconds to mix the fluid, set the level down -- out of the sunlight and away from heating or air conditioning vents -- and let it rest for 10 or 15 minutes before adjusting the vial as Leigh and Joe describe. (Incidentally, I'm with Joe on the importance of checking the iron for burrs and stoning off any you find . . . especially on a used instrument.)
After you've adjusted the vial tilt, "roll" the instrument around its long axis. You should be able to rotate the instrument through +/- 15 degrees or so without having the bubble change lengthwise position in the vial, which only happens if the vial's long axis is parallel to the long axis of the iron body. If the vial is rotated away from parallel to the body, the bubble will move lengthwise in the vial when the instrument is rolled.
It should go without saying, but the bubble moves to the high side of the vial. To adjust, loosen the top adjustment-clamp nut and move the now-loose end of the vial carrier end cap sideways to correct the error. I don't recall ever seeing a Starrett 98 that couldn't be adjusted within the factory clearance between the adjustment stud and end-cap hole, but I've had to open the hole in the end cap or file away a portion of the stud's thread to align the vial on some other reputable-brand millwright levels.
Incidentally, some instruments have very small setscrews in lateral holes in the vial carrier end cap. These setscrews bear against the sides of the adjustment stud, and are used to jack the vial carrier around when setting the vial axis parallel to the edge of the iron body.
Obviously you'll need to retighten the top adjustment-clamp nut and recheck the vertical and in-plane adjustment of the vial before you can consider your instrument properly adjusted.
Now let's go back and reconsider the thermal distortion of the vial for a moment. With the instrument set on a stable surface with the bubble reasonably centered in the vial, reach out and rest the tip of your index finger on the glass vial at one end of the see-the-vial slot in the vial carrier. In only a few seconds you'll see the bubble move, significantly.
Pull your finger away as soon as your see the bubble run.
Now just sit and watch. Note how much time it takes for the vial to reach thermal equilibrium agains, when the bubble will return to the earlier reasonably-centered position in the vial.
If you've not done this finger-on-vial demonstration before, I can just about guarantee you'll be amazed.
John
An aside to Leigh:
The vial in your Mitutoyo -- and most other European and Asian instruments -- is graduated in 2 millimeter increments, while that in a Starrett 199 -- and most other US and British instruments -- is graduated in 1/10 inch increments. For the same apparent specification (in terms of tilt / division), a vial with 2 mm graduations has a significantly shorter radius of curvature than a vial with 1/10 inch graduations.
Your Mitutoyo is more sensitive than a Starrett 199Z, but "twice as sensitive" probably overstates the difference.
John